APC somatic mosaicism in a patient with Gardner syndrome carrying the E1573X mutation: report of a case

Paired Somatic-Germline Testing of 15 Polyposis and Colorectal Cancer-Predisposing Genes Highlights the Role of APC Mosaicism in de Novo Familial Adenomatous Polyposis

Familial adenomatous polyposis (FAP) is an autosomal dominant syndrome responsible for 1% of colorectal cancers (CRCs). Up to 90% of classic FAPs are caused by inactivating mutations in APC, and mosaicism has been previously reported in 20% of de novo cases, usually linked to milder phenotypic manifestations. This study aimed to explore the prevalence of mosaicism in 11 unsolved cases of classic FAP and to evaluate the diagnostic yield of somatic testing. Paired samples of colorectal polyps, tumors, and/or mucosa were analyzed using a custom next-generation sequencing panel targeting 15 polyposis and CRC-predisposing genes. Whenever possible, the extension of mosaicism to blood or sperm was also examined. Of 11 patients with classic adenomatous polyposis, a mosaic pathogenic variant in APC was identified in 7 (64%). No other altered genes were identified. In two of seven patients (29%), mosaicism was found restricted to colonic tissues, whereas in five of seven patients (71%), it was extended to the blood. Germline affectation was confirmed in one patient. We report the first analysis at a somatic level of 15 genes associated with CRC susceptibility, which highlights the role of APC mosaicism in classic FAP etiology. The results further reinforce the importance of testing target tissues when blood test results are negative.

APC c.4621C>T variant causing Gardner's syndrome in a Han Chinese family may be inherited through maternal mosaicism

Gardner's syndrome is a rare autosomal dominant hereditary disease that is characterized by multiple colorectal polyps combined with extra-colonic presentation (such as osteoma or desmoid tumors) of familial adenomatous polyposis syndrome. Gardner's syndrome is caused by the mutation of the adenomatous polyposis coli (APC) gene, which is located at 5q21. The aim of the current study was to investigate the APC gene mutations present in a Han Chinese family diagnosed with Gardner's syndrome. The 38-year-old proband presented with clinical symptoms, and was later diagnosed with Gardner's syndrome. Genomic DNA was extracted from the peripheral venous blood of 150 normal controls as well as the family members of the proband. Analysis of the respective APC gene sequences was performed using PCR amplification and Sanger sequencing. Pathogenesis associated with the APC mutation was investigated using reverse-transcription quantitative PCR and determined through bioinformatics approaches. Haplotype analysis was performed to identify the genetic source of the mutation(s). In the initial screening for APC variants, the APC c.4621C>T variant was detected in the proband and his son, but was not detected in the proband's affected mother. The mRNA expression changed significantly according to age and the presence of the mutation in the blood of the patients. Haplotype analysis suggested the presence of maternal mosaicism for this mutation. Haplotype analysis revealed that the APC c.4621C>T variant in a patient with Gardner's syndrome was most likely derived from his mother through mosaicism. These results indicate the necessity to verify the possibility of gonadal mosaicism when a proband diagnosed with Gardner's syndrome appears to exhibit a de novo mutation.

Mosaicism in Patients With Colorectal Cancer or Polyposis Syndromes: A Systematic Review

BACKGROUND & AIMS

Somatic mosaicism, in which variants arise post-zygotically and are therefore not present in all cells in the body, may be an underestimated cause of colorectal cancer (CRC) and polyposis syndromes. We performed a systematic review to provide a comprehensive overview of somatic mosaicism in patients with CRC and polyposis syndromes.

METHODS

We searched PubMed through March 2018 to identify reports of mosaicism in patients with CRC or polyposis syndromes. We divided the final set of studies into 3 subgroups describing APC mosaicism, mosaicism in other CRC susceptibility genes, and epigenetic mosaicism.

RESULTS

Of the 232 articles identified in our systematic search, 46 met the criteria for further analysis. Of these, 35 studies described mosaic variants or epimutations in patients with CRC or polyposis syndromes. Nineteen studies described APC mosaicism, comprising a total of 57 patients. Six described mosaicism in genes associated with familial CRC syndromes, such as Lynch and Cowden syndromes. Ten studies described epigenetic mosaicism, sometimes resulting from a germline variant (such as deletion of EPCAM).

CONCLUSIONS

We found that somatic mosaicism is underdiagnosed but critical for determining the clinical management of patients with de novo polyposis who possibly carry mosaic APC variants, and present a decision tree for the clinical management of these patients. Mosaicism in genes associated with susceptibility to CRC contributes to development of other familial CRC syndromes. Heritable epigenetic mosaicism is likely underestimated and could have a dominant pattern of inheritance. However, the inheritance of primary mosaic epimutations, without an underlying genetic cause, is complex and not fully understood.

Low-level APC mutational mosaicism is the underlying cause in a substantial fraction of unexplained colorectal adenomatous polyposis cases

BACKGROUND

In 30-50% of patients with colorectal adenomatous polyposis, no germline mutation in the known genes APC, causing familial adenomatous polyposis, MUTYH, causing MUTYH-associated polyposis, or POLE or POLD1, causing polymerase-proofreading-associated polyposis can be identified, although a hereditary aetiology is likely. This study aimed to explore the impact of APC mutational mosaicism in unexplained polyposis.

METHODS

To comprehensively screen for somatic low-level APC mosaicism, high-coverage next-generation sequencing of the APC gene was performed using DNA from leucocytes and a total of 53 colorectal tumours from 20 unrelated patients with unexplained sporadic adenomatous polyposis. APC mosaicism was assumed if the same loss-of-function APC mutation was present in ≥ 2 anatomically separated colorectal adenomas/carcinomas per patient. All mutations were validated using diverse methods.

RESULTS

In 25% (5/20) of patients, somatic mosaicism of a pathogenic APC mutation was identified as underlying cause of the disease. In 2/5 cases, the mosaic level in leucocyte DNA was slightly below the sensitivity threshold of Sanger sequencing; while in 3/5 cases, the allelic fraction was either very low (0.1-1%) or no mutations were detectable. The majority of mosaic mutations were located outside the somatic mutation cluster region of the gene.

CONCLUSIONS

The present data indicate a high prevalence of pathogenic mosaic APC mutations below the detection thresholds of routine diagnostics in adenomatous polyposis, even if high-coverage sequencing of leucocyte DNA alone is taken into account. This has important implications for both routine work-up and strategies to identify new causative genes in this patient group.