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Regulated Expression of a FANCL Splicing Variant as a Potential Modifier of DNA Repair Activity

St-Laurent Pedneault C, Plourde K, Bélanger S, Ouellette G, Bouffard F, Labrie Y and Durocher F

Fifteen Fanconi Anemia (FA) proteins work altogether to repair DNA interstrand cross-links (ICLs) in response to DNA damage. Monoallelic mutation associated with breast cancer predisposition has been demonstrated for the FANCD1/BRCA2, FANCJ/BRIP1, FANCN/PALB2 and FANCO/RAD51C genes. The monoubiquitination of FANCD2, which represents the key event of the FANC/BRCA repair pathway is catalyzed by FANCL, which is the only FA protein known to possess an ubiquitin ligase activity. We have identified and studied the expression as well as the impact of a splicing isoform of the FANCL gene (FANCL∆4) on DNA repair processes as well as its cellular localization.

A FANCL splicing isoform showing the skipping of exon 4 has been isolated in non-BRCA1/2 breast cancer patients and is expressed in several human cell lines. The efficient translation of the FANCL∆4 isoform into a functional protein has been shown using the polyribosomal fractions. Although in silico analyses did not reveal any potential implication of any sequence variation, the expression of this isoform has been demonstrated to be under the control of a specific genomic variant localized 11 nucleotides upstream of exon 4. Experiments performed in FANCL-deficient cells transfected with the FANCL∆4 isoform demonstrated a decreased cell survival, G2/M cell cycle blockade and absence of FANCD2 monoubiquitination. The FANCL∆4 protein remains localized in the cytoplasm in transfected HEK293T cells following mitomycin C treatment, unlike the FANCL protein that migrates to the nucleus under such conditions.

A splicing isoform of a given FA gene may impact on the severity of the clinical manifestations when occurring in a FA mutation background, such as what is seen with FA and breast cancer patients. Evaluation of the sensitivity of this FANCL∆4 isoform to chemotherapeutic agents commonly used in cancer treatment would be of interest in the context of long-term management of affected individuals and their families.