Abstracts

A form of dwarfism known as Meier-Gorlin syndrome (MGS) is caused by recessive mutations in one of six different genes (ORC1, ORC4, ORC6, CDC6, CDT1, and MCM5). These genes encode components of the pre-replication complex that assembles at origins of replication prior to S phase. Also, variants in two additional replication initiation genes have joined the list of causative mutations for MGS (Geminin and CDC45). The identity of the causative MGS genes strongly suggests that some aspect of replication is amiss in MGS patients; however, no evidence has yet been obtained regarding what aspect of replication is faulty nor is there a specific hypothesis for how a problem with replication might produce the specific phenotypes of MGS patients. This dissertation aims to characterize the cellular and molecular phenotype of MGS mutations to better understand how they might give rise to the developmental phenotypes observed in humans with this condition. Since the site of one of the missense mutations in the human ORC4 alleles is conserved between humans and yeast, I sought to determine in what way this single amino acid change affects the process of chromosome replication, by introducing the comparable mutation into yeast (orc4Y232C). I found that orc4Y232C yeast cells have a prolonged S phase due to compromised replication initiation at the ribosomal DNA (rDNA) locus located on chromosome XII. The inability to initiate replication at the rDNA locus results in chromosome breakage and a severely reduced rDNA copy number in the survivors, presumably helping to ensure complete replication of chromosome XII. Although reducing rDNA copy number may help ensure complete chromosome replication, orc4Y232C cells struggle to meet the high demand for ribosomes. This finding provides evidence linking two pathways that are not typically thought of as being connected, DNA replication and ribosome biogenesis. Furthermore, it raises the possibility that the phenotypes observed in humans with MGS are a result of defects in a totally unexpected pathwayribosome biogenesis.Advisors/Committee Members: Brewer, Bonita J (advisor), Raghuraman, M K (advisor).