Reduced functional MMR efficiency as an indicator of Lynch syndrome

Mismatch repair (MMR) genes, MSH2, MSH6, MLH1 and PMS2, are central to post-replicative DNA error avoidance.

Inherited defects in these genes result in significantly increased cancer susceptibility (up to 80% risk), a condition known as Lynch syndrome (LS). For long, the functional in vitro assessment of recombinant proteins carrying mutations found in LS patients has been used to understand the effect of mutations on MMR capability. Here, the effect of reduced MMR gene expression on the MMR capability and efficiency has been functionally characterized (Fig 1) and the results are (MSH2 and MSH6) or will be used (MLH1 and PMS2) for development of the diagnostic test.

Figure 1. Stable MMR gene deficient knockdown (KD) cell lines are created by transfecting MSH2, MSH6, MLH1 and PMS2 specific shRNA into fibroblasts. For each gene, the cells are expanded for protein and RNA extractions. Quantitative RT-PCR is used for assessing the mRNA expression of the gene of interest, while Western blotting is used for determining their protein expression levels. The mismatch repair efficiency of selected cell lines is studied using the in vitro MMR assay (Nyström-Lahti et al 2002).

The significance and role of each gene in the MMR mechanism and Lynch syndrome characteristics such as the age of cancer onset, tumor spectrum and cancer penetrance is unique. Similarly, we have observed that whether and how decreased gene expression levels effect on the MMR efficiency is dependent on the MMR gene. The MMR function is sensitive to already slightly reduced MSH2 and MSH6 levels, while not so sensitive to MLH1 expression decrease. Interestingly, the effect of reduced PMS2 expression is the most severe at carrier level (~50%).

Kansikas M, Kasela M, Kantelinen J, Nyström M. 2014. Assessing how reduced expression levels of the mismatch repair genes MLH1, MSH2, and MSH6 affect repair efficiency. Human Mutation, 35:1123-7. doi: 10.1002/humu.22605.

Kasela M, Nyström M, Kansikas M. 2019. PMS2 expression decrease causes severe problems in mismatch repair. Human Mutation, 40:904-7. doi: 10.1002/humu.23756