Further delineation of putative ACTB loss-of-function variants: A 4-patient series

Abstract

ACTB encodes β-cytoplasmic actin, an essential component of the cytoskeleton. Based on chromosome 7p22.1 deletions that include the ACTB locus and on rare truncating ACTB variants, a phenotype resulting from ACTB haploinsufficiency was recently proposed. We report putative ACTB loss-of-function variants in four patients. To the best of our knowledge, we report the first 7p22.1 microdeletion confined to ACTB and the second ACTB frameshifting mutation that predicts mRNA decay. A de-novo ACTB p.(Gly302Ala) mutation affects β-cytoplasmic actin distribution. All four patients share a facial gestalt that is distinct from that of individuals with dominant-negative ACTB variants in Baraitser-Winter cerebrofrontofacial syndrome. Two of our patients had strikingly thin and sparse scalp hair. One patient had sagittal craniosynostosis and hypospadias. All three affected male children have attention deficits and mild global developmental delay. Mild intellectual disability was present in only one patient. Heterozygous ACTB deletion can allow for normal psychomotor function.

Keywords: ACTB; intellectual disability; loss-of-function; sparse scalp hair; β-cytoplasmic actin.

Figure 1

Clinical findings, ACTB variant compilation, and effects of the ACTB p.(Gly302Ala) variant. (a) Patient 1 at the age of 11 years. (b) Patient 2 at the age of 35 years. (c) Patient 3 at the age of 6 years. (d) Patient 4 at the age of 4 years. Shared facial dysmorphism consists of wavy interrupted eyebrows, dense eyelashes, wide nose, wide mouth, prominent cheeks, and chin. Sparse scalp hair in patients 2 and 4. (e) Compilation of novel and reported ACTB variants associated with BWCFF, putative ACTB loss‐of‐function and ACTB‐AST (chromosomal order of genes and exons not drawn to scale). Modeling of ACTB residue Gly‐302 (f) and variant Ala‐302 (g). (h) The relative ACTB messenger RNA expression in a healthy donor (C) and Patient 4 (P4) was assessed by quantitative real‐time polymerase chain reaction using the ${\mathrm{\Delta }\mathrm{\Delta }C}_{\mathrm{t}}$ method. Values were normalized to the amount of human ribosomal protein L32 mRNA. Immunoblot analysis of ACTB and α‐tubulin (TUBA) in total lysates, NP‐40 soluble (S) and insoluble (INS) fractions of blood mononuclear cells from healthy individuals (C1 and C2) and Patient 4 (P4). GAPDH and histone H3 were used as loading controls (i,j). (k) TUBA acetylation was analyzed in total lysates (S + INS) and NP‐40 soluble (S) fraction obtained from control and Patient 4 blood mononuclear cells using anti‐Ac‐TUBA and anti‐TUBA antibodies. BWCFF, Baraitser‐Winter cerebrofrontofacial syndrome