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Keywords:

  • hyperpigmentation;
  • melanin;
  • melanocyte;
  • pigmentation pathway;
  • syndrome

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Hyperpigmentation is a key feature in a variety of inherited and acquired syndromes. Nonetheless, determining the exact diagnosis only on the clinical phenotype can be challenging, and a detailed search for associated symptoms is often of crucial importance. As pigmentation pathways are regulated by complex signaling transduction cascades (e.g. MSH/cAMP, KIT signaling pathways), the underlying defects leading to elevated melanin production are numerous. With regard to treatment, limited therapeutic options exist, each with specific side effects. In acquired hyperpigmentation, the melanin deposition may, however, be reversible after adequate therapy of the underlying disorder or even disappear spontaneously. In this review, we provide an overview of the biology of hyperpigmentation syndromes classified according to the main underlying defect that deregulates physiological melanogenesis. The identification of novel genes or key players involved in hyperpigmentary disorders is becoming increasingly important in view of the development of safer and more efficient treatments.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Hyperpigmentation is a frequent reason for consultation in clinical practice. It affects quality of life of patients and remains often a persistent burden due to the sparse and limited efficacy of available treatments. Hyperpigmentation disorders range from physiological phenomena to genetic inherited disorders. In other diseases, darkening of the skin is a side phenomenon from pathologic transduction cascades coincidentally interacting with pigmentation pathways. As such, hyperpigmentation can be a first sign of an underlying genetic, metabolic, or neoplastic disorder. An increased deposition of melanin in the skin can be caused by a wide range of disorders, and especially diffuse hyperpigmentation can pose a diagnostic challenge. A detailed understanding of the underlying biological phenomena in these diseases may aid in further insights into the development of new therapies. Furthermore, the pathogenesis of these disorders sheds light on the functional biology of melanocytes.

Melanocytes are specialized cells originating from a subset of multipotent stem cells at the neural crest. The precursor cells, called melanoblasts, express early specific markers such as microphthalmia-associated transcription factor (MITF), paired box gene 3a (Pax3a), and Sry-related HMG-box (Sox10). MITF has been shown to be crucial for the survival of melanoblasts. Subsequently, the melanoblasts migrate from the dorsal midline of the neural tube in a dorsolateral way to their final destination at the basal layer of the epidermis, where they acquire a mature phenotype (Thomas and Erickson, 2008). Epidermal melanocytes are the exclusive pigment producing cells offering protection against DNA-damaging UV light. They produce melanin in lysosome-related organelles, called melanosomes, which are transported from the nucleus to the peripheral border of the melanocyte by a complex melanosomal transport mechanism. Ultimately, melanosomes are transferred from the dendritic tips of the melanocytes to the surrounding keratinocytes, giving the skin its natural pigmentation (Van Gele et al., 2009).

This review focuses on the biology/pathogenesis of hyperpigmentary diseases with the aim to provide the reader a more detailed insight into the genetic causes leading to specific hyperpigmentary phenotypes. The disorders were classified based on affected genes involved in melanocyte development or involved in common cellular processes or pathways associated with melanogenesis. However, it has become clear that in most hyperpigmentation syndromes multiple pathways that regulate melanoblast differentiation/migration, melanogenesis, and melanocyte proliferation are affected simultaneously. Nonetheless, the disorders were classified according to the presumed main deregulated factor leading to the observed hyperpigmentation. Hyperpigmentation due to benign or malignant melanocytic lesions (e.g. naevi, melanoma) are distinct entities associated with increased melanin production which were not included in this review.

Pathways influencing pigmentation

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Melanin synthesis is triggered by hydroxylation of L-phenylalanine to L-tyrosine or directly from L-tyrosine. Tyrosinase hydroxylates L-tyrosine, resulting in 3,4-L-dihydroxyphenylalanine (L-DOPA), which further undergoes oxidation to dopaquinone. Thereafter, two main pathways diverge leading in the end to production of black-brown eumelanin and yellow-red pheomelanin. Microphthalmia-associated transcription factor (MITF) is a central factor in melanogenesis that upregulates the expression of tyrosinase (TYR), tyrosinase-related protein 1 (TRP1), and tyrosinase-related protein 2 (TRP2) (also called dopachrome tautomerase) (Bertolotto et al., 1998). The pivotal role of MITF is further substantiated by its role on the transport of melanosomes to the dendritic tips (Chiaverini et al., 2008). The expression of MITF is mainly controlled by the KIT and melanocyte-stimulating hormone (MSH)/ ATP to 3′,5′-cyclic adenosine monophosphate (cAMP) signaling pathways, which will be briefly discussed below.

MSH/cAMP signaling pathway

The melanocortin system involves the melanocortin peptides α-, β-, and γ-MSH and adrenocorticotropic hormone (ACTH). They are formed by proteolytic cleavage of pro-opiomelanocortin (POMC) (Grantz and Fong, 2003). α-MSH exerts its effects on pigmentation by binding the melanocortin-1 receptor (MC1R) at the cell membrane of melanocytes (Figure 1) (Yang et al., 1997). The ACTH molecule shares the first 13 amino acid sequences with MSH and exerts an activity similar to MSH. Activated melanocortin 1 receptor (MC1R) is able to bind Gαs proteins, which subsequently activate adenylate cyclase (AC) (Costin and Hearing, 2007). In turn, AC catalyzes the conversion of ATP to cAMP.

image

Figure 1. Pigmentation pathways with hyperpigmentation syndromes located according to the involved signaling cascade. AC, adenylate cyclase; ACTH, adrenocorticotropic hormone; ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; CREB, cAMP response element-binding protein; MAPK, mitogen-activated protein kinase; MC1R, melanocortin 1 receptor; MSH, melanocyte-stimulating hormone; mTOR, mammalian target of rapamycin; PI3K, phosphoinositide 3-kinase; PKA, protein kinase A; SCF, stem cell factor; TYR, tyrosinase; TRP-1, tyrosinase-related protein 1; TRP-2, tyrosinase-related protein 2.

