A review of mismatch repair gene transcripts: issues for interpretation of mRNA splicing assays

Many mismatch repair (MMR) gene disease-causing mutations identified in cancer patients result in aberrant messenger RNA (mRNA) splicing. However, mRNA assay interpretation can be complicated by the existence of naturally occurring alternative mRNA transcripts, most of which have not been formally described or fully characterized. Here, we provide a comprehensive catalogue of all MMR transcripts described to date, and a review of MMR nucleotide variants associated with an apparent upregulation of alternatively spliced transcripts. This work sets reference starting points for designing and interpreting MMR RNA analyses. Our database and literature searches retrieved 30 MLH1, 22 MSH2, 4 MSH6 and 9 PMS2 alternative transcripts, many predicted to introduce premature termination codons. Furthermore, we collected information on 66 MLH1, 24 MSH2 and 6 PMS2 nucleotide variants reported to be associated with altered expression of at least one of these alternative transcripts, and in many instances reported as splicing mutations. This review shows that there are many alternatively spliced MMR transcripts, which have potential to confound interpretation of splicing assays. These findings highlight the need to perform RNA analysis of patients systematically in parallel with control individuals, and call for the implementation of quantitative assessment of transcript levels for informed interpretation of mRNA assays.