Expanding the clinical and molecular spectrum of TUBB2B through distinct variants identified across multiple families

Novel heterozygous truncating variant in TUBB associated with thrombocytopaenia and neurological abnormalities

Tubulinopathies constitute a growing and heterogeneous class of disorders caused by pathogenic variants in genes encoding for tubulins, the structural components of microtubules. These conditions encompass a broad spectrum of neurological manifestations, most commonly including structural brain malformations and, less frequently, neurodegenerative disorders. In addition to brain diseases, tubulinopathies also include non-neurological conditions, such as female meiotic infertility and bleeding disorders. Among tubulin genes, TUBB , encoding the class I β-tubulin, has been associated with a wide range of clinical conditions, primarily including neurodevelopmental delay, structural brain abnormalities, and skin creases. More recently, TUBB has also been linked to inherited macrothrombocytopaenia, suggesting a broader, yet still incompletely defined, role of this β-tubulin in haematopoiesis and platelet biogenesis. In this study, we aim to investigate pathogenic mechanisms of a novel frameshift variant in TUBB (c.628dup; p.Ile210AsnfsTer40), identified in a subject affected by thrombocytopaenia and neurological involvement. Functional analyses demonstrated that the p.Ile210AsnfsTer40 TUBB mutation leads to the production of a truncated β-tubulin with reduced protein stability and aberrant subcellular localization. The mutant protein showed impaired interaction with α-tubulin and kinesin family member 1 A (KIF1A), suggesting defective microtubule assembly and microtubule-mediated intracellular transport. Furthermore, the inhibition of the proteasome degradative pathway resulted in a marked increase of the mutant TUBB, indicating altered proteostasis and enhanced degradation of the truncated tubulin. Finally, we showed that the p.Asp249Val substitution in TUBB , previously associated with macrothrombocytopaenia, similarly reduces protein stability, weakens interaction with α-tubulin, and impairs TUBB incorporation into microtubules. This study associates the novel heterozygous truncating p.Ile210AsnfsTer40 variant in TUBB with a multisystem disorder characterized by haematological and neurological manifestations, expanding the phenotypic spectrum of TUBB -related diseases. Our findings indicate that reduced protein stability and functional impairment of β-tubulin represent key pathogenic mechanisms underlying this condition. Lastly, our experimental evidence further supports the implication of TUBB dysfunctions in haematological abnormalities, offering additional insight into the molecular basis and clinical variability of tubulin-related disorders.

Genotypic and phenotypic heterogeneity in tubulinopathies: insights from a Turkish multicenter cohort

Cross-species functional analysis of a de novo DCLK1 variant associated with a neurodevelopmental disorder

The findings implicate DCLK1 in a previously unrecognized progressive neurodevelopmental disorder and demonstrate the power of integrative cross-species functional genomics in resolving ultra-rare disease variants.

Phenotypic Expansion of PPP1R12A-Related Syndrome: A Novel Splicing Variant Associated with Hearing Loss and Inner Ear Malformations

Background: Pathogenic variants in the PPP1R12A gene have been associated with a malformation syndrome involving the brain and the genitourinary systems (GUBS, MIM #618820). To date, neither hearing loss (HL) nor inner ear malformations have been reported in affected individuals, and these features are therefore not currently regarded as part of the PPP1R12A-related phenotype. Moreover, functional evidence supporting the pathogenicity of several reported variants remains limited. Methods: We investigated a 12.5-year-old patient presenting with profound bilateral sensorineural hearing loss associated with inner ear malformations, genitourinary and central nervous system abnormalities. The patient underwent comprehensive clinical, audiological and radiological assessments, followed by genetic testing via trio-based whole-exome sequencing (WES). The molecular consequences of the identified variant were evaluated through minigene splicing assay and RT–PCR analysis on RNA extracted from peripheral blood cells. Results: WES identified a novel heterozygous splicing variant (c.792+3A>C) in the PPP1R12A gene (NM_002480.3). Functional studies demonstrated that this variant causes complete skipping of exon 5, resulting in a frameshift and the introduction of a premature termination codon. RT–PCR analysis confirmed the presence of the alternatively spliced transcript lacking exon 5. In addition, an extensive review of the literature indicated that no clear genotype–phenotype correlation has yet been established for PPP1R12A-related disorders and, whereas the majority of previously reported patients share brain and genitourinary malformations our patient additionally presented with profound bilateral sensorineural HL and inner ear malformations. Conclusions: Our findings suggest that PPP1R12A-related disorders may exhibit a broader phenotypic variability than previously recognized and we further propose that HL and inner ear malformations may represent novel features associated with this clinical spectrum. Moreover, our study underscores the importance of functional studies for accurately defining the molecular consequences of novel variants and establishing appropriate clinical correlations.

A case report of a novel de novo variant in PPP2CA causing a neurodevelopmental disorder and epilepsy

The findings support the pathogenicity of this variant and further expand the pathogenic variant spectrum of the PPP2CA gene, and the observed genotype–phenotype correlation provides valuable information for prognosis and genetic counseling.

The AP5B1 p.(Leu785Pro) variant is a frequent cause of late-onset macular dystrophy with variable extraocular manifestations.

Inherited retinal diseases (IRDs) represent a large group of genetically heterogeneous disorders that often cause progressive visual loss. The fifth adaptor protein (AP-5) complex, which contributes to endolysosomal trafficking and lysosomal homeostasis, has previously been implicated in neurodegenerative syndromes, and more recently biallelic variants in three of its subunits were found in families with macular dystrophy including four with variants in AP5B1. Here, we describe 22 affected individuals from 20 families with AP5B1-associated IRD, all carrying the recurrent missense variant NM_138368.5:c.2354T>C p.(Leu785Pro), either in the homozygous state (16 families) or in trans with another rare heterozygous AP5B1 missense or loss-of-function variant. Five families were of Ashkenazi Jewish ancestry and 15 families of European ancestry. Clinically, affected individuals presented with a predominantly late-onset macular dystrophy that frequently progressed to cone-rod degeneration. Characteristic retinal findings included foveal sparing, early peripapillary involvement, and a reticular pattern best observed by fundus autofluorescence imaging in the mid- or peripheral retina. The typical presenting symptom was decreased visual acuity. Age at onset ranged from 27 to 74 years, with most individuals becoming symptomatic after the fifth decade of life. Some individuals also presented with extraocular manifestations, most notably hearing loss, which was reported in 8 cases. These findings further support AP5B1 as a cause of macular dystrophy, identify p.(Leu785Pro) as a relatively frequent pathogenic allele in individuals of European and Ashkenazi Jewish ancestry, and expand the associated phenotypic spectrum to include both isolated macular dystrophy and possible syndromic presentations.