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cAMP signaling is an essential regulator of pigmentation. cAMP directly activates protein kinase A (PKA) which is then able to transfer to the nucleus. Once arrived, PKA becomes phosphorylated and upregulates cAMP-responsive element-binding proteins (CREB) (Costin and Hearing, 2007). CREB binds to the CRE domain present in the promotor of the MITF gene. As such, transcription the MITF gene becomes initiated (Im et al., 1998). cAMP is also known to improve the binding affinity of MITF to the M-box of the TYR gene (Tachibana, 2000).

KIT signaling pathway

The KIT signaling pathway upregulates MITF by phosphorylating mitogen-activated protein kinase (MAPK). Stem cells factor (SCF) binds to the c-KIT receptor located on the cell membrane of melanocytes (Figure 1). This triggers dimerization of two subunits followed by autophosphorylation of tyrosine. Subsequently a variety of signal transduction cascades are initiated. In melanocytes, phosphorylated c-KIT receptor recruits adapter proteins such as GRB2 (growth factor receptor-bound protein 2), SHC (Src homology 2 domain-containing transforming protein 1), SOS (son of sevenless), and SHP2 (SH2 domain-containing protein tyrosine phosphatase), followed by activation of the Ras-MAPK pathway that stimulates MITF (Vance and Goding, 2004).

Wnt signaling pathway

The Wnt signaling pathway has besides important functions in embryogenesis and cancer development also a role in melanogenesis. Wnt proteins bind to the frizzled family cell surface receptors. As a result, the dishevelled family protein is activated, which inhibits a multiprotein complex consisting of axin, adenomatous polyposis coli (APC), casein kinase Iα (CKIα), and glycogen synthase kinase-3β (GSK3β). Normally this protein complex stimulates the degradation of β-catenin (Rubinfeld et al., 1996). During embryogenesis, optimal levels of β-catenin have a crucial role in the development of mature melanocytes. Moreover, activated GSK3β synergizes with MITF to enhance the expression of TYR (Khaled et al., 2002).

Hyperpigmentation due to gene defects affecting early melanocyte development and function

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Dyschromatosis symmetrica hereditaria

Patients with dyschromatosis symmetrica hereditaria (DSH) (reticulate acropigmentation of Dohi) have small hyper- and hypopigmented macules on the dorsal side of hands and feet. In a number of cases, it extends to the dorsal sides of the limbs. The disturbed skin pigmentation develops in most cases (73%) before the age of 6. The hyperpigmented macules tend to progress until adolescence. DSH is inherited by autosomal-dominant mutations in the adenosine deaminase RNA-specific (ADAR) gene (Table 1). It has been put forward that an aberrant editing of RNA during the migration of melanoblasts results in areas of hyperactive and hypoactive melanocytes with different capacity of melanin production (Cui et al., 2005; Miyamura et al., 2003). On histological examination, the melanocytes in the hyperpigmented macules are increased in size but not in number (Kono and Akiyama, 2013).

Table 1. Overview of affected genes in genetic hyperpigmentary syndromes
DiseaseGene(s)Gene locusGene functionCaused defect
Genes involved in early melanocyte development and function
Dyschromatosis symmetrica hereditaria (DSH, OMIM #127400) ADAR 1q21.3Catalyzing the deamination of adenosine to inosine in dsRNA substratesFailure of correct RNA editing during melanoblast migration: irregular distribution of hyper- or hypoactive melanocytes in the skin
Incontinentia pigmenti (IP, OMIM #308300) NEMO Xq28Essential for NF-ΚB activationMutations or deletions: abnormal self-destruction (apoptosis) of MCs
Neurofibromatosis type I (NF1, OMIM # 162200) NF1 17q11.2Gene involved in melanosome localizationDisruption of NF1-dynein heavy chain 1 interaction: increased trafficking of melanosomes in melanocytes of café au-lait macules
Genes involved in pigmentation and related pathways
Familial progressive hyperpigmentation (FPH, OMIM #145250) KITLG 12q22MelanogenesisGain-of-function mutations: increased TYR activity
Carney complex (CNC1, OMIM #160980) PRKAR1A 17q24.2Crucial signaling factor in cAMP pathwayMutated PRKAR1A: increased cAMP activity
McCune–Albright syndrome (MAS, OMIM #174800) GNAS1 20q13.32α-subunit of Gs protein (GTPase)Gain-of-function mutations: increased cAMP activity
Deregulation of the tyrosinase gene
Fanconi's anemia
(FANCA, OMIM #227650) FANCA 16q24.3DNA repair maintenance of chromosomal integrityOveractivation of TYR activity due to reduced thioredoxin levels
(FANCB, OMIM #300514) FANCB Xp22.2
(FANCC, OMIM #227645) FANCC 9q22.32
(FANCD1, OMIM #605724) BRCA2 13q13.1
(FANCD2, OMIM #227646) FANCD2 3p25.3
(FANCE, OMIM #600901) FANCE 6p21.31
Genes involved in cell structure and metabolism
Naegeli–Franceschetti–Jadassohn syndrome (NFJ, OMIM #161000) KRT14 17q21.2Melanosome uptake in keratinocytesAbnormal melanin distribution in keratinocytes
Dowling–Degos syndrome (DGS, OMIM #179850) KRT5 12q13.13

