Pleiotropic cancer manifestations of germline CDH1 mutations: Risks and management

Abstract Germline CDH1 defects are related with the development of multiple cancers due its pleiotropic nature. These several conditions are associated with various risks of penetrance and with different clinical management strategies. In this clinical review, we described the penetrance risks of gastric, breast, prostate, and colorectal cancers, in CDH1 carriers, within as well as outside the familial setting, and the best approaches to manage each risk, using either prophylactic surgery or surveillance.

family members had died from this disease. 3 Due to the high penetrance of GC in this ethnicity, current international guidelines recommend CDH1 genetic screening in all Māori individuals. 4 To date, the roughly 500 germline CDH1 mutations that have been detected worldwide have revealed a significantly heterogenous distribution at a global level. 5 Further studies have reported mutations also in cancers other than a gastric tumor. Indeed, breast, prostate, and colorectal cancer, as well as some congenital malformations, 6 have also been found to associate with germline CDH1 mutations. If GC is not the only phenotype associated with an altered CDH1 genotype, additional considerations must be made particularly in relation to clinical management.
Herein, we discuss the multiple cancer phenotypes that are currently known to be associated with germline CDH1 pathogenic mutations and their possible implications for risk containment. RNA that encodes a 120 kDa protein called E-cadherin. 7 This macromolecule is a transmembrane glycoprotein expressed in the epithelial tissue and is responsible for calcium-dependent, cell-to-cell adhesion. 8 Other well-known members of the cadherin family are N-cadherin (neuronal) and P-cadherin (placental).

| E-cadherin protein structure
E-cadherin has been demonstrated to be critical for establishing and maintaining polarized and differentiated epithelia through the formation of intercellular adhesion complexes. Its structure comprises three major domains, namely: a "signal peptide" comprising 27 amino acids encoded by exons 1 and 2, a "precursor peptide" consisting of 154 amino acids encoded by exons 2 to 4, and a "mature protein" containing 728 amino acids encoded by exons 4-16.
The "mature protein" is comprised of an intracellular domain, a transmembrane domain, and an extracellular domain. The latter is formed by five tandem cadherin repeats known as "cadherin domains" interacts with cytoskeleton actin filaments through catenins (α-, βand γ-catenin and p120 ctn ) to regulate intracellular signaling pathways. In particular, β-catenin attaches to the C-terminal region of E-cadherin and then to α-catenin, thus linking the complex to the actin cytoskeleton. p120 ctn binds to a juxta-membrane site in E-cadherin cytoplasmic tail. 11 The cadherin-catenins complex is involved in intracellular signaling, and, when deregulated, promotes tumor growth through the Wnt-signaling pathway. 12  E-cadherin loss results in abnormal activation of the EGFR and Notch pathways, with consequences on cell motility, invasion, and resistance to apoptotic stimuli. 15 It was demonstrated that mutations affecting the extracellular domain of E-cadherin lead to the activation of EGFR upon EGF stimulation as well as of its downstream effectors (RhoA, Src kinase, and p38 MAPK). 16,17 Importantly, HDGC patients with mutations in exons 4-13 of the CDH1 gene may benefit from treatment with EGFR inhibitors. 16 In GC setting, it was demonstrated that patients carrying somatic CDH1 alterations were associated with poor survival and worse prognosis, thus confirming CDH1 as a prognostic/predictive molecular biomarker. 18 In relation to breast cancer (BC), regular E-cadherin functions as an inhibitor of metastasis. It has been shown that somatic E-cadherin inactivation is associated with an aggressive pattern of BC, particularly lymphovascular invasion and metastasis in the axillary lymph nodes. 19,20 3 | CLINICAL CRITERIA In 1999, the International Gastric Cancer Linkage Consortium (IGCLC) defined families with the HDGC syndrome associated with CDH1 germline mutations as those fulfilling one of the following criteria 1 : (a) two or more documented cases of Diffuse Gastric Cancer (DGC) in first-or second-degree relatives, with at least one diagnosed before the age of 50 years; (b) three or more cases of documented DGC in first-or second-degree relatives, independent of the age of onset. However, due to the increase in the CDH1 germline mutation rate, those initial criteria have been recognized insufficient and too stringent. According to recent literature, two independent cancer conditions, associated with DGC and Lobular Breast Cancer (LBC) respectively, can be distinguished.

