Complete loss of microcephaly protein CPAP leads to severe and complex features in developing mouse brain

Primary microcephaly (MCPH) is an autosomal recessive disorder. This neurodevelopmental condition is usually presented with mild to severe retardation with smaller brain size. Previous studies showed that the centrosome has at least four core proteins necessary for centriole duplication. When the genes that encode these proteins are mutated, it causes MCPH in humans. The implication of this is that the biogenesis of centriole plays an important role in MCPH, but the mechanism is yet to be identified. CPAP (also known as CENPJ) has been identified as a human functional homolog of SAS4 (one of the four core centrosomal proteins) and it has been reported to contribute to mitosis and centriole biogenesis. Human CPAP is structurally involved in maintaining the normal spindle morphology and integrity of the centrosome, and regulating centriole length. In recent studies, a Cpap/Cenpj/Sas-4 hypomorphic mice were generated and displayed clinical features similar to those seen in microcephalic primordial dwarfism disorder. Moreover, studies have shown that when the exon 5 in Cenpj mutant mice was deleted via Nestin-Cre-mediated conditional deletion, the mice had clinical features similar to those seen in human microcephaly. This implied that centrosomal dysfunction plays an important role in cortical neurogenesis.

In a recent research published in Journal of Cell Science, scientists at Institute of Biomedical Sciences at Academia Sinica in Taiwan:  Dr. Yi-Nan Lin, Ying-Shan Lee, Shu-Kuei Li and Professor Tang K. Tang identified the link between CPAP loss and microcephaly. They found that a complete loss of CPAP resulted in complex and severe clinical features in developing mouse brain.

In their study, the research team developed a different mutant of the Cenpj mouse line that possesses a formerly unidentified Cpap conditional knockout allele (Cpap cKO) with specific deletion of exons 6 and 7 and a frame shift stop codon in the CPAP protein. In the developing brain of the Cpap cKO mouse, they observed that loss of CPAP impaired the formation of cilium, suppressed duplication of centriole and induced neuronal cell death that was dependent on p53. However, the removal of one of the two p53 alleles from the Cpap cKO embryos (p53^+/−;Cpap^flox/−;Nestin-Cre+) reduced the severity of the neuronal cell death.

In in vitro cultured p53; Cpap double knockout [hereafter dKO (p53^−/−; Cpap^flox/−;Nestin-Cre+)] neural progenitor cells, it was noticed that loss of CPAP resulted in mitotic spindle defects in mostly monopolar spindles. The authors also found that treatment of control neural progenitors with centrinone, a centriole duplication inhibitor, caused a neuronal cell death enhanced by time and associated with an increase in p53 expression and a decrease in centriole numbers. However, in dKO or p53-knockout neural progenitors, treatment with centrinone resulted in minimal or no cell death.

The researchers noticed that deletion of CPAP caused a reduction in radial glia progenitors in cKO mice. In dKO mice, it led to the abnormal localization of radial glia progenitors, disruption of junction integrity and profound heterotopia in dKO embryonic cortex. The authors carried out an in vitro pair-cell assay whose result suggested that CPAP loss in radial glia progenitors induces premature neuronal differentiation. The dKO mice did not survive past 3 or 4 weeks postnatally and their brains showed signs of severe hydrocephalus and cerebellar hypoplasia. In addition, some of the clinical features displayed by these mice were associated with ataxia, like growth retardation with feeding difficulty and loss of motor coordination.

In a series of elegant and carefully designed studies the Academia Sinica scientists demonstrated the relationship between the loss of CPAP and microcephaly. This will advance our knowledge of causes of MCPH.