Impaired p53-mediated DNA damage response contributes to microcephaly in Nijmegen Breakage Syndrome patient-derived cerebral organoids

Nijmegen Breakage Syndrome (NBS) is a rare autosomal recessive genetic disorder caused by mutations within NBN, a DNA-damage repair protein. Hallmarks of NBS include several clinical manifestations such growth retardation, chromosomal instability, immunodeficiency and progressive microcephaly. However, the etiology of microcephaly in NBS patients remains elusive. Here, we employed induced pluripotent stem cell-derived brain organoids from two NBS patients to analyze the underlying mechanisms of microcephaly. We show that NBS-organoids carrying the homozygous 647del5 NBN mutation are significantly smaller in size with disrupted cyto-architecture Patient-derived organoids exhibit premature differentiation together with neuronatin (NNAT) overexpression and key pathways related to DNA damage response and cell cycle are differentially regulated compared to controls. Moreover, we show that after exposure to bleomycin, NBS organoids undergo a delayed p53-mediated DNA damage response and aberrant trans-synaptic signalling, which ultimately leads to neuronal apoptosis. Our data provide insights into how mutations within NBN alters neurogenesis in NBS patients, thus providing a proof of concept that cerebral organoids are a valuable tool for studying DNA damage-related disorders.