Melanosome uptake in keratinocytes

Organelle transport

Deficient melanin aggregation in keratinocytes
Peutz–Jeghers syndrome (PJS, OMIN #175200) STK11 19p13.3Serine/threonine kinase regulates cell polarity and tumor suppressor function

Dysregulation of mTOR pathway

Dysregulation of Wnt pathway

LEOPARD syndrome (LS, OMIM #151100) PTPN11 12q24.13Tyrosine phosphataseDysregulation of mTOR pathway increased RAS activity
Genes involved in DNA repair or senescence
Xeroderma pigmentosum
(XPA, OMIM #278700) XPA 9q22.33DNA repair geneFailure of nucleotide excision repair machinery
(XPB, OMIM #610651) ERCC3 2q14.3
(XPC, OMIM #278720) XPC 3p25.1
(XPD, OMIM #278730) ERCC2 19q13.32
(XPE, OMIM #278740) DDB2 11p11.2
(XPF, OMIM #278760) ERCC4 16p13.12
(XPG, OMIM #278780) ERCC5 13q33.1
Dyskeratosis congenita
(DKCX, OMIM #305000) DKC1 Xq28Components of the telomerase complexAccumulation of DNA damage increased melanin synthesis in senescent melanocytes
(DKCA1, OMIM #127550) TERC 3q26.2
(DKCA2, OMIM #613989) TERT 5p15.3

Incontinentia pigmenti

Incontinentia pigmenti (IP) is an X-linked, dominant genetic disorder which is almost only observed in females as being lethal in males before birth. It affects ectodermal structures (skin, teeth, eyes, nails) and the nervous system caused by mutations in the nuclear factor kappa-B (NF-κB) essential modulator (NEMO) gene. NEMO (Xq28) is expressed in epidermal cells, and the transcribed protein offers protection against tumor necrosis factor alpha (TNF-α)-triggered apoptosis. NEMO is a part of the regulatory subunit of the IκB kinase (IKK) complex. IKK dephosphylates IκB proteins which normally inhibit NF-κB signaling. IP is characterized by a blaschkoid hyperpigmentation due to highly skewed patterns of X-chromosome inactivation (lyonization). Four stages have been identified: during the first months of life a vesiculobullous stage is observed which is most marked on the extremities. The second stage is verrucous, followed by a hyperpigmented phase which is typically localized on the trunk and intertriginous areas. This stage takes from 3 months of age until adolescence. The last phase involves hypopigmented and atrophic lesions on the calves (Jabbari et al., 2010). This clinical phenotype can be explained by inflammatory lesions, which heal into hyperpigmented lesions resulting from melanin incontinence. Thereafter, melanin is taken up by macrophages which explains the final hypopigmented atrophic stage (Ehrenreich et al., 2007).

Linear and whorled nevoid hypermelanosis

Linear and whorled nevoid hypermelanosis (LWNH) is an uncommon sporadic skin disorder. LWNH patients have multiple swirls and whorls of hyperpigmented macules organized along the Blaschko lines. The macules have a reticulated pattern and can be segmental or diffuse. LWNH is mostly detected from birth or during the first 2 yrs and stabilizes beyond the age of 2. It has been associated with several congenital problems including neurodevelopmental delay, skeletal malformations, growth retardation, cardiac defects, and ocular abnormalities (Mehta et al., 2011). It can be differentiated from IP by the absence of an inflammatory phase with blister formation or verrucous lesions. A genetic mosaicism has been proposed as the underlying mechanism leading to areas of melanocytes with a different capacity to produce melanin (Maruani et al., 2012). A variety of chromosomal abnormalities (e.g. trisomy 7, 14, 18, 20, X-chromosome mosaicism) have been detected in patients with LWNH, especially when associated with extracutaneous symptoms (Hong et al., 2008). The large majority of documented chromosomal defects overlap with at least one gene involved in pigmentation although in a subset of cases the responsible genetic locus remains to be elucidated (Taibjee et al., 2004).

Neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is a well-known genetic disorder caused by mutations in the NF1 gene. Typical symptoms of NF1 are neurofibromas, pigmented lesions, lisch nodules, neurological abnormalities, development delay, and skeletal deformations. An increased risk for malignant peripheral nerve sheat tumors and other cancer types (gliomas, leukemia) has also been observed (Korf, 2000). Café-au-lait macules (CALMs) are a hallmark feature of this disorder being present in almost all patients with NF1. Second-hit NF1 mutations in NF1 have been proposed in lesional melanocytes (Boyd et al., 2010). However, haploinsufficiency for neurofibromin may already affect melanocytes in a specific stage of early development. This can lead to elevated presence of melanocytes derived from schwann cell precursors (Deo et al., 2013). Moreover, NF1-mutant melanocytes have an increased survival capacity in absence of growth factors such as mast cell growth factor (Wehrle-Haller et al., 2001). CALMs in NF1 patients have been shown to exhibit an increased epidermal melanization and increased amount of melanocytes (De Schepper et al., 2006). Axillary and inguinal freckling is another key feature of NF1. The NF1 gene encodes neurofibromin which is a tumor suppressor gene. Differences in melanosome size and distribution of melanocytes have been found in patients with NF1. As NF1 is a regulating factor in the Ras signaling pathway an enhanced Ras activation in melanocytes seems an obvious explanation for increased melanin production in the melanocytes of CALMs. However, normal levels of Ras-GTP have been found in these melanocytes (Griesser et al., 2000). Neurofibromin has also a regulatory capacity on the MAPK and cAMP pathway (Busca and Balotti, 2000). In absence of NF1, NF1-dynein heavy chain 1 (DHC) interaction is lost. This results in increased trafficking of melanosomes to the tips of melanocytes whereby the transfer of melanin to the surrounding keratinocytes is enhanced (Arun et al., 2013). In addition, neurofibromin colocalizes with amyloid precursor protein in melanosomes. Amyloid precursor protein functions as a kinesin-1 cargo receptor probably involved in the anterograde migration of melanosomes. In patients with NF1, this complex is lost, which may impair melanosome transport or result in formation of macromelanosomes (De Schepper et al., 2006).

