Identification of frequently mutated genes with relevance to nonsense mediated mRNA decay in the high microsatellite instability cancers.

Frameshift mutations at coding mononucleotide repeats (cMNR) are frequent in high-microsatellite instability (MSI-H) cancers. Frameshift mutations in cMNR result in the formation of a premature termination codon (PTC) in the transcribed mRNA, and these abnormal mRNAs are generally degraded by nonsense mediated mRNA decay (NMD). We have identified novel genes that are frequently mutated at their cMNR by blocking NMD in two MSI-H cancer cell lines. After blocking NMD, we screened for differentially expressed genes using DNA microarrays, and then used database analysis to select 28 candidate genes containing cMNR with more than 9 nucleotide repeats. cMNR mutations have not been previously reported in MSI-H cancers for 15 of the 28 genes. We analyzed the cMNR mutation of each of the 15 genes in 10 MSI-H cell lines and 21 MSI-H cancers, and found frequent mutations of 12 genes in MSI-H cell lines and cancers, but not in microsatellite stable (MSS) cancers. Among these genes, the most frequently mutated in MSI-H cell lines were MLL3 (70%), PHACTR4 (70%), RUFY2 (50%) and TBC1D23 (50%). MLL3, which has already been implicated in cancer, had the highest mutation frequency in MSI-H cancers (48%). Our combined approach of NMD block, database search, and mutation analysis has identified a large number of genes mutated in their cMNR in MSI-H cancers. The identified mutations are expected to contribute to MSI-H tumorigenesis by causing an absence of gene expression or low gene dosage effects.