HCFC1 loss-of-function mutations disrupt neuronal and neural progenitor cells of the developing brain

Abstract

Both gain- and loss-of-function mutations have recently implicated HCFC1 in neurodevelopmental disorders. Here, we extend our previous HCFC1 over-expression studies by employing short hairpin RNA to reduce the expression of Hcfc1 in embryonic neural cells. We show that in contrast to over-expression, loss of Hcfc1 favoured proliferation of neural progenitor cells at the expense of differentiation and promoted axonal growth of post-mitotic neurons. To further support the involvement of HCFC1 in neurological disorders, we report two novel HCFC1 missense variants found in individuals with intellectual disability (ID). One of these variants, together with three previously reported HCFC1 missense variants of unknown pathogenicity, were functionally assessed using multiple cell-based assays. We show that three out of the four variants tested result in a partial loss of HCFC1 function. While over-expression of the wild-type HCFC1 caused reduction in HEK293T cell proliferation and axonal growth of neurons, these effects were alleviated upon over-expression of three of the four HCFC1 variants tested. One of these partial loss-of-function variants disrupted a nuclear localization sequence and the resulting protein displayed reduced ability to localize to the cell nucleus. The other two variants displayed negative effects on the expression of the HCFC1 target gene MMACHC, which is responsible for the metabolism of cobalamin, suggesting that these individuals may also be susceptible to cobalamin deficiency. Together, our work identifies plausible cellular consequences of missense HCFC1 variants and identifies likely and relevant disease mechanisms that converge on embryonic stages of brain development.

Transcriptional Elongation of HSV Immediate Early Genes by the Super Elongation Complex Drives Lytic Infection and Reactivation from Latency

“…Recently, point mutations in HCF-1 were determined to be a basis for both neurological (X-linked Intellectual Disability) and biochemical (cblX) syndromes (Jolly et al, 2015; Yu et al, 2013). …”

A novel HCFC1 variant in male siblings with intellectual disability and microcephaly in the absence of cobalamin disorder

“…These male siblings also exhibited dysmorphic features and microcephaly. Experimental evidence from the examination of patient-derived cell lines and in vitro assays suggest that mutations within kelch domains result in almost complete loss of HCFC1 function and result in cobalamin disorder, while HCFC1 mutations in other regions result in much milder effects on protein function and cobalamin metabolism (8,10). Loss-of-function of the zebrafish homolog of HCFC1 has also been associated with craniofacial abnormalities through dysregulation of MMAHC (11).…”

“…Loss-of-function of the zebrafish homolog of HCFC1 has also been associated with craniofacial abnormalities through dysregulation of MMAHC (11). It has been suggested that mutations in non-kelch domains of HCFC1, which result in partial loss-of-function, cause neurological disorders in the absence of cobalamin deficiency due to HCFC1 having other gene targets that are important for neurological development (8,10).…”

The Necessity for In Vivo Functional Analysis in Human Medical Genetics.

“…However, one might expect that the mechanistic basis of each disorder is completely unique because MMACHC is an enzyme in the cobalamin pathway (Hannibal et al, 2009) and HCFC1 is a transcriptional co-factor that regulates nearly 5000 different genes (Michaud et al, 2013, p. 1). In order to determine the distinct mechanisms regulating each phenotype, researchers have performed elegant functional analyses which have provided evidence that some phenotypes might be related, while others are likely completely unique (Gérard et al, 2015a, p. 1; Jolly et al, 2015, p. 1; Quintana et al, 2014, p. 1). These animal model studies substantiate clinical findings that demonstrate that a single treatment does not necessarily alleviate all phenotypes (Weisfeld-Adams et al, 2013).…”