Genes involved in pigmentation and related pathways

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Deregulation of the KIT pathway

Familial progressive hyperpigmentation

Familial progressive hyperpigmentation (FPH) represents a rare congenital disorder displaying a congenital diffuse hyperpigmentation. It is inherited in an autosomal-dominant or recessive manner. At birth or early in life, hyperpigmented patches become apparent, which enhance in size and number during life leading to a substantial amount of hyperpigmented skin over time including the face, neck, trunk, limbs, lips, oral mucosa, palms, and soles. On microscopic examination, an increased amount of melanin pigment can be detected in the epidermis, in particular in the stratum corneum. Outside of the skin, no other organs are affected. FPH is probably a heterogeneous disease from which most underlying molecular defects remain still to be discovered (Rojek and Niedziela, 2013). Gain-of-function mutations of kit ligand (KITL) have been found in families with FPH. KIT and KITLG genes play a crucial role in the development, survival, and proliferation of the melanocyte and melanin synthesis. KITLG is produced in the human epidermis by keratinocytes and endothelial cells (Wang et al., 2009). Interestingly, overexpression of kit ligand a (Kitla), binding to the Kita receptor, in wild-type zebrafish embryos resulted in hyperpigmented embryos with more and larger melanocytes (Hultman et al., 2007). Although the exact mechanism leading to increased numbers of melanocytes is currently not known, the authors concluded that during normal embryonic development Kitla is rate-limiting, and a correct amount of its expression is needed to produce the wild-type pigment pattern. Further studies are warranted to gain more insight into the role of Kitla/Kita signaling in cell number, cell size control, and directing melanocyte migration.

Hyperactivation of the cAMP pathway

Carney complex

Carney complex is a rare dominant inherited disorder consisting of a combination of hyperpigmented spots (lentigines, freckling, blue nevi and café-au-lait spots), schwannomas, myxomas, endocrine overactivity, and endocrine tumors. Lentigines are typically situated bilateral at the medial side of the canthi and genital mucosa. Mutations in the protein kinase, cAMP-dependent, regulatory, type I, alpha (PRKAR1A) gene have been identified as the causal factor. PRKAR1A encodes the subunit type 1-α of protein kinase A (PKA). PKA is a crucial signaling factor in the cAMP pathway, and loss of PRKAR1A function leads to an increased cAMP activity. No increased prevalence of melanoma has been observed in these patients (Lodisch and Stratakis, 2011).

McCune–Albright syndrome

McCune–Albright syndrome is characterized by café-au-lait spots, fibrous dysplasia, sexual precocity, and hyperfunction of multiple endocrine glands. The cAMP pathway is activated by a somatic mutation explaining the symptoms of the McCune–Albright syndrome. This overactive cAMP activity stimulates several cell types including melanocytes (Weinstein et al., 1991). Mutations in the GNAS gene, which encodes the alpha subunit of the G protein, have been found in patients with McCune–Albright. A constitutive activation of the cAMP pathway is caused by a missense mutation which impairs the GTPase activity of the stimulatory G protein (GSα). The phenotype of the disease is believed to be dependent on varying degrees of mosaicism. Somatic mutations in G protein signaling cascades modulate cell proliferation (Parma et al., 1994). Café-au-lait patches or linear epidermal naevi are typical dermatological signs. A marked pigmentation on the nape of the neck is also frequently observed (Millington, 2006). The hyperpigmented areas are unilateral with a sharp lining bordering the midline of the body which is a sign of hyperpigmentation caused by somatic mosaicism. Melanocytes migrate during embryogenesis from the dorsal edge of the neural crest in a ventrolateral way. The migration pathway of melanoblasts ends at the midline of the body. This results in hyperpigmentary diseases due to somatic mosaicism with a sharp midline demarcation.

Elevated stimulation of MC1R

Endocrine disorders

Addison's disease or primary adrenal insufficiency results from a deficiency to synthesize glucocorticoid and mineralocorticoid hormones. In response, adrenocorticotropin hormone (ACTH) is increased in an attempt to reactivate the adrenal gland production (Tsatmali et al., 2000). It is characterized by a diffuse hyperpigmentation especially on sun-exposed areas which is due to the elevated levels of ACTH and α-MSH. The hyperpigmentation is more pronounced on flexural areas, skin folds, areas of friction, recent scars, the vermillion border of the lips and the genital skin (Nieman and Chanco Turner, 2006). A similar mechanism takes place in Cushing's syndrome, where an overproduction of ACTH can be explained by a pituitary corticotroph adenoma or an ectopic non-pituitary tumor (Bertagna et al., 2009). Hyperpigmentation is a feature of hyperthyroidism. The pigmentation can only be apparent on the face and sun-exposed areas or be generalized, similar to Addison's disease (Stefanato and Bhawan, 1997). Excessive pigmentation in patients with hyperthyroidism is caused by increased ACTH secretion. Depigmented areas are also frequently found in patients with thyroid disease, but are mostly a sign of associated vitiligo, which has a high prevalence in patients with thyroid disorders (van Geel et al., 2013a). Phaeochromocytoma is another cause of hyper-ACTH production, which can be associated with hyperpigmentation. Dark skin can be a paraneoplastic phenomenon by release of α-MSH or analogues that bind on MC1 receptors (Zawar and Walvekar, 2004).

