Mutations in CIT, encoding citron rho-interacting serine/threonine kinase, cause severe primary microcephaly in humans

Abstract

Primary microcephaly is a clinical phenotype in which the head circumference is significantly reduced at birth due to abnormal brain development, primarily at the cortical level. Despite the marked genetic heterogeneity, most primary microcephaly-linked genes converge on mitosis regulation. Two consanguineous families segregating the phenotype of severe primary microcephaly, spasticity and failure to thrive had overlapping autozygomes in which exome sequencing identified homozygous splicing variants in CIT that segregate with the phenotype within each family. CIT encodes citron, an effector of the Rho signaling that is required for cytokinesis specifically in proliferating neuroprogenitors, as well as for postnatal brain development. In agreement with the critical role assigned to the kinase domain in effecting these biological roles, we show that both splicing variants predict variable disruption of this domain. The striking phenotypic overlap between CIT-mutated individuals and the knockout mice and rats that are specifically deficient in the kinase domain supports the proposed causal link between CIT mutation and primary microcephaly in humans.

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.…”

“…Second, it will be important to establish tools, such as small-molecule inhibitors and analogue-sensitive mutants, to address the role of its kinase activity in vivo. Third, we do not know why CIT-K is necessary in every tissue in fly (D'Avino et al, 2004;Naim et al, 2004;Shandala et al, 2004), whereas it appears to be more important for the normal development and function of the CNS in mammals (Basit et al, 2016;Di Cunto et al, 2000Harding et al, 2016;Li et al, 2016;Shaheen et al, 2016), despite the fact that its depletion causes cytokinesis failure in different types of human cultured cell ( . This discrepancy between in vivo an ex vivo model systems might simply reflect a difference in the requirement of CIT-K and midbody formation in cultured cells versus whole tissues.…”

“…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.…”

“…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;.…”

“…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).…”