A recurrent de novo variant in NUSAP1 escapes nonsense‐mediated decay and leads to microcephaly, epilepsy, and developmental delay

NUSAP1 encodes a cell cycle‐dependent protein with key roles in mitotic progression, spindle formation, and microtubule stability. Both over‐ and under‐expression of NUSAP1 lead to dysregulation of mitosis and impaired cell proliferation. Through exome sequencing and Matchmaker Exchange, we identified two unrelated individuals with the same recurrent, de novo heterozygous variant (NM_016359.5 c.1209C > A; p.(Tyr403Ter)) in NUSAP1. Both individuals had microcephaly, severe developmental delay, brain abnormalities, and seizures. The gene is predicted to be tolerant of heterozygous loss‐of‐function mutations, and we show that the mutant transcript escapes nonsense mediated decay, suggesting that the mechanism is likely dominant‐negative or toxic gain of function. Single‐cell RNA‐sequencing of an affected individual's post‐mortem brain tissue indicated that the NUSAP1 mutant brain contains all main cell lineages, and that the microcephaly could not be attributed to loss of a specific cell type. We hypothesize that pathogenic variants in NUSAP1 lead to microcephaly possibly by an underlying defect in neural progenitor cells.