Published Online: 27 JAN 2006
Copyright © 2001 John Wiley & Sons, Ltd. All rights reserved.
How to Cite
Lohmann, D. R. and Gallie, B. L. 2006. Retinoblastoma. eLS. .
- Published Online: 27 JAN 2006
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Retinoblastoma is a malignant tumor that originates from the developing retina. The estimated incidence is between one in 15000 and one in 20000 live births. Diagnosis is based on clinical signs and symptoms, and is usually made in children under the age of 5 years. The first presenting sign is most often a white pupillary reflex (leucocoria). Strabismus is the second most common sign and may accompany or precede leucocoria. The diagnosis of retinoblastoma is usually established by examining the fundus of the eye using indirect ophthalmoscopy. Additional diagnostic tools such as computed tomography, magnetic resonance imaging and ultrasonography may be required for differential diagnosis and staging. If tumor material has been obtained, histopathology can confirm the diagnosis of retinoblastoma.
Most patients (about 60%) have retinoblastoma in one eye only (unilateral retinoblastoma), and occasionally multiple tumor foci can be found (unilateral multifocal retinoblastoma). Patients with unilateral retinoblastoma most often have sporadic disease, that is, no other case of retinoblastoma has been noted in their family. In about 40% of patients, both eyes are affected (bilateral retinoblastoma), usually with more than one focus per eye (bilateral multifocal retinoblastoma). In children with bilateral retinoblastoma, diagnosis is made earlier than in children who develop retinoblastoma in only one eye (median age at diagnosis 11 and 22 months respectively). In only 10% of patients is there a family history of retinoblastoma (familial retinoblastoma). However, examination of the retina in all first-degree relatives of retinoblastoma patients is required to identify retinal scars or quiescent tumors (retinomas) because these lesions also indicate familial disease.
The treatment of retinoblastoma depends on tumor stage, the number of tumor foci (unifocal, unilateral multifocal or bilateral disease), the localization and size of the tumor(s) within the eye, the presence of vitreous seeding and the age of the child. Treatment options include enucleation, external-beam radiation, cryotherapy, photocoagulation, brachytherapy with episcleral plaques and systemic chemotherapy combined with local therapy. If tumor cells have not yet invaded the extraocular tissues, treatment is successful in most patients. Metastasizing retinoblastoma is fatal in most patients. Following successful treatment, children require frequent follow-up examinations for the early detection of new intraocular tumors.
Patients with bilateral retinoblastoma have an increased risk of specific neoplasms outside the eye (second tumors). The spectrum of second tumors includes osteogenic sarcoma, soft tissue sarcoma and malignant melanoma. The risk for a second tumor developing is increased in patients who have received external beam radiation for treatment of bilateral retinoblastoma.
Genetics of Retinoblastoma
The development of retinoblastoma is initiated by two mutations (two-hit model) that impair the function of both alleles of the retinoblastoma gene (RB1). The origin of the first of the two oncogenic mutations is important in terms of the genetic risk to offspring and the clinical presentation of the retinoblastoma.
In patients with nonhereditary retinoblastoma, the first oncogenic mutation has occurred in a somatic cell, and the mutant allele is not present in any of the patient's germ-line cells. Development of a tumor focus is initiated by loss of the normal allele (second mutation). With rare exceptions, patients with nonhereditary retinoblastoma have unilateral retinoblastoma. About 90% of patients with isolated unilateral tumors, and in addition a few patients with isolated bilateral tumors, have nonhereditary retinoblastoma.
Retinoblastoma can be inherited as an autosomal dominant trait (hereditary retinoblastoma) that is caused by germ-line mutations in the RB1 gene. Patients who are heterozygous for a germ-line mutation are predisposed to retinoblastoma because just one second RB1 gene mutation in a retinal progenitor cell is sufficient to initiate the development of a single tumor focus. In most patients with isolated bilateral retinoblastoma, the predisposing RB1 gene mutation has occurred de novo. Typically, patients with hereditary retinoblastoma have multiple tumor foci in both eyes. Phenotypic expression, however, is dependent on the nature of the predisposing mutation. Specific RB1 gene mutations are associated with milder expression (unilateral retinoblastoma) or incomplete penetrance in families (see below). In addition to patients with familial retinoblastoma and almost all patients with isolated bilateral retinoblastoma, some patients with isolated unilateral retinoblastoma also have hereditary retinoblastoma.