Melasma

Local or diffuse hyperpigmentation can be seen in a subset of women probably due to hormonal factors. Melasma is the most common presentation with hyperpigmented macules on the face which become more pronounced after sun exposure. The number of melanocytes is not increased but they become enlarged and more dendritic, suggesting a hypermetabolic state. This is highlighted by increased melanin deposition in the epidermis and dermis (Grimes et al., 2005; Kosmadaki and Kontochristopoulos, 2011). Pregnancy and oral contraceptives have been linked to increased skin pigmentation. It has been speculated that this is due to increased levels of estrogen and progesterone stimulating the activity of melanocytes (Wong and Ellis, 1984). In third-trimester pregnancy, increased levels of estrogen, progesterone, and MSH have been associated with melasma. Melanocytes express estrogen receptors and increased levels of estradiol stimulate enzymes involved in melanogenesis, in particular TYR, TRP1, and TRP2 (Kippenberger et al., 1998). In lesional skin, α-MSH is elevated compared to perilesional skin. It has been speculated that persistent overexpression of α-MSH following UV exposure plays an important role in the development of melasma (Im et al., 2007). Nonetheless, the exact pathogenesis remains to be elucidated. Other proposed hypotheses include upregulation of Wnt pathway modulator genes and prostaglandins (Kang et al., 2011). Even non-coding RNA (H19 gene) could be involved in the pathogenesis of melasma (Kim et al., 2010). UV-mediated elevations in inducible nitric oxide synthase (iNOS) levels is another plausible option which can activate the AKT-NFκB pathway (Jo et al., 2009; Passeron, 2013).

Renal failure

A diffuse, brown color of the skin can be found in patients with renal failure. The duration and severity of kidney disease has been linked to the development of pigmentation (Khanna et al., 2010). Most prominent areas include the face, the palms, and soles (Pico et al., 1992). This seems more pronounced in patients with hepatitis C (Choi et al., 2003). High levels of β-MSH have been found in patients with hyperpigmentation and renal failure. β-MSH becomes accumulated due to decreased renal clearance (Smith et al., 1975). This eventually results in an increased activation of MC1R, followed by elevated levels of cAMP inducing an excessive production of melanin. Deposition of lipochromes and carotenoids in the skin may also contribute to its appearance.

Hyperpigmentation as a result of tyrosinase deregulation

Fanconi's anemia

Fanconi's anemia (FA) is a rare autosomal recessive disorder with a progressive pancytopenia, an increased risk of cancer, several congenital defects, a mottled hyperpigmentation, and café-au-lait spots. A reduced antioxidative capacity is believed to be the underlying cause of FA. FA cells have an increased sensitivity for TNF-α induced apoptosis. FA cells have decreased levels of thioredoxin (Kontou and Adelfalk, 2002). Thioredoxin regulates an adequate activity of ribonucleotide reductase inside cells. Low thioredoxin levels lead to a decreased function of ribonucleotide reductase. This ultimately results in an increased chromosomal breakage, which is typical in FA. Beside an antioxidative capacity, thioredoxin is also involved in the pigmentation pathway. Thioredoxin regulates the function of TYR, a key enzyme in melanin synthesis. Reduced thioredoxin levels will lead to an aberrantly increased activity of TYR, resulting in hyperpigmentation of the skin (Rupptisch et al., 1998). To date, FA has been divided into 14 groups according to the affected genes. Most patients (85%) carry defects in FANC, FANCC or FANCG genes (Table 1) (Kaddar and Carreau, 2012).

Post-inflammatory hyperpigmentation

Post-inflammatory hyperpigmentation (PIH) is a very common observed entity especially in patients of darker skin type. It can develop after a wide range of endogenous or exogenous inflammatory conditions. A subset with only epidermal elevations of melanin and a subset with both epidermal and dermal melanin deposition due to pigment accumulation in melanophages located in the superficial dermis have been identified (Pandya and Guevara, 2000). Arachidonic acid-derived mediators are believed to play a central etiologic role in this condition, ultimately elevating melanin production and transfer. Especially prostaglandins (PGE2 and PGF2alpha), leukotrienes (e.g LTC4 and LTD4), and thromboxanes (TXs) have been discovered as the main inducers of tyrosinase (Yamaguchi and Hearing, 2009). In vitro stimulation of melanocytes with prostaglandins and leukotrienes leads to a swollen and dendritic aspect of melanocytes which is more suited for melanin transfer. Melanocytes express multiple types of PG receptors, such as the prostaglandin F (FP) receptors which bind PGF2α and subsequently stimulate the mitogen-activated protein kinase (MAPK)/ protein kinase C (PKC) pathway and activate phospholipase C-induced phosphoinositide turnover (Breyer et al., 1996; Costin and Hearing, 2007).

Vitamin B12 deficiency

Vitamin B12 deficiency may be associated with anemia and can cause hyperpigmentation. The brown spots are most marked on the hands and feet, in particular on the interphalangeal joints and nails. It is more commonly found in patients of dark-skinned origin (Baker et al., 1963). The exact mechanism causing hyperpigmentation is still unknown, although it is believed that vitamin B12 deprivation decreases intracellular-reduced glutathione. This leads to an increased tyrosinase activity (Hoffman et al., 2003; Mori et al., 2001).

