Mutations in Citron Kinase Cause Recessive Microlissencephaly with Multinucleated Neurons

Abstract

Primary microcephaly is a neurodevelopmental disorder that is caused by a reduction in brain size as a result of defects in the proliferation of neural progenitor cells during development. Mutations in genes encoding proteins that localize to the mitotic spindle and centrosomes have been implicated in the pathogenicity of primary microcephaly. In contrast, the contractile ring and midbody required for cytokinesis, the final stage of mitosis, have not previously been implicated by human genetics in the molecular mechanisms of this phenotype. Citron kinase (CIT) is a multi-domain protein that localizes to the cleavage furrow and midbody of mitotic cells, where it is required for the completion of cytokinesis. Rodent models of Cit deficiency highlighted the role of this gene in neurogenesis and microcephaly over a decade ago. Here, we identify recessively inherited pathogenic variants in CIT as the genetic basis of severe microcephaly and neonatal death. We present postmortem data showing that CIT is critical to building a normally sized human brain. Consistent with cytokinesis defects attributed to CIT, multinucleated neurons were observed throughout the cerebral cortex and cerebellum of an affected proband, expanding our understanding of mechanisms attributed to primary microcephaly.

Citron kinase – renaissance of a neglected mitotic kinase

“…Indeed, four recent studies have reported that mutations in CIT are responsible for some cases of primary microcephaly in humans (Basit et al, 2016;Harding et al, 2016;Li et al, 2016;Shaheen et al, 2016), thereby, further supporting this hypothesis and, for the first time, linking CIT-K to human disease. In both rodents and humans, the defects in CNS development were associated with failure of cytokinesis and severe apoptosis.…”

“…The CIT-K-binding partner ASPM is one of the most frequently mutated genes in autosomal-recessive human microcephaly and one of the main determinants of human brain size (Nicholas et al, 2009), suggesting that a common cellular mechanism underlies the cause of primary microcephaly. The presence of multinucleated neurons in patients carrying CIT mutations (Harding et al, 2016;Li et al, 2016) and the established roles for both CIT-K and ASPM in cytokinesis (Higgins et al, 2010;Paramasivam et al, 2007) suggest that cytokinesis failure and, consequently, apoptosis is one such possible mechanism. However, as both proteins also cooperate in spindle orientation , defects during this step of mitosis cannot be discounted as a possible cause.…”

“…In both rodents and humans, the defects in CNS development were associated with failure of cytokinesis and severe apoptosis. Mutations of CIT that were found to be associated with human microcephaly mapped to different locations within the gene, and included point mutations in conserved amino acids and splice sites, as well as deletions (Basit et al, 2016;Harding et al, 2016;Li et al, 2016;Shaheen et al, 2016). Some of the mutations identified in the kinase domain abolish the kinase activity of CIT-K in vitro (Li et al, 2016), clearly indicating that the kinase activity is essential for brain development and cytokinesis.…”

“…Therefore, it did not come as a total surprise that, as discussed below, deregulation and loss of CIT-K were recently found to be involved in human diseases (Basit et al, 2016;Harding et al, 2016;Li et al, 2016;Shaheen et al, 2016).…”

“…Subsequent studies, however, have revealed that, instead, CIT-K has an evolutionarily conserved role in later stages of cytokinesis, after completion of furrow ingression (Bassi et al, 2011(Bassi et al, , 2013D'Avino et al, 2004;Echard et al, 2004;Gai et al, 2011;Gruneberg et al, 2006;Naim et al, 2004;Shandala et al, 2004;Watanabe et al, 2013), and recent evidence indicates that CIT-K is also important in early mitosis . Furthermore, mutations in CIT-K have been recently found to cause human primary microcephaly, and this kinase has been proposed as a target in anti-cancer therapy (Basit et al, 2016;Fu et al, 2011;Harding et al, 2016;Li et al, 2016;Shaheen et al, 2016). Finally, there is growing evidence that CIT-K is subject to multiple levels of regulation by both intra-and extracellular mechanisms (Jungas et al, 2016;.…”