Whole Exome Sequencing Identifies APCDD1 and HDAC5 Genes as Potentially Cancer Predisposing in Familial Colorectal Cancer

Germline mutations in predisposition genes account for only 20% of all familial colorectal cancers (CRC) and the remaining genetic burden may be due to rare high- to moderate-penetrance germline variants that are not explored. With the aim of identifying such potential cancer-predisposing variants, we performed whole exome sequencing on three CRC cases and three unaffected members of a Polish family and identified two novel heterozygous variants: a coding variant in APC downregulated 1 gene ( , p.R299H) and a non-coding variant in the 5' untranslated region (UTR) of histone deacetylase 5 gene ( ). Sanger sequencing confirmed the variants segregating with the disease and Taqman assays revealed 8 additional variants in a cohort of 1705 familial CRC patients and no further variants. Proliferation assays indicated an insignificant proliferative impact for the variant. Luciferase reporter assays using the variant resulted in an enhanced promoter activity. Targeting of transcription factor binding sites of SNAI-2 and TCF4 interrupted by the variant showed a significant impact of TCF4 on promoter activity of mutated . Our findings contribute not only to the identification of unrecognized genetic causes of familial CRC but also underline the importance of 5'UTR variants affecting transcriptional regulation and the pathogenesis of complex disorders.