Hyperpigmentation due to gene defects involved in cell structure and metabolism

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Defects in keratin genes

Dowling–Degos syndrome

Dowling–Degos syndrome (DGS) presents with multiple pigmented reticulated macules on the flexural areas. In some patients dark, plugged follicles are visible and atrophic pits around the perioral area. Loss-of-function mutations (haploinsufficiency) in keratin 5 (KRT5) are the underlying cause of the Dowling–Degos syndrome (Batycka-Baran et al., 2010; Pickup and Mutasim, 2011). As explained above, keratins are believed to play an important role in melanosome uptake into keratinocytes. Mutations in KRT5 have also been found in patients with EBS-MP. These mutations have been suggested to cause a deficient melanin granule aggregation explaining the clinical hyperpigmentation (Garcia et al., 2011).

Recently, involvement of the Notch pathway has been demonstrated in DDS. A mutation in the POFUT1 gene was found. POFUT1 acts by supplementing O-linked fucose to epidermal growth factor-like repeats of Notch receptors. As a result, Notch ligands bind to Notch receptors and the Notch Intracellular Domain (NICD) is released. Knockdown of POFUT1 leads to reduced expression of NOTCH1, NOTCH2, HES1, and KRT5. The Notch pathway plays a key role in the early development of melanocytes (Stahl et al., 2008; Yao et al., 2001). Loss of POFUT1 leads to decreased levels of TYR and MITF, explaining the hypopigmented macules, whereas the reticular hyperpigmentation results from an abnormal melanin distribution in the epidermis (Li et al., 2013).

Naegeli–Franceschetti–Jadassohn syndrome

Naegeli–Franceschetti–Jadassohn syndrome (NFJ) is a rare genetic syndrome which affects skin pigmentation, sweat glands (resulting in hypohidrosis), nails, hair, and teeth. The genetic defect lies in the KRT14 gene and is inherited in a dominant way. A reticular hyperpigmentation can be observed, and dermatoglyphs are absent. Diffuse palmoplantar keratoderma can also be present. The hyperpigmentation starts at an early age and is, in contrast to incontinentia pigmenti, not preceded by an inflammatory phase. The hyperpigmentation is situated on the trunk, proximal extremities, skin folds, and periocular and perioral regions. The hyperpigmented areas become more pronounced until approximately the age of 10. They fade again from 15 yrs of age leaving in the aged NFJ patients almost no clear hyperpigmented areas (Itin and Burger, 2010). Dermatopathia pigmentosa reticularis (DPR) is another dominantly inherited disorder closely related to NFJ and also linked to mutations in KRT14. Goh et al. (2009) reported a patient with a KRT14 mutation. DPR has a reticulate pigmentation combined with alopecia, nail changes, palmoplantar hyperkeratosis, and loss of dermatoglyphics.

Interestingly, a missense mutation has also been detected in the α-helical rod domain of KRT14 in a patient with a rare subtype of epidermolysis bullosa, namely epidermolysis bullosa showing mottled pigmentation (EBS-MP) (Harel et al., 2006). Genetic studies with mouse models provided supporting evidence for a novel role of keratins in regulating skin pigmentation (Gu and Coulombe, 2007). Although the exact mechanisms remain to be elucidated, it is proposed that, besides KRT5, KRT14 is also involved in the regulation of melanosome import and melanin distribution in keratinocytes explaining the reticulate or mottled hyperpigmentation of the skin. The group of Barsh performed some interesting studies on mice with dark skin. Mutations in a keratin gene were one of the few non-melanocyte-restricted genetic events involved in hyperpigmentation. A particular mutation was discovered in Keratin 2e (Thr500Pro), which is presumed to impair intermediate filament assembly. Interestingly, in these mice, the hyperkeratosis precedes the epidermal melanocytosis, suggesting a role of paracrine secretion from keratinocytes that stimulate melanocyte proliferation (Fitch et al., 2003). To further elucidate this topic, it would be interesting in future experiments to investigate the role of paracrine factors on melanocytes secreted by keratinocyte cell lines overexpressing keratin genes.

Aberrant regulation of the Ras pathway (RASopathies)

Lentiginosis

Lentigines are small (<0.5 cm) pigmented macules on the skin and mucosa. On histological examination, lentigines exhibit marked epidermal thickening and basal cell hyperpigmentation. Melanocyte hyperplasia is also a feature. Several familial syndromes exist, which are linked to endocrine, neural, and mesenchymal tumors. The most common forms are Peutz–Jeghers syndrome, Carney complex, lentiginoses, PTEN hamartoma tumor syndrome, and LEOPARD syndrome. Most of these syndromes show aberrations in the signaling pathways, leading to a dysregulation of PKA, Ras-MAP kinase, and the mammalian target of rapamycin (mTOR) (Bauer and Stratakis, 2005).

LEOPARD syndrome

LEOPARD is an acronym for a genetic disorder with lentigines, electrocardiographic conduction abnormalities, ocular hypertelorism, pulmonary stenosis, abnormal genitalia, growth retardation, and sensorineural deafness. The upper trunk and face are the most important predilection areas for the lentigines. In contrast to patients with Peutz–Jeghers syndrome, involvement of the lips is absent. No obvious increased risk of melanoma has been discovered to date. Germline mutations in the PTPN11 gene, which take part in the mTOR pathway, have been found. PTPN11 codes for the tyrosine phosphatase SHP2 which stimulates RAS/ERK signaling. Patients with LEOPARD syndrome have an impaired SHP2 phosphatase activity. The catalytic activity of Shp2 is essential for activation of the Ras/Erk cascade that is involved in neural crest specification and migration via Foxd3 and Sox10. However, unexpectedly, loss of this protein tyrosine phosphatase (PTP)-dependent pathway of shp2 in zebrafish embryos results in a similar phenotype as LEOPARD syndrome with an increased amount of pigment cells and craniofacial dysmorphia (Lodisch and Stratakis, 2011; Stewart et al., 2010).