| Hereditary diffuse gastric cancer
Recently, novel international guidelines for CDH1 genetic screening have been published as follows 4 :

| Hereditary lobular breast cancer (HLBC)
In 2020, the IGCLC recognized that the HLBC syndrome presents possible independent traits from the classic HDGC spectrum. 4 In 2018, more specific clinical criteria had been already introduced to select LBC patients for CDH1 genetic screening. For HLBC, the panel established the following criteria: (a) bilateral LBC with or without family history of LBC, with age at onset <50 years; and (b) unilateral LBC with family history of LBC, with age at onset <45 years. When adopting these criteria, the probability to identify germline CDH1 mutations is estimated to be around 3% in high-risk LBC patients, 21 and 0.5% in unselected LBCs. 22 4 | DIFFUSE GASTRIC CANCER

| Penetrance risk
DGC is the main cancer phenotype unequivocally associated with germline E-cadherin pathogenic mutations. To date, it is assessed that about 80%-90% of GCs appear as sporadic forms, while 10%-20% are within a familial setting. However, only 1%-3% of them are related to documented germline alterations. 23 In a recent study, the majority of HDGC families segregated only for DGC, without association with other cancer phenotypes (Figure 2). 5 Indeed, 95 families, accounting for about 66% of all screened pedigrees, were found to present a classic HDGC phenotype (unpublished data, personal archive). Penetrance risk for DGC development in germline CDH1 mutation carriers is not "fixed," but appears to vary depending on several factors: country of origin (high-vs. low-risk areas for GC), mutation subtypes (truncating vs. nontruncating mutations), family history (positive vs negative history), adopted clinical criteria (stringent vs. broader).

| Risk-reducing measures
The measures that can be taken to contain GC risk are either prophylactic total gastrectomy (PTG) or gastric endoscopic surveillance. In HDGC with exclusive DGC manifestation, endoscopic surveillance seems insufficient to detect early gastric lesions associated with CDH1 mutations, because the tumor is often multifocal, tumor cells infiltrate the mucosa, the epithelium presents a normal surface, and each focus is usually less than 1 mm in diameter at most. 28

| Penetrance risk
Mutation rate detection in LBC is increasing. 22,30,32 It is interesting to note that the majority of the screened LBC families were not associated with the DGC spectrum. The exact GC risk in HLBC, and, In the case of mixed HDGC syndrome, the risk of BC for females is 42% (95% CI, 23%-68%), in accord with the IGCLC criteria. 25 Roberts et al. have reported a similar penetrance risk (55%) when analyzing families with at least one case of GC in their history. 26 Unselected BCs have also a similar penetrance risk of 42.9% (Table 1). 27 In the case of HLBC, the exact risk of developing LBC is still unknown, due to insufficient evidence on penetrance.

| Risk-reducing measures
Although rare, CDH1 is the only gene that has been so far associated  (Figure 3). 34 To date, out of the 15 families that have been identified with this spectrum, 10 have shown an association with GC history, and five have not (Figure 2). 35 These data are too limited to provide an explanation for PC-GC association with germline CDH1 mutations, and no indications are available on risk containment measures.

| COLORECTAL CANCER (CRC)
CRC is the fourth most common cancer phenotype in the world, but a very rare event in germline CDH1 mutation carriers with HDGC syndrome. 35 Only six families harboring seven different CDH1 mutations have been so far described to have developed CRC ( Figure 2). Penetrance risk is low and has been estimated to be 7% for males and 4% for females (Table 1). 26 The available evidence is insufficient to recommend additional colorectal cancer screening in addition to adherence to national population screening guidelines. 4

| CONCLUSION
According to the literature, about 7% of all the so-far identified germline CDH1 mutations are present in non-gastric tumors. 35