Previously undescribed phenotypic findings and novel ACTG1 gene pathogenic variants in Baraitser-Winter cerebrofrontofacial syndrome

Hearing Loss in Baraitser-Winter Syndrome: Case Reports and Review of the Literature

Background: Baraitser-Winter Syndrome (BRWS) is a rare autosomal dominant condition associated with hearing loss (HL). In the literature, two types of this condition are reported, Baraitser-Winter type 1 (BRWS1) and type 2 (BRWS2) produced by specific pathogenetic variants of two different genes, ACTB for BRWS1 and ACTG1 for BRWS2. In addition to syndromic BRWS2, some pathogenic variants in ACTG1 are associated also to another pathologic entity, the "Autosomal dominant non-syndromic hearing loss 20/26". In these syndromes, typical craniofacial features, sensory impairment (vision and hearing) and intellectual disabilities are frequently present. Heart anomalies, renal and gastrointestinal involvement and seizure are also common. Wide inter- and intra-familial variety in the phenotypic spectrum is reported. Some phenotypic aspects of these syndromes are not yet fully described, such as the degree and progression of HL, and better knowledge of them could be useful for correct follow-up and treatment. Methods and Results: In this study, we report two cases of children with HL and diagnosis of BRWS and a review of the current literature on HL in these syndromes.

A likely pathogenic ACTG1 variant in a child showing partial phenotypic overlap with Baraitser-Winter syndrome

Baraitser-Winter syndrome (BRWS) is a rare autosomal dominant disease (AD) caused by heterozygous variants in ACTB (BRWS1) or ACTG1 (BRWS2) genes. BRWS features developmental delay/intellectual disability of variable degree and craniofacial dysmorphisms. Brain abnormalities (especially pachygyria), microcephaly, epilepsy, as well as hearing impairment, cardiovascular and genitourinary abnormalities may be present. We report on a 4-year-old female, who was addressed to our institution because of psychomotor delay associated with microcephaly and dysmorphic features, short stature, mild bilateral sensorineural hearing loss, mild cardiac septal hypertrophy, and abdominal swelling. Clinical exome sequencing detected a c.617G>A p.(Arg206Gln) de novo variant in ACTG1 gene. Such variant has been previously reported in association with a form of AD nonsyndromic sensorineural progressive hearing loss and we classified it as likely pathogenic according to ACMG/AMP criteria, despite our patient's phenotype only partially overlapped BWRS2. Our finding supports the extreme variability of the ACTG1-related disorders, ranging from classical BRWS2 to nuanced clinical expressions not fitting the original description, and occasionally featuring previously undescribed clinical findings.

Identification of a De Novo Heterozygous Missense ACTB Variant in Baraitser–Winter Cerebrofrontofacial Syndrome

Baraitser–Winter cerebrofrontofacial syndrome (BWCFF, OMIM: 243310) is a rare autosomal-dominant developmental disorder associated with variants in the genes ACTB or ACTG1. It is characterized by brain malformations, a distinctive facial appearance, ocular coloboma, and intellectual disability. However, the phenotypes of BWCFF are heterogenous, and its molecular pathogenesis has not been fully elucidated. In the present study, we conducted detailed clinical examinations on a Chinese patient with BWCFF and found novel ocular manifestations including pseudoduplication of the optic disc and nystagmus. Targeted gene panel sequencing and Sanger sequencing identified a de novo heterozygous missense c.478A > G (p.Thr160Ala) variant in ACTB. The mRNA and protein expression of ACTB was assessed by quantitative reverse transcription PCR and Western blots. Furthermore, the functional effects of the pathogenic variant were analyzed by protein structure analysis, which indicated that the variant may affect the active site for ATP hydrolysis by the actin ATPase, resulting in abnormal filamentous actin organization in peripheral blood mononuclear cells. This discovery extends the ACTB variant spectrum, which will improve genetic counseling and diagnosis, and may contribute to understanding the pathogenic mechanisms of actin-related diseases.

De Novo ACTG1 Variant Expands the Phenotype and Genotype of Partial Deafness and Baraitser-Winter Syndrome

Actin molecules are fundamental for embryonic structural and functional differentiation; γ-actin is specifically required for the maintenance and function of cytoskeletal structures in the ear, resulting in hearing. Baraitser–Winter Syndrome (B-WS, OMIM #243310, #614583) is a rare, multiple-anomaly genetic disorder caused by mutations in either cytoplasmically expressed actin gene, ACTB (β-actin) or ACTG1 (γ-actin). The resulting actinopathies cause characteristic cerebrofrontofacial and developmental traits, including progressive sensorineural deafness. Both ACTG1-related non-syndromic A20/A26 deafness and B-WS diagnoses are characterized by hypervariable penetrance in phenotype. Here, we identify a 28th patient worldwide carrying a mutated γ-actin ACTG1 allele, with mildly manifested cerebrofrontofacial B-WS traits, hypervariable penetrance of developmental traits and sensorineural hearing loss. This patient also displays brachycephaly and a complete absence of speech faculty, previously unreported for ACTG1-related B-WS or DFNA20/26 deafness, representing phenotypic expansion. The patient’s exome sequence analyses (ES) confirms a de novo ACTG1 variant previously unlinked to the pathology. Additional microarray analysis uncover no further mutational basis for dual molecular diagnosis in our patient. We conclude that γ-actin c.542C > T, p.Ala181Val is a dominant pathogenic variant, associated with mildly manifested facial and cerebral traits typical of B-WS, hypervariable penetrance of developmental traits and sensorineural deafness. We further posit and present argument and evidence suggesting ACTG1-related non-syndromic DFNA20/A26 deafness is a manifestation of undiagnosed ACTG1-related B-WS.

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

Since the early 2000s, an ever-increasing subset of missense pathogenic variants in the ACTG1 gene has been associated with an autosomal-dominant, progressive, typically post-lingual non-syndromic hearing loss (NSHL) condition designed as DFNA20/26. ACTG1 gene encodes gamma actin, the predominant actin protein in the cytoskeleton of auditory hair cells; its normal expression and function are essential for the stereocilia maintenance. Different gain-of-function pathogenic variants of ACTG1 have been associated with two major phenotypes: DFNA20/26 and Baraitser-Winter syndrome, a multiple congenital anomaly disorder. Here, we report a novel ACTG1 variant [c.625G>A (p. Val209Met)] in an adult patient with moderate-severe NSHL characterized by a downsloping audiogram. The patient, who had a clinical history of slowly progressive NSHL and tinnitus, was referred to our laboratory for the analysis of a large panel of NSHL-associated genes by next generation sequencing. An extensive review of previously reported ACTG1 variants and their associated phenotypes was also performed.