Missense and nonsense mutations of the zebrafishhcfc1agene result in contrasting mTor and radial glial phenotypes

Abstract

Mutations in theHCFC1transcriptional co-factor are the cause ofcblXsyndrome and X-linked intellectual disability (XLID).cblXis the more severe disorder associated with intractable epilepsy, abnormal cobalamin metabolism, facial dysmorphia, cortical gyral malformations, and intellectual disability.In vitro, Hcfc1regulates neural precursor (NPCs) proliferation and number, which has been validated in zebrafish. However, conditional deletion ofHcfc1in Nkx2.1+ NPCs increased cell death, reducedGfapexpression, and reduced numbers of GABAergic neurons. Thus, the role of HCFC1 in brain development is not completely understood. Recently, knock-in of acblX(HCFC1) andcblX-like (THAP11) were created in mice. Knock-in of thecblX-like allele was associated with increased expression of proteins required for ribosome biogenesis. However, the brain phenotypes were not comprehensively studied due to sub-viability and therefore, a mechanism underlying increased ribosome biogenesis was not described. We used a missense, a nonsense, and two conditional zebrafish alleles to further elucidate this mechanism during brain development. We observed unique subtypes ofcblXsyndrome, each with a unique signature (activation or repression) of Akt/mTor signaling. Differential regulation of mTor was associated with allele dependent radial glial cell phenotypes. Liquid chromatographmass spectrometry and chromatin immunoprecipitation revealed at least two downstream target genes of HCFC1,asxl1andywhab, whose divergent expression correlates with mTor activity in an allele specific manner. These data suggest that differential regulation of mTor across individualcblXmutations may underlie the clinical heterogeneity of patient phenotypes.Summary StatementWe uncovered a novel mechanism whereby HCFC1 regulates brain development through differential regulation of mTor activity and acts as a putative biomarker for subtypes ofcblXsyndrome.