Clinically severe CACNA1A alleles affect synaptic function and neurodegeneration differentially

Abstract

Dominant mutations in CACNA1A, encoding the α-1A subunit of the neuronal P/Q type voltage-dependent Ca2+ channel, can cause diverse neurological phenotypes. Rare cases of markedly severe early onset developmental delay and congenital ataxia can be due to de novo CACNA1A missense alleles, with variants affecting the S4 transmembrane segments of the channel, some of which are reported to be loss-of-function. Exome sequencing in five individuals with severe early onset ataxia identified one novel variant (p.R1673P), in a girl with global developmental delay and progressive cerebellar atrophy, and a recurrent, de novo p.R1664Q variant, in four individuals with global developmental delay, hypotonia, and ophthalmologic abnormalities. Given the severity of these phenotypes we explored their functional impact in Drosophila. We previously generated null and partial loss-of-function alleles of cac, the homolog of CACNA1A in Drosophila. Here, we created transgenic wild type and mutant genomic rescue constructs with the two noted conserved point mutations. The p.R1673P mutant failed to rescue cac lethality, displayed a gain-of-function phenotype in electroretinograms (ERG) recorded from mutant clones, and evolved a neurodegenerative phenotype in aging flies, based on ERGs and transmission electron microscopy. In contrast, the p.R1664Q variant exhibited loss of function and failed to develop a neurodegenerative phenotype. Hence, the novel R1673P allele produces neurodegenerative phenotypes in flies and human, likely due to a toxic gain of function.

Cognitive impairment in children with CACNA1A mutations

“…The frequency of episodes varied widely among the patients, ranging from some per year up to almost daily episodes (600 per year in one patient). Hemiplegic migraine occurred as a first phenotype in four patients (patients [15][16][17][18], and followed another phenotype in one patient (patient 9). The median age of the start of hemiplegic migraine was 9 years (IQR 7-10y; range 4-14y).…”

“…[4][5][6][7][8][9][10] Ataxia, with or without cerebellar atrophy, has been reported in adults, and in single case reports of children with CAC-NA1A mutations. 6,[11][12][13][14][15][16] Among the chronic neurological manifestations associated with heterozygous CACNA1A mutations, cognitive dysfunction has been suggested in recent years, including developmental delay, learning difficulties, autism, attention-deficit/hyperactivity disorder, intellectual impairment, and epileptic or non-epileptic encephalopathy. 6,[17][18][19][20][21][22][23] Until now, the cognitive profile of children with CACNA1A mutations has not been studied systematically in a series of patients.…”

Genetic strategies to tackle neurological diseases in fruit flies

“…Based on previously identified lethal mutations and neurodegenerative phenotypes caused by mutations in the fly homologue, cac [61], the authors tested the rescuing ability of a wild type 80 kb P[acman] clone and clones derived from this BAC in which the human variants were inserted by recombineering [62]. Whereas the wild type P[acman] clone fully rescued the observed phenotypes, the two variants tested showed very different phenotypes in the flies providing a better understanding of the nature of the patient variants in CACNA1A [60]. …”

CRISPR-Cas9 human gene replacement and phenomic characterization inCaenorhabditis elegansto understand the functional conservation of human genes and decipher variants of uncertain significance

“…Genetically tractable model organisms are critical for discovering novel gene 32 functions and the functional consequences of disease-associated genetic variants 33 (Dunham and Fowler, 2013;Lehner, 2013;Manolio et al, 2017;Wangler et al, 2017). 34…”

“…Governmental and private funding agencies are increasingly commissioning large-scale 35 collaborative programs to use diverse genetic model organisms to decipher variants of 36 uncertain significance (Chong et al, 2015;Gahl et al, 2016;Wangler et al, 2017). 37…”