Mutation analysis of multiple pilomatricomas in a patient with myotonic dystrophy type 1 suggests a DM1-associated hypermutation phenotype

Myotonic dystrophy type 1 (DM1) is an inherited neuromuscular disease which results from an expansion of repetitive DNA elements within the 3’ untranslated region of the DMPK gene. Some patients develop multiple pilomatricomas as well as malignant tumors in other tissues. Mutations of the catenin-β gene (CTNNB1) could be demonstrated in most non-syndromic pilomatricomas. In order to gain insight into the molecular mechanisms which might be responsible for the occurrence of multiple pilomatricomas and cancers in patients with DM1, we have sequenced the CTNNB1 gene of four pilomatricomas and of one pilomatrical carcinoma which developed in one patient with molecularly proven DM1 within 4 years. We further analyzed the pilomatrical tumors for microsatellite instability as well as by NGS for mutations in 161 cancer-associated genes. Somatic and independent point-mutations were detected at typical hotspot regions of CTNNB1 (S33C, S33F, G34V, T41I) while one mutation within CTNNB1 represented a duplication mutation (G34dup.). Pilomatricoma samples were analyzed for microsatellite instability and expression of mismatch repair proteins but no mutated microsatellites could be detected and expression of mismatch repair proteins MLH1, MSH2, MSH6, PMS2 was not perturbed. NGS analysis only revealed one heterozygous germline mutation c.8494C>T; p.(Arg2832Cys) within the ataxia telangiectasia mutated gene (ATM) which remained heterozygous in the pilomatrical tumors. The detection of different somatic mutations in different pilomatricomas and in the pilomatrical carcinoma as well as the observation that the patient developed multiple pilomatricomas and one pilomatrical carcinoma over a short time period strongly suggest that the patient displays a hypermutation phenotype. This hypermutability seems to be tissue and gene restricted. Co-translation of the mutated DMPK gene and the CTNNB1 gene in cycling hair follicles might constitute an explanation for the observed tissue and gene specificity of hypermutability observed in DM1 patients. Elucidation of putative mechanisms responsible for hypermutability in DM1 patients requires further research. Author summary Less than 10% of patients with myotonic dystrophy type 1 (DM1), an inherited and the most common neuromuscular disorder, develop pilomatricomas, often as multiple tumors. Pilomatricomas are benign skin tumors deriving from hair matrix cells, and they are very rare in the general population. Recently it could be demonstrated that DM1 patients also harbour and enhanced risk for benign and malignant tumors in various other tissues. DM1 is characterized genetically by an expansion of trinucleotide repeats within the 3’ untranslated region of the DMPK gene (DM1 protein kinase). It could be demonstrated that these expanded CTG-repeats are transcribed into RNA and that this non-translated repetitive RNA forms aggregates with various splicing regulators, which in turn impair transcription of multiple genes in various tissues. Following the gain-of-function-RNA hypothesis, Mueller and colleagues suggested in 2009 that the untranslated repetitive RNA directly enhances expression of β-catenin resulting in pilomatricomas as well as in various cancers which rely on activation of the WNT/APC/β-catenin pathway. In order to prove or to reject this hypothesis we have sequenced the CTNNB1 gene of four pilomatricomas and of one pilomatrical carcinoma which developed in one patient with molecularly proven DM1 within 4 years. Somatic and independent point-mutations were detected at typical hotspot regions of CTNNB1 (S33C, S33F, G34V, T41I) while one mutation within CTNNB1 represented a duplication mutation (G34dup.). We further analyzed the pilomatrical tumors for microsatellite instability but no mutated microsatellites could be detected and expression of mismatch repair proteins MLH1, MSH2, MSH6, PMS2 was not perturbed. NGS analysis in 161 cancer-associated genes only revealed one heterozygous germline mutation c.8494C>T; p.(Arg2832Cys) within the ataxia telangiectasia mutated gene (ATM) which remained heterozygous in the pilomatrical tumors. The detection of different somatic mutations in different pilomatricomas and in the pilomatrical carcinoma does not support the hypothesis that untranslated repetitive RNA directly enhances expression of β-catenin resulting in pilomatricomas. In contrast, our results strongly suggest that the patient displays a tissue and gene restricted hypermutation phenotype. One putative mechanism for the assumed gene and tissue restriction could be co-translation of the mutated DMPK gene and the CTNNB1 gene in cycling hair follicles.