Loss of the human BRIP1 C terminus does not cause Fanconi anemia

Fanconi anemia (FA) is a genetic disorder characterized by congenital anomalies, progressive bone marrow failure, and an increased cancer risk. FA patient cells are hypersensitive to DNA interstrand crosslinks (ICLs), which cause chromosomal and genomic instability if left unrepaired. We identified 11 individuals from seven families who are homozygous for one of two C-terminally truncating variants in BRIP1 (previously known as FANCJ), c.2947del (p.Ile983Leufs^∗2) (n = 3) or c.2992_2995del (p.Lys998Glufs^∗60) (n = 8). None of them exhibited hematologic manifestations of FA, including bone marrow failure, myelodysplasia, or any FA-associated early-onset cancer types. Patient-derived blood cells showed stable levels of the BRIP1 p.Ile983Leufs^∗2 variant and were resistant to ICL-inducing agents. In contrast, cells containing the BRIP1 p.Lys998Glufs^∗60 variant displayed partial BRIP1 degradation and a mild cellular phenotype, evidenced by intermediate ICL sensitivity and a moderately elevated rate of ICL-induced chromosomal breaks. An isogenic BRIP1 knockout cell model expressing either wild-type or variant BRIP1 fully recapitulated these phenotypes. Introducing a stop codon at position 998 (p.Lys998^∗) resulted in wild-type-like protein stability and ICL resistance, demonstrating that the aberrant frameshift extension, rather than C-terminal truncation, causes the observed cellular ICL sensitivity. Further deletion analyses revealed that the BRIP1 helicase domain (amino acids 1-888) is required and sufficient for cellular ICL resistance. BRCA1 interaction with phosphorylated BRIP1 Ser990 is dispensable for ICL repair. Our findings indicate that FA can result from the loss of the N-terminal helicase domain of BRIP1 but not from the loss of its C-terminal functions.