In some patients, the first oncogenic RB1 gene mutation has occurred during embryonic development. In these patients, the mutation is not present in all cells (mutational mosaicism), and clinical presentation as well as the transmissibility of the retinoblastoma will depend on the number and the types of cell that carry the oncogenic mutation. It is reasonable to assume that mutational mosaicism is relatively common among patients with isolated unilateral retinoblastoma. Available data suggest that about 8% of patients with isolated bilateral retinoblastoma carry mosaicism.
Although mutations in both alleles of the RB1 gene are a prerequisite for the development of retinoblastoma, there is reason to believe that additional genetic alterations are required for progression to a malignant tumor. Retinoma, which is a nonmalignant tumor that is identified in some carriers of oncogenic RB1 gene mutations, might be regarded as a precursor lesion, and the malignant transformation of such a tumor has been observed. Moreover, investigation of retinoblastoma by cytogenetic analysis or comparative genomic hybridization (CGH) has revealed recurrent genetic alterations, including isochromosome of the short arm of chromosome 6 (i(6p)), in about 60% of tumors.
The retinoblastoma gene (RB1) is located on chromosome 13q14. It consists of 27 exons that are scattered over 183 kb of genomic sequence (Figure 1a). At its 5′ end, the RB1 gene has a CpG island that is normally unmethylated. The promoter region contains binding motifs for transcription factors Spl and ATF but no TATA or CAAT elements. In tissues investigated so far, the gene is transcribed into a 4.7 kb messenger ribonucleic acid (mRNA) with no convincing evidence for alternative splicing. The open reading frame, which starts in exon 1 and is terminated in exon 27, has 2.7 kb and is followed by a 2-kb untranslated region (Figure 1b). Homologs of the human RB1 gene have been identified in a wide variety of organisms and show a high level of sequence similarity in translated regions. The part of the gene that encodes the domains for the A/B pocket (see below) has a homolog also in higher plants (mat3).
The RB1 gene encodes the retinoblastoma protein (pRb), a 928 amino acid nuclear phosphoprotein that migrates at 110 kDa in SDS-PAGE when hypophosphorylated (Figure 1c). pRb is part of a small family of nuclear proteins that includes p107 and p130. These proteins share significant sequence similarity in two discontinuous areas that constitute the A/B pocket (pocket proteins). Conditional on the phosphorylation status at multiple serine and threonine residues in other regions of the protein, the A/B pocket can bind to members of the E2F family of transcription factors as well as to transforming proteins of deoxyribonucleic acid tumor viruses (e.g. adenovirus E1a, simian virus 40 T antigen and human papillomavirus E7) and endogenous nuclear proteins that contain the LxCxE peptide motif (such as histone deacetylases 1 and 2). The C-terminal region of pRb contains a nuclear localization signal and a cyclin–cyclin-dependent kinase (cdk) interaction motif that enables it to be recognized and phosphorylated by cyclin–cdk complexes. The C-terminal region can also bind to the nuclear c-Abl tyrosine kinase and to MDM2, which are proteins with oncogenic properties.
The role of pRb that is understood best is its function as a gatekeeper that negatively regulates progression through G1 phase of the cell cycle. During the G1 phase of the cell cycle, pRb is hypophosphorylated. Unphosphorylated Rb can bind E2F and cause a repression of E2F-mediated transcription. Beginning in late G1 and continuing into the M phase, pRb is phosphorylated by G1 cdks. Upon phosphorylation of pRb, E2F is released and promotes the expression of genes that are required for cell division. Consequently, pRb controls cell cycle phase transition by transcriptional repression. In addition to phosphorylation, cell cycle-dependent acetylation has been found to control pRb function. Acetylation hinders the phosphorylation of pRb and enhances binding to the MDM2 oncoprotein. Besides cell cycle regulation, pRb probably has a variety of roles including control of apoptosis and stimulation of differentiation.
Spectrum of RB1 gene mutations
Conventional cytogenetic analysis of peripheral blood lymphocytes shows deletions and rearrangements involving 13q14 in about 8% of patients with bilateral and up to 5% of patients with sporadic unilateral retinoblastoma. Large interstitial deletions are often associated with facial dysmorphism and developmental delay (13q deletion syndrome).
About 17% of patients with bilateral or familial retinoblastoma have subcytogenetic deletions of parts of the RB1 gene. Until now, no evidence for recurrent rearrangements or hot spots of deletion breakpoints has emerged.
