Molecular impacts of Meier-Gorlin syndrome mutations on human origin licensing

Meier-Gorlin syndrome (MGS) is a form of primordial dwarfism linked to mutations in DNA replication initiation factors. Many MGS variants affect proteins required for the first step of replication initiation-the licensing of replication origins-during which the origin recognition complex (ORC), CDC6, and CDT1 cooperatively load MCM2-7 complexes onto DNA as an MCM double hexamer. The specific impacts of MGS mutations on origin licensing remain poorly understood. In this study, we systematically analyze the effects of MGS-linked missense mutations in core domains of human origin licensing factors in a fully reconstituted in vitro MCM loading system. Our results show that MGS mutations inhibit origin licensing by blocking MCM recruitment or loading at discrete but distinct stages of the reaction. MGS mutations in ORC and CDC6 impair MCM recruitment by abrogating ATP-dependent DNA binding or the maturation of recruited MCM into a loaded single hexamer. MGS variants of CDT1 specifically reduce MCM recruitment, whereas disease mutations in MCM subunits support ORC-mediated MCM hexamer recruitment but hinder their stable deposition onto DNA. Our findings establish how MGS mutations perturb specific origin licensing steps and provide mechanistic insights into the molecular basis of MGS pathogenesis.