Peutz–Jeghers syndrome

Peutz–Jeghers syndrome (PJS) is characterized by lentigines, gastrointestinal polyps, and several neoplasias (including gastrointestinal tract, pancreas, breast, ovary, and uterus). The key dermatological findings are mucosal lentigines which are found on the lips and both oral and bowel mucosa. They are also present on the ventral side of hands and feet, eyes, peri-anal region, and nares. The lentigines tend to become lighter with age making this diagnostic hallmark feature less evident in older patients. Despite the higher incidence of several tumor types, no elevated risk of melanoma is present. Mutations in the serine threonine kinase gene STK11/LKB1 are found in most patients with PJS. STK11 is a tumor suppressor gene that loses its function due to a germline mutation in combination with a somatic mutation or loss of heterozygosity of the normal allele according to the classic second-hit hypothesis. SKT11 inhibits AMP-activated protein kinase (AMPK) that inhibits mTOR [member of the phosphatidylinositide 3-kinase (PI3K) family]. As such, control over cell growth, proliferation and metabolism is lost. The lentigines are likely to represent small benign tumors (Lodisch and Stratakis, 2011). LKB1 mutants have been shown to abrogate the capacity of LKB1 to upregulate several genes in the Wnt signaling pathway [including glycogen synthase kinase-3 beta (GSK-3β) kinase] (Lin-Marq et al., 2005).

Hyperpigmentation due to gene defects involved in DNA repair or early senescence

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Dyskeratosis congenita

Dyskeratosis congenita (DKC) is characterized by a triad of mucosal leukoplakia, reticulate hyperpigmentation with poikiloderma, and nail dystrophy being present in 80–90% of the patients. Patients with DKC are at increased risk for development of bone marrow failure, pulmonary fibrosis, and several types of cancer (including a very high risk for squamous cell carcinoma of the head, neck and anogenital region) (Savage and Alter, 2009). The disease is caused by short telomeres and an aberrant stem cell function. DKC can be inherited in different ways: X-linked recessive, autosomal dominant, and autosomal recessive. The X-linked variant (DKCX) has been linked to the dyskeratosis congenita 1 (DKC1) gene which codes for dyskerin (Heiss et al., 1998). Dyskerin interacts with human telomerase RNA, which is important in the maintenance of telomeres (Ashbridge et al., 2009). Autosomal-dominant DKC (DKCA1) has been associated with the telomerase RNA component (TERC) gene which leads to the protein product telomerase component 3 (Vulliamy et al., 2001). Other telomerase-associated genes, such as telomerase reverse transcriptase (TERT), TERF1-interacting nuclear factor 2 (TINF2), NHP2 ribonucleoprotein (NHP2), and NOP10 ribonucleoprotein (NOP10), are mutated in the autosomal recessive form of DKC (DKCA2). A mouse model with deficiency of the shelterin component POT1b exhibits similar features as patients with DKC. The shelterin complex protects telomeres dampening DNA damage responses. The melanin distribution in this POT1b−/− model was similar to the pattern observed after ultraviolet exposure in human skin. The abnormal pigmentation in DKC could therefore represent a response to DNA damage (Hockemeyer et al., 2008; Kirwan and Dokal, 2008). Hyperpigmentation of the skin usually appears around 10 yrs of age. It is believed that the skin pigmentation is due to an accumulation of DNA damage. This correlates with the highly increased risk of epidermal tumors in these patients and the observed poikiloderma (Kirwan and Dokal, 2009).

Xeroderma pigmentosum

Xeroderma pigmentosum is an archetypical example of a genetic disease with a deficient DNA repair mechanism. These patients exhibit a highly increased sensitivity for UV-induced skin damage. Lentiginosis on sun-exposed areas is frequently seen which progresses and darkens over time. Hypopigmented macules may also arise in increased photoaged skin and become evident with advanced age. Several genes involved in repair of ultraviolet-induced DNA damage have been found that impair the nucleotide excision repair (NER) machinery (Lehmann et al., 2001). The observed freckling and later development of lentigines is due to early damage to melanocytes leading to changes in pigmentation (DiGiovanna and Kraemer, 2012). Increased pigmentation after chronic sun exposure is not due to an elevated number of melanocytes. In fact, it has been shown that the amount of DOPA-positive melanocytes decreases with age. However, senescent melanocytes, caused by chronic UV-damage, exhibit a greater functional activity. This increased pigment production can be additionally stimulated by an elevated production of endothelin-1 (ET-1), hepatocyte growth factor (HGF), keratinocyte growth factor (KGF), and stem cell factor (SCF) as shown in solar lentigines (Imokawa et al., 1992; Lin et al., 2010). The higher epidermal ET-1 production was associated with the level of TYR in melanocytes. ET-1 binds to its corresponding endothelin type B receptor (ETBR), initiating a signaling process and leading to PKC and MAPK activation. Induction of KGF in photoaged fibroblasts can regulate the expression of SCF in keratinocytes (Kovacs et al., 2010). As outlined in the introduction, SCF plays a key activating role in the KIT signaling pathway. Increasing evidence demonstrates that DNA damage can directly induce skin hyperpigmentation. Small DNA fragments (e.g. thymine dinucleotides) enhance TYR mRNA values and increase the response to MSH (Eller et al., 1996). The pigmentation can further be induced by p53 which can upregulate POMC/MSH expression (Cui et al., 2007). Using mouse dark skin mutants bearing mutations in genes encoding ribosomal proteins (Rps19 or Rps20), McGowan et al. (2008) were able to show that these mutations increased pigmentation when present in keratinocytes, but not in melanocytes. Reduced Rps gene dosage in keratinocytes induced apparently p53 stabilization and/or activity, leading to increased expression of Kitl and finally resulting in epidermal melanocytosis via a paracrine mechanism.

