The Splicing Factor CCAR1 Regulates the Fanconi Anemia/BRCA Pathway

The twenty-three Fanconi Anemia proteins cooperate in a DNA repair pathway, called the FA/BRCA pathway, required for the monoubiquitination of FANCD2 and the excision of interstrand DNA crosslinks (ICLs). The CCAR1 (cell division cycle and apoptosis regulator 1) protein is also a regulator of ICL repair, though its possible function in the FA/BRCA pathway remains unknown. Here, we demonstrate that CCAR1 plays a unique upstream role in the FA/BRCA pathway and is required for FANCA protein expression and FANCD2 monoubiquitination. Interestingly, loss of CCAR1 results in retention of a poison exon in the FANCA transcript, thereby leading to reduced FANCA protein expression. A unique domain of CCAR1, the so-called EF-hand domain, binds to the spliceosome SF3B complex and is required for its mRNA splicing activity. Taken together, CCAR1 is a unique splicing factor, required for normal splicing of the FANCA mRNA and perhaps other mRNAs involved in various cellular pathways. To evaluate the possible genome-wide effect of CCAR1 knock-out, we performed RNA sequencing using RPE p53-/- parental and CCAR1 knockout cells.