Two Deafness-Causing Actin Mutations (DFNA20/26) Have Allosteric Effects on the Actin Structure

Abstract

Point mutations in γ-cytoplasmic actin have been shown to result in autosomal-dominant, nonsyndromic, early-onset deafness. Two mutations at the same site, K118M and K118N, provide a unique opportunity to compare the effects of two dissimilar amino acid substitutions that produce a similar phenotype in humans. K118 resides in a helix that runs from K113 to T126, and mutations that alter the position, dynamics, and/or biochemistry of this helix can result in a wide range of pathologies. Using a combination of computational and experimental studies, both employing yeast actin, we find that these mutations at K118 result in changes in the structure and dynamics of the DNase-I loop, alterations in the structure of the H73 loop as well as the side-chain orientations of W79 and W86, changes in nucleotide exchange rates, and significant shifts in the twist of the actin monomer. Interestingly, in the case of K118N, the twist of the monomer is nearly identical to that of the F-actin protomer, and in vitro polymerization assays show that this mutation results in faster polymerization. Taken together, these results indicate that mutations at this site give rise to a series of small changes that can be tolerated in vivo but result in misregulation of actin assembly and dynamics.

Essential nucleotide- and protein-dependent functions ofActb/β-actin

“…Clinically, spontaneous missense mutations in both ACTB and ACTG1 have been associated with Baraitser-Winter syndrome, a rare disease with complex phenotypes, including facial dysmorphism, learning disabilities, and deafness (24). Some of the mutations affect the polymerization properties of actin or interaction with actin binding proteins to cause a gain-of-function or a dominant-negative effect (25)(26)(27). Interestingly, the more severe forms of Baraitser-Winter syndrome are associated with mutations in ACTB over ACTG1 (4).…”

Newly described recessive MYH11 disorder with clinical overlap of Multisystemic smooth muscle dysfunction and Megacystis microcolon hypoperistalsis syndromes

“…In addition, congenital sensorineural hearing loss was present. While mutations in both myosin and actin genes have been documented to cause non-syndromic early onset sensorineural hearing loss (Jepsen, Kruth, Rubenstein, & Sept, 2016;Kim et al, 2016), mutations in MYH11 have not. The impact of the patient's myosin mutation on his vasculature remains unknown in light of his relatively limited survival.…”

DFNA20/26 and Other ACTG1-Associated Phenotypes: A Case Report and Review of the Literature

“…Gamma actin is the predominant actin protein in the cytoskeleton of auditory hair cells, where it is found in stereocilia, the cuticular plate, and adherens junctions [2]; its normal expression and function are essential for the stereocilia maintenance but are not required for auditory hair cell development [3,4]. The molecular pathogenic effect of missense variants detected in the first families described in the literature was evaluated using a variety of biochemical analyses, studies with yeast and hair cells of mouse cochlear explants, and Actg1 knock-in mouse models [4,19,[43][44][45][46]. These results suggest a gain-of-function mode of pathogenesis, with missense mutations giving rise to a series of small changes that result in misregulation of actin assembly and dynamics; these alterations progressively damage auditory hair cells but are tolerated in other cell types [4].…”

“…Anyway, the spectrum of pathogenic variants observed in patients with DFNA20/26 does not overlap with the one observed in patients with a typical BWCFF syndrome (Supplementary Tables S2 and S3) [6,7,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66]. In addition, the results of preliminary studies with lymphoblastoid cell lines from BWCFF syndrome patients, as well as the results of in vitro and in vivo studies on DFNA20/26-associated variants, suggest that the phenotypic impact of missense mutations may differ in a mutation-specific way; the available functional studies highlighted that each ACTG1 variant led to specific alterations in terms of actin organization and polymerization, interaction with actin-binding proteins, and cell morphology [4,6,7,19,[43][44][45][46]. Given that HL is a common feature of BWCFF syndrome, DFNA20/26 may represent the mild end and BWCFF syndrome the severe end of a wide spectrum of ACTG1-associated phenotypes [6].…”