Hyperpigmentation due to accumulation of pigmented metabolites or substances

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Besides the above discussed disorders, darkening of the skin can also be due to non-melanocyte-mediated mechanisms or by a combination of factors. Metabolic storage disorders such as ochronosis are examples of non-melanin pigment, giving the skin a tinted color. Hereditary ochronosis is a genetic metabolic disorder caused by a deficiency of homogentisate 1,2 dioxygenase, which normally takes part in the degradation of tyrosine. As a result, homogentisic acid accumulates and binds to collagen, causing a black hyperpigmentation often first visible on skin overlying cartilaginous structures and the sclera (Garcia et al., 1999; Phornphutkul et al., 2002). Another example of such syndrome includes Gaucher's disease (Grabowski, 2008).

In other diseases, the hyperpigmentation is caused by a combination of pigmented substance and elevated melanin deposition. Hemochromatosis is an autosomal recessive hereditary condition with marked iron overload. In this condition, the bronze skin color is caused by the combination of iron accumulation and increased iron-induced melanin production (Kim and Kang, 2002).

Drug-induced hyperpigmentation can, besides drug-induced melanogenesis and drug-mediated post-inflammatory hyperpigmentation, also be explained by accumulation of drug and its metabolites in the skin. Nonetheless, the causal relation between drugs and hyperpigmentation is often substantiated with little evidence (Krause, 2013).

Conclusion

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References

Hyperpigmentation is a characteristic sign in a variety of disorders. In contrast to several hypopigmentation disorders (e.g. TYR in albinism, MITF in Waardenburg syndrome, KIT in piebaldism), the majority of hyperpigmentary diseases carry in general no mutation in key genes involved in melanogenesis (van Geel et al., 2013b). Instead, melanin production is in general stimulated by enhancing ligands of key receptors (e.g. for MC1R) or elevating crucial transduction signals (e.g. cAMP, MAPK, PKA). The keratin machinery essential for melanosome transfer and melanin distribution in keratinocytes can also be affected. Therefore, the reason for the clinical skin darkening often lies in multiple triggering events that take place at the same time. In various disorders, the exact signaling cascades have not been clarified exactly (including some very common encountered disorders such as post-inflammatory hyperpigmentation and melasma). Nonetheless, identification of key molecular events becomes increasingly important and will offer new treatment opportunities, giving the current progress in targeted therapies. In contrast to other skin diseases, little advances have been made in the hyperpigmentation field during the last decades, which is illustrated by the rather aspecific working mechanisms of the current treatment options (such as hydroquinone, retinoids, peelings, sunscreens…).

In conclusion, this review provides an overview of the biology of hyperpigmentation syndromes with a focus on disorders caused by deregulated pigmentation pathways. This offers insights into the complex nature of melanocytes and the interplay of numerous signaling factors that control normal melanogenesis. To date, several key players (including MSH/MC1R pathway, TYR, RAS and keratins) have been discovered, which may provide opportunities for the development of new targeted therapies. Such therapies would be very suitable for targeting melanocyte-specific genes or keratinocyte-specific genes (keratins) involved in pigmentation and/or for restoring disrupted pigmentation pathways in certain hyperpigmentary diseases. In the past, RNA interference has been successfully used for therapeutic purposes in cutaneous therapy (Geusens et al., 2009a) As such, siRNA molecules directed against TYR could be an attractive method to inhibit TYR function or activity. Some groups illustrated already that siRNA-mediated specific downregulation of TYR inhibited melanin synthesis in fish embryos (Boonanuntanasarn et al., 2003) and in in vitro cultured human melanocytes (An et al., 2009). Additionally, we recently showed that silencing of TYR in a human reconstructed skin model resulted in a strong reduction of pigmentation (Van Gele et al., 2011). Similar siRNA-based experiments could be used to inhibit the expression of MC1R or RAS in melanocytes in order to prove an effect on pigmentation.

Another important step toward the development of targeted therapies for hyperpigmentary skin diseases is the development of a suitable carrier able to penetrate deep into the skin and to deliver the siRNA in a sufficient amount to the target cells. Our group recently developed ultradeformable nanosomes, named SECosomes, which were able to deliver siRNA into the viable epidermis of ex vivo human excised skin when topically applied (Geusens et al., 2009b). Such approach would be useful to test the effect of siRNAs directed against TYR, MC1R or RAS, encapsulated in these SECosmes, and applied onto excised hyperpigmented lesions, provided that the skin explants can be kept viable throughout the experiment. If not, one could apply the topical siRNA-formulations onto commercially available pigmented skin equivalents (e.g. MatTek's Melanoderm model). For topical delivery purposes, this setting is preferable over the use of animal models, as the observations arising from their use can be confounding due to interspecies variations in morphology and permeability. Further improvement (penetration capacity, efficacy …) of existing deformable liposomal carriers, to ensure that they can target melanocytes to deliver efficiently their payload, will be a challenging task in the near future, but nevertheless a very important one before starting clinical trials on patients with hyperpigmentary diseases.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Pathways influencing pigmentation
  5. Hyperpigmentation due to gene defects affecting early melanocyte development and function
  6. Genes involved in pigmentation and related pathways
  7. Hyperpigmentation due to gene defects involved in cell structure and metabolism
  8. Hyperpigmentation due to gene defects involved in DNA repair or early senescence
  9. Hyperpigmentation due to accumulation of pigmented metabolites or substances
  10. Conclusion
  11. Acknowledgements
  12. References