Frameshift mutations of TAF1C gene, a core component for transcription by RNA polymerase I, and its regional heterogeneity in gastric and colorectal cancers

Initiation of transcription for ribosomal RNA (rRNA) by RNA polymerase I requires TATA-binding protein (TBP) and TBP-associated factors (TAF1A, TAF1B and TAF1C). p53 tumour suppressor inhibits rRNA transcription by blocking TAF1C-UBF interaction, but alterations of TAF1C itself in tumorigenesis remain unknown. The aim of this study was to explore whether TAF1C gene was mutated in gastric (GC) and colorectal cancers (CRC). In a public database, we found that TAF1C gene had a mononucleotide repeat (C8) in the coding sequences that might be a mutation target in the cancers with microsatellite instability (MSI). We analysed 79 GC and 124 CRC by single-strand conformation polymorphism and DNA sequencing analyses. In this study, we found TAF1C frameshift mutations (8.8% of GC and 10.1% of CRC with MSI-H), which were not found instable MSI/low MSI (MSS/MSI-L) (0/90). In addition, we analysed intratumoural heterogeneity (ITH) of TAF1C frameshift mutations in 16 CRC and found that three CRC (18.8%) harboured regional ITH of the TAF1C frameshift mutations. Our results indicate that TAF1C gene harboured not only somatic frameshift mutations but also the mutational ITH, which together might play a role in tumourigenesis of GC and CRC. Our data also suggest that multi-regional mutation analysis is needed for a better evaluation of the mutation status in CRC.