LAUSR

Nijmegen breakage syndrome caused by biallelic pathogenic variants of the DNA‐damage response gene NBN, is characterized by severe microcephaly, cancer proneness, infertility, and karyotype abnormalities. We previously reported NBN variants in siblings suffering from fertility defects. Here, we identify a new founder NBN variant (c.442A>G, p.(Thr148Ala)) in Lebanese patients associated with isolated infertility. Functional analyses explored preserved or altered functions correlated with their remarkably mild phenotype. Transcript and protein analyses supported the use of an alternative transcript with in‐frame skipping of exons 4–5, leading to p84‐NBN protein with a preserved forkhead‐associated (FHA) domain. The level of NBN was dramatically reduced and the MRN complex delocalized to the cytoplasm. Interestingly, ataxia–elangiectasia mutated (ATM) also shifted from the nucleus to the cytoplasm, suggesting some interaction between ATM and the MRN complex at a steady state. The ATM pathway activation, attenuated in typical patients with NBS, appeared normal under camptothecin treatment in these new NBN‐related infertile patients. Cell cycle checkpoint defect was present in these atypical patients, although to a lesser extent than in typical patients with NBS. In conclusion, we report three new NBN‐related infertile patients and we suggest that preserved FHA domain could be responsible for the mild phenotype and intermediate DNA‐damage response defects.