Expanding the β-III Spectrin-Associated Phenotypes toward Non-Progressive Congenital Ataxias with Neurodegeneration

Abstract

(1) Background: A non-progressive congenital ataxia (NPCA) phenotype caused by β-III spectrin (SPTBN2) mutations has emerged, mimicking spinocerebellar ataxia, autosomal recessive type 14 (SCAR14). The pattern of inheritance, however, resembles that of autosomal dominant classical spinocerebellar ataxia type 5 (SCA5). (2) Methods: In-depth phenotyping of two boys studied by a customized gene panel. Candidate variants were sought by structural modeling and protein expression. An extensive review of the literature was conducted in order to better characterize the SPTBN2-associated NPCA. (3) Results: Patients exhibited an NPCA with hypotonia, developmental delay, cerebellar syndrome, and cognitive deficits. Both probands presented with progressive global cerebellar volume loss in consecutive cerebral magnetic resonance imaging studies, characterized by decreasing midsagittal vermis relative diameter measurements. Cortical hyperintensities were observed on fluid-attenuated inversion recovery (FLAIR) images, suggesting a neurodegenerative process. Each patient carried a novel de novo SPTBN2 substitution: c.193A > G (p.K65E) or c.764A > G (p.D255G). Modeling and protein expression revealed that both mutations might be deleterious. (4) Conclusions: The reported findings contribute to a better understanding of the SPTBN2-associated phenotype. The mutations may preclude proper structural organization of the actin spectrin-based membrane skeleton, which, in turn, is responsible for the underlying disease mechanism.

Keywords: SPTBN2 gene; neurodegeneration; non-progressive congenital ataxia; β-III spectrin.

Figure 1

The protein β-III spectrin. (A) Protein structure (NP_008877.2) and location of its clinical mutations. Previously reported variants are in black, while the ones related to the non-progressive cerebellar ataxia (NPCA) phenotype are highlighted in red, including our two new mutations in bold font. CH1−CH2: calponin homology domains; β1−β17: β-spectrin domains; PH: pleckstrin homology domain.

Figure 2

Brain magnetic resonance imaging. Upper panels (A–D) correspond to patient 1 (MD-219; p.K65E) and bottom panels (E–H) to patient 2 (MD-207; p.D255G). (A) Quantitative analysis using midsagittal vermis relative diameter (MVRD). The ratio (vermis diameter/total posterior cranial fossa diameter) was used to express the proportion of both values. The first image (A) corresponds to patient 1 at 4 months old; MVRD resulted in 54%. (B–G) Midline sagittal T1 demonstrating cerebellar vermian atrophy at the age of 12 (B) and 19 months (C) with an MVRD of 48% and 43% respectively, in patient 1; and at the age of 22 months (E), 29 months (F), and 6 years (G), with an MVRD of 59%, 57%, and 50%, respectively, in patient 2. (D,H) coronal fluid-attenuated inversion recovery (FLAIR) images exhibiting global cerebellar atrophy and predominantly superior cerebellar cortical hyperintensity (D) at 19 months old in patient 1, and (H) at 29 months old in patient 2.

Figure 3

Structural homology model corresponding to the calponin homology (CH) domains of the human β-III spectrin and mutants. (A,B) Orthogonal views of the CH1 and CH2 domains (residues 47 to 291) highlighting the mutated residues K65 and D255 in red (in ball and stick representation). Surrounding residues and secondary structure elements are labeled. (C,D) Conservation of the mutated residues K65E and D255G, and close-up views of the mutants K65E and D255G (in red) and surrounding residues.

Figure 4

Western blot demonstrating the relative level of β-III spectrin in HeLa cells transfected with the different protein isoforms (WT or the p.K65E, p.L253P, and p.D255G mutants). (A) β-III spectrin immunodetected with an antibody against HaloTag, using tubulin as a load control. (B) Western blot-based quantification of β-III spectrin. Error bars: SEM; * p value < 0.05; ** p value < 0.01; ns: not significant.