A de novo mutation in the β-tubulin gene TUBB4A results in the leukoencephalopathy hypomyelination with atrophy of the basal ganglia and cerebellum

Abstract

Hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) is a rare hereditary leukoencephalopathy that was originally identified by MRI pattern analysis, and it has thus far defied all attempts at identifying the causal mutation. Only 22 cases are published in the literature to date. We performed exome sequencing on five family trios, two family quartets, and three single probands, which revealed that all eleven H-ABC-diagnosed individuals carry the same de novo single-nucleotide TUBB4A mutation resulting in nonsynonymous change p.Asp249Asn. Detailed investigation of one of the family quartets with the singular finding of an H-ABC-affected sibling pair revealed maternal mosaicism for the mutation, suggesting that rare de novo mutations that are initially phenotypically neutral in a mosaic individual can be disease causing in the subsequent generation. Modeling of TUBB4A shows that the mutation creates a nonsynonymous change at a highly conserved asparagine that sits at the intradimer interface of α-tubulin and β-tubulin, and this change might affect tubulin dimerization, microtubule polymerization, or microtubule stability. Consistent with H-ABC's clinical presentation, TUBB4A is highly expressed in neurons, and a recent report has shown that an N-terminal alteration is associated with a heritable dystonia. Together, these data demonstrate that a single de novo mutation in TUBB4A results in H-ABC.

Figure 1

MRI Findings in H-ABC Axial T2-weighted images (first three columns) and sagittal T1-weighted images (fourth column) of individual HA107 at the age of 3.5 years (A–D), individual HA27 at 21 years (E–H), and an unaffected individual at 14 years (I–L). We included the unaffected individual to demonstrate the low signal on T2-weighted images of normal myelinated white matter, the normal volume of the putamen, and the normal volume of the cerebellum. Note the relatively high T2 signal of the cerebral white matter in the two affected individuals (A, B, E, and F); this indicates lack of myelin. No putamen is visible in the affected individuals (B and F) (arrows are where the putamen should be). The cerebellar atrophy is already present in the younger individual (D) but is worse in the older individual (H).

Figure 2

The Highly Conserved TUBB4A Residue Asp249 Sits at the Intradimer Interface of α-Tubulin and β-Tubulin (A) The genomic structure of TUBB4A shows the region encoding Arg2 in exon 1 and the region encoding Asp249 in exon 4. (B) Multiple-sequence alignment of a section of four human β-tubulins, a zebrafish β-tubulin, and the Saccharomyces cerevisiae β-tubulin. Residue Asp249 is marked in bold text, residues that form the T7 loop are boxed, and asterisks indicate invariant residues. Sequences are labeled with their Uniprot ID. (C) Structure of bovine tubulin heterodimer. Guanosine triphosphate (GTP; green) can be seen bound at the interface of α-tubulin (magenta) and β-tubulin (blue). Residue Asp249 is shown as red spheres, its interacting partner Arg2 is shown as blue spheres, and the remainder of the T7 loop is shown in yellow. (D) A magnified view of the intradimer interface demonstrates the roll of Asp249 in coordinating the interaction between the T7 loop and the α-tubulin-bound GTP.