Mutations in the gene encoding 3-hydroxyisobutyryl-CoA hydrolase results in progressive infantile neurodegeneration

Abstract

Only a single patient with 3-hydroxyisobutyryl-CoA hydrolase deficiency has been described in the literature, and the molecular basis of this inborn error of valine catabolism has remained unknown until now. Here, we present a second patient with 3-hydroxyisobutyryl-CoA hydrolase deficiency, who was identified through blood spot acylcarnitine analysis showing persistently increased levels of hydroxy-C(4)-carnitine. Both patients manifested hypotonia, poor feeding, motor delay, and subsequent neurological regression in infancy. Additional features in the newly identified patient included episodes of ketoacidosis and Leigh-like changes in the basal ganglia on a magnetic resonance imaging scan. In cultured skin fibroblasts from both patients, the 3-hydroxyisobutyryl-CoA hydrolase activity was deficient, and virtually no 3-hydroxyisobutyryl-CoA hydrolase protein could be detected by western blotting. Molecular analysis in both patients uncovered mutations in the HIBCH gene, including one missense mutation in a conserved part of the protein and two mutations affecting splicing. A carefully interpreted acylcarnitine profile will allow more patients with 3-hydroxyisobutyryl-CoA hydrolase deficiency to be diagnosed.

Figure  1.

Catabolism of L-valine. The names of the intermediates in the pathway for catabolism of L-valine are shown on the left, with solid arrows indicating enzymatic reactions. The structures of methacrylyl-CoA, 3-hydroxyisubutyryl-CoA, and 3-hydroxyisobutyrate are depicted on the left. The names of the metabolites that are increased as result of HIBCH deficiency are shown by dashed arrows on the right.

Figure  2.

MRI scans made when patient 2 was 14 mo old (T2-weighted images). There is signal abnormality in the brain regions indicated by the white arrows: in the globi pallidi (upper panel) and in the midbrain with asymmetrical involvement of the cerebral peduncles (R→L) (lower panel). The appearances were considered likely to represent a neurometabolic disorder. No structural abnormality was noted.

Figure  3.

Immunoblot analysis of HIBCH in fibroblast lysates and human liver homogenates. 25 μg of human liver (HL) protein and equal amounts of fibroblast protein (100 μg) of two control subjects (C1 and C2), the index patient (P1), and the newly identified patient (P2) were subjected to SDS-PAGE and transferred onto nitrocellulose by semidry blotting. Polyclonal antibodies raised against purified rat liver HIBCH were used at a dilution of 1:5,000. Antigen-antibody complexes were visualized with goat anti-rabbit IgG-alkaline phosphatase conjugate. As a control for transfer of protein, each blot was reversibly stained with Ponceau S before the incubation with antibodies.

Figure  4.

Cryptic splice acceptor site in patient 1. A, Sequence analysis of the genomic DNA region from which the 8-bp insertion originates and amplified by PCR from control subjects identified an intron of 2,183 bp (small lettering) with consensus splice donor– and splice acceptor–site sequences (italics and underlined) and branch point (underlined). The complete nucleotide sequence of the intron can be obtained from GenBank (accession number NT_005403). B, In patient 1, a T→G mutation (*) at the −9 position of the authentic splice acceptor site (underlined) results in alternative splicing at a cryptic splice acceptor site located 9 bp 5′ of the authentic splice acceptor site and preceded by a consensus branch point sequence. The 8-bp intron sequence, which is retained as a result of the aberrant splicing, is indicated in bold. Original exon sequences are indicated in capitals. Consensus sequences are defined as follows: splice donor site, [exon]..(C/A)AG↓gt(a/g)agt..[intron]; branch point, (t/c)n(t/c)t(g/a)ac (18–40 bp 5′ of splice acceptor site); splice acceptor site, [intron]..(t/c)₆n(t/c)ag ↓ G(G/T)..[exon].