Cdk5rap2 regulates centrosome function and chromosome segregation in neuronal progenitors

Abstract

SUMMARYMicrocephaly affects ~1% of the population and is associated with mental retardation, motor defects and, in some cases, seizures. We analyzed the mechanisms underlying brain size determination in a mouse model of human microcephaly. The Hertwig's anemia (an) mutant shows peripheral blood cytopenias, spontaneous aneuploidy and a predisposition to hematopoietic tumors. We found that the an mutation is a genomic inversion of exon 4 of Cdk5rap2, resulting in an in-frame deletion of exon 4 from the mRNA. The finding that CDK5RAP2 human mutations cause microcephaly prompted further analysis of Cdk5rap2 an/an mice and we demonstrated that these mice exhibit microcephaly comparable to that of the human disease, resulting from striking neurogenic defects that include proliferative and survival defects in neuronal progenitors. Cdk5rap2 an/an neuronal precursors exit the cell cycle prematurely and many undergo apoptosis. These defects are associated with impaired mitotic progression coupled with abnormal mitotic spindle pole number and mitotic orientation. Our findings suggest that the reduction in brain size observed in humans with mutations in CDK5RAP2 is associated with impaired centrosomal function and with changes in mitotic spindle orientation during progenitor proliferation.

Small organelle, big responsibility: the role of centrosomes in development and disease

“…Therefore, mitotic delay triggered by aberrant centrosomes could obliterate long-term proliferative capacity of progenitors leading to MCPH. In support of this scenario, reduced proliferation is the shared phenotype of CDK5RAP2 mutation and WDR62-or ASPM-depletion in the mouse neocortex [24,57,59]. Premature entry into mitosis has also been implicated in MCPH [60].…”

“…Of these, mitotic delay and aneuploidy are more likely to account for the proliferative impairment than apoptosis, because p53 disruption does not restore normal brain size in PLK4-overexpressing mice, and elevated apoptosis is not consistently observed in mouse models of MCPH. Multipolar spindles are also seen in CDK5RAP2 mutant mouse neuroepithelium, but a link to aneuploidy is yet to be established in this system [57].…”

“…In fact, mitotic delay is the major phenotype seen after depletion of WDR62, ASPM and STIL in developing zebrafish retinal neuroepithelium [56]. Mutations in CDK5RAP2 also cause mitotic delay in the mouse brain [57]. A recent study suggests that cells sense the duration of mitosis; in particular, prolonged prometaphase in the mother cell causes daughter cells to exit the cell cycle, even if mitosis is completed normally [58].…”

The Centrosome and Its Duplication Cycle

“…Mutations in CPAP (MCPH6) and STIL (MCPH7) also affect spindle orientation (Kitagawa et al 2011b;Brito et al 2012). Other genes for centrosomal proteins that are mutated in microcephaly include the CNN homolog, CDK5RAP2 (MCPH3), required for centrosome maturation and DNA damage-induced G 2 arrest (Barr et al 2010;Lizarraga et al 2010), and CEP63 with its partner Cep152, which participate in centriole assembly (Sir et al 2011). Centriole amplification has also been shown to be one cause of microcephaly in human patients that is triggered by mutants that stabilize STIL by removing a KEN destruction box (Arquint and Nigg 2014).…”

Translational potential of human brain organoids

“…Loss‐of‐function mutation in CDK5RAP2 has been implicated in microcephaly, but mouse models failed to reproduce the full spectrum of human phenotypes 39, 40. When brain organoids were generated from microcephaly patients harboring truncating mutations of CDK5RAP2 , they were found to be significantly smaller than the ones generated from controls 27.…”