Deletions or insertions of one or a few base pairs are identified in about 30% of patients with bilateral or familial retinoblastoma. These mutations are found in all coding regions and splice sites except the 3′-terminal exons. A few sites in the RB1 gene that contain repetitive sequence motifs show a higher mutation frequency. Most small-length mutations result in a premature termination codon because of either a frameshift or a disruption of splice signals. In-frame deletions are rare and are almost restricted to the regions that code for the A/B pocket.
Single-base substitutions are found in about 50% of patients with hereditary retinoblastoma and have been identified in all coding regions and splice sites except the two 3′-terminal exons. In addition, transcription factor binding sites upstream of the start-ATG can be disrupted by base substitutions. Most single-base substitutions are nonsense mutations. Among these, recurrent CpG transitions at 12 of the 15 CGA codons within the open reading frame are most frequent. In addition, the CpG dinucleotide contained in the 5′ splice site of intron 12 (AACgta to AACata) is a target of recurrent transitions. About 10% of reported substitutions result in missense changes and most of them affect the A/B pocket. A relatively frequent missense mutation is a CpG transition that causes an R661W in exon 20.
Oncogenic mutations in retinoblastoma
With two important exceptions, the spectrum of oncogenic mutations in retinoblastomas corresponds to that observed in constitutional cells. In about 65% of tumors from patients that are heterozygous for an RB1 gene mutation, the normal allele is lost because of chromosomal mechanisms that also result in a loss of heterozygosity at polymorphic loci (LOH). Most of these tumors are homozygous for the mutant allele and show complete or partial isodisomy because of chromosomal nondisjunction and mitotic recombination respectively. Hypermethylation of the CpG-rich island at the 5′ end of the RB1 gene, which is normally unmethylated, is observed in about 10% of retinoblastomas and results in transcriptional silencing and thus loss of function.
Heterozygous carriers of an oncogenic RB1 gene mutation can show variable phenotypic expression. This is to be expected because the second mutation that initiates tumor formation is a chance event. Analysis of phenotypic variation has shown that, within most families, the proportion of mutation carriers with bilateral, unilateral and no tumors complies with the ratios that are expected if the second mutation events follow a Poisson distribution. However, penetrance and expressivity can vary between families. A significant proportion of interfamilial variance of penetrance and expressivity can be explained by allelic heterogeneity.
In most families with retinoblastoma, penetrance is complete (100%) and, with rare exceptions, all mutation carriers who have inherited the mutant allele show bilateral retinoblastoma (Figure 2a, Figure 2b). Typically, retinoblastoma predisposition in these families is caused by mutant alleles with premature termination codons or deletions of significant parts or the whole RB1 gene. In RNA from constitutional cells of heterozygous mutation carriers, nonsense transcripts are less abundant than transcripts from the normal allele. This indicates that mutant RB1 transcripts can be subject to nonsense-mediated decay and may explain why oncogenic alleles with premature termination codons in any of exons 2–25 show a similar phenotypic expression.
In a few families, penetrance is incomplete, and, between families, observed values may vary from 20% to 80% (low-penetrance retinoblastoma; Figure 2c, Figure 2d). In most of these families, incomplete penetrance is accompanied by mild expressivity (unilateral retinoblastoma). Families with incomplete penetrance and mild expressivity have mutant alleles that result in reduced levels of structurally normal transcript (promoter mutations) or result in only partial loss of function because of missense or in-frame alterations (Figure 1d). Most of these mutations involve the regions that encode the A/B pocket of pRb.
Carriers of an oncogenic RB1 gene mutation also show variable phenotypic expression with regard to second tumors. Although a higher incidence of second tumors in patients who were exposed to external beam radiation for the treatment of retinoblastoma indicates that environmental factors are important, there might be an influence of genetic variation. A link between specific oncogenic RB1 gene mutations and a higher risk for second tumors has not been established. Available data suggest that carriers of mutations associated with low penetrance might have a lower incidence of second tumors compared to carriers of null mutations.
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- Retinoblastoma 1 (including osteosarcoma) (RB1); Locus ID 5925. LocusLink: http://www.ncbi.nlm.nih.gov/LocusLink/LocRpt.cgi?=5925
- Retinoblastoma 1 (including osteosarcoma) (RB1); MIM number: 180200. OMIM: http://www.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?180200