Mutations in ORC1, encoding the largest subunit of the origin recognition complex, cause microcephalic primordial dwarfism resembling Meier-Gorlin syndrome

Abstract

Studies into disorders of extreme growth failure (for example, Seckel syndrome and Majewski osteodysplastic primordial dwarfism type II) have implicated fundamental cellular processes of DNA damage response signaling and centrosome function in the regulation of human growth. Here we report that mutations in ORC1, encoding a subunit of the origin recognition complex, cause microcephalic primordial dwarfism resembling Meier-Gorlin syndrome. We establish that these mutations disrupt known ORC1 functions including pre-replicative complex formation and origin activation. ORC1 deficiency perturbs S-phase entry and S-phase progression. Additionally, we show that Orc1 depletion in zebrafish is sufficient to markedly reduce body size during rapid embryonic growth. Our data suggest a model in which ORC1 mutations impair replication licensing, slowing cell cycle progression and consequently impeding growth during development, particularly at times of rapid proliferation. These findings establish a novel mechanism for the pathogenesis of microcephalic dwarfism and show a surprising but important developmental impact of impaired origin licensing.

Meier–Gorlin syndrome genotype–phenotype studies: 35 individuals with pre-replication complex gene mutations and 10 without molecular diagnosis

“…In vitro studies using lymphoblastoid cell lines or skin fibroblast cell lines from two individuals with mutations in ORC1 showed reduced levels of ORC1, ORC2, and MCM2 in chromatin-enriched cell fractions. 13 Furthermore, the S-phase entry/progression in both individuals was delayed.…”

“…13 As individuals with mutations in ORC1 showed overlapping features with MGS, mutation analysis of ORC1 was performed in 33 individuals with MGS and revealed mutations in 4 individuals from three families. 13 Mutation analysis of other genes of this prereplication complex showed mutations in ORC4, ORC6, CDT1, and CDC6 in 14 individuals from nine families with MGS. 14 Simultaneously, beginning with a family-based mapping approach in individuals with MGS with a founder effect, Guernsey et al 15 identified mutations in ORC1, ORC4, and CDT1 in eight individuals from five families with MGS.…”

“…In vitro studies using lymphoblastoid cell lines or skin fibroblast cell lines from two individuals with mutations in ORC1 showed reduced levels of ORC1, ORC2, and MCM2 in chromatin-enriched cell fractions. 13 Furthermore, the S-phase entry/progression in both individuals was delayed.The five genes known to cause MGS are part of the same pre-replication complex. We posed the question whether mutations in the different genes from the pre-replication complex have a different effect on the MGS phenotype.…”

“…Recently, mutations in ORC1, a pre-replication complex member gene, were identified in 5 out of 204 individuals with microcephalic primordial dwarfism. 13 As individuals with mutations in ORC1 showed overlapping features with MGS, mutation analysis of ORC1 was performed in 33 individuals with MGS and revealed mutations in 4 individuals from three families. 13 Mutation analysis of other genes of this prereplication complex showed mutations in ORC4, ORC6, CDT1, and CDC6 in 14 individuals from nine families with MGS.…”

Comment l’approche génomique aide à comprendre le processus d’initiation de la réplication

“…Malheureusement il n'existe, pour l'heure, aucune cartographie de cette marque dans les cellules humaines qui puisse être comparée à la position des sites ORC. À noter que les mutations affectant le domaine BAH sont retrouvées chez des patients atteints du syndrome Meier-Gorlin, un syndrome rare qui se caractérise par une petite taille, de petites oreilles et des rotules sous-développées [33,34]. À n'en pas douter la cartographie des différentes formes méthylées de la lysine 20 de l'histone H4 sera une étape importante pour notre compréhension du mécanisme de sélection des origines de réplication.…”