Abstract 4221: Mutations in Rad51C in colon tumors.
Authors: Kalvala Arjun , Gao Li , Barnwel Brittany , Otterson Greg , Villalona Calero Miguel A , Duan Wenrui 
Published: August 2014

Fanconi anemia (FA) is a rare inherited chromosomal instability genetic disorder characterized by congenital musculoskeletal abnormalities, bone marrow failure and cancer susceptibility. At the genetic level, monoubiquitination of FANCD2 and I by an FA core complex (FANCA, B, C, E, F, G M and L) is impaired. Monoubiquitinated FANCD2 and I co-localize with DNA damage response proteins such as Rad51C (FANCO) to mediate homologous recombination (HR) repair. Defects in Rad51C have been documented as the cause of FA complementation group O (FANCO) disorder.In cells the effects include chromosomal instability, hypersensitivity to DNA damaging agents and defective DNA damage repair. Germline mutations for Rad51C gene in patients have been reported in breast and ovarian cancers. Although germline mutations for Rad51C gene are known to cause an FA like phenotype, somatic mutations for Rad51C in colon or other tumors have not been previously described.

We evaluated 40 formalin fixed paraffin embedded (FFPE) colorectal adenocarcinomas by an immunofluorescence based triple staining method (FATSI) that we have developed to assess for potential somatic functional deficiency of the FA pathway. Thirteen of the 40 (33%) FFPE samples had been noted to lack FANCD2 foci formation. Among the 40 tumors, we analyzed 31 available frozen tumors that included all 13 foci negative tumors and matching non-tumor tissues for mutations in Rad51C coding region. The tumor and non-tumor tissue samples were procured following an IRB protocol of the Ohio State University. RNA and DNA were isolated from the fresh frozen tissue samples with TRIzol reagent. Direct sequencing of PCR amplicons was performed.

Two of the 31 tumor DNA samples contained point mutation at the codon 319 (from CGA to TGA, a stop codon) which technically results in a truncated FANCO protein. The Rad51C protein domain shows amino acid 125 to132 (exon 2 to 5) is important for exerting single stranded DNA dependent ATPase activity, a functional nuclear localization signal located from aa 366 to 370 (exon-9). The mutation substitutes arginine at codon 319 to a premature stop codon terminating the protein abruptly, without producing a full length Rad51C protein. Thus the altered protein is unable to localize in the nucleus. Of interest, both tumors with RAD51C mutations lacked formation of FANCD2 foci by FATSI staining. In both cases, the mutation was absent from the matched non-tumor tissues, suggesting a somatic mutation. The underlining molecular alterations that cause deficiency in FANCD2 foci formation in other 11 foci negative tumors are still under investigation currently.

Given that the FA pathway plays essential role in response to DNA interstrand cross-link damage agent, and that cancers with defective FA pathway are expected to be more sensitive to cross-link based therapy, our findings have the potential significance of identifying a subpopulation among colorectal cancer patients specially susceptible to these type of treatments.

Citation Format: Arjun Kalvala, Li Gao, Brittany Barnwel, Greg Otterson, Miguel A Villalona Calero, Wenrui Duan. Mutations in Rad51C in colon tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4221. doi:10.1158/1538-7445.AM2013-4221