Semic, A.; Yuen Yu Chan, K.; Bernardi, P.; Silveira, K. C.; Silveira, C.; Cavalcanti, D. P.; Kannu, P.; Stumpff, J.

Spondyloepimetaphyseal dysplasia with joint laxity, type 2 (SEMDJL2) is a rare skeletal disorder caused by pathogenic variants in KIF22, a mitotic chromokinesin that generates polar ejection forces (PEF) to ensure proper chromosome alignment and segregation. Although prior work showed that SEMDJL2-associated KIF22 hotspot variants impair chromosome segregation in epithelial cells, how these variants affect chondrocyte mitosis remains incompletely understood. Here, we analyzed the effects of the hotspot variant R149Q, a recently reported recessive variant R49Q, and two newly identified heterozygous variants, P144T and E222Q, in human chondrocytes. Both novel variants were identified in individuals with classic SEMDJL2 features. P144T and E222Q retained PEF-generating activity, whereas R49Q displayed reduced PEFs, consistent with their respective inheritance patterns. Live cell imaging revealed that all variants disrupted mitosis. The heterozygous variants (P144T, E222Q, R149Q) dominantly impeded anaphase chromosome segregation and spindle pole separation, supporting reclassification of P144T and E222Q as likely pathogenic. In contrast, R49Q caused milder, partially penetrant segregation defects, consistent with reduced and dysregulated motor activity. Together, our results define two mechanistic classes of KIF22 dysregulation: constitutive activation in heterozygous variants, which fail to down-regulate KIF22 at anaphase onset, and mixed-state dysregulation in the recessive R49Q variant, which exhibits partial loss of polar ejection force activity coupled with incomplete inactivation during anaphase. These findings broaden the mechanistic framework for how KIF22 variants perturb mitosis in chondrocytes and expand the genotypic landscape associated with SEMDJL2.