HIBCH mutations can cause Leigh-like disease with combined deficiency of multiple mitochondrial respiratory chain enzymes and pyruvate dehydrogenase

Abstract

Background: Deficiency of 3-hydroxy-isobutyryl-CoA hydrolase (HIBCH) caused by HIBCH mutations is a rare cerebral organic aciduria caused by disturbance of valine catabolism. Multiple mitochondrial respiratory chain (RC) enzyme deficiencies can arise from a number of mechanisms, including defective maintenance or expression of mitochondrial DNA. Impaired biosynthesis of iron-sulphur clusters and lipoic acid can lead to pyruvate dehydrogenase complex (PDHc) deficiency in addition to multiple RC deficiencies, known as the multiple mitochondrial dysfunctions syndrome.

Methods: Two brothers born to distantly related Pakistani parents presenting in early infancy with a progressive neurodegenerative disorder, associated with basal ganglia changes on brain magnetic resonance imaging, were investigated for suspected Leigh-like mitochondrial disease. The index case had deficiencies of multiple RC enzymes and PDHc in skeletal muscle and fibroblasts respectively, but these were normal in his younger brother. The observation of persistently elevated hydroxy-C4-carnitine levels in the younger brother led to suspicion of HIBCH deficiency, which was investigated by biochemical assay in cultured skin fibroblasts and molecular genetic analysis.

Results: Specific spectrophotometric enzyme assay revealed HIBCH activity to be below detectable limits in cultured skin fibroblasts from both brothers. Direct Sanger sequence analysis demonstrated a novel homozygous pathogenic missense mutation c.950G <A; p.Gly317Glu in the HIBCH gene, which segregated with infantile-onset neurodegeneration within the family.

Conclusions: HIBCH deficiency, a disorder of valine catabolism, is a novel cause of the multiple mitochondrial dysfunctions syndrome, and should be considered in the differential diagnosis of patients presenting with multiple RC deficiencies and/or pyruvate dehydrogenase deficiency.

Figure 1

HIBCH deficiency leads to accumulation of hydroxy-C4-carnitine. (a) Valine degradation pathway. 3-Hydroxy-isobutyrylCoA hydrolase (HIBCH) catalyses the fifth step of valine catabolism. HIBCH deficiency leads to accumulation of 3-hydroxy-isobutyryl carnitine, which is detected as hydroxy-C4-carnitine by tandem mass spectrometry. (b) Plasma acylcarnitine analysis by tandem mass spectrometry. Left panel: normal acylcarnitine profile; Right panel: acylcarnitine profile from Patient 2 with accumulating hydroxy-C4-carnitine indicated by arrow.

Figure 2

Magnetic resonance imaging of the brain in HIBCH deficiency. (a) Patient 2 at 9 months: axial T2 weighted image (left panel) shows bilateral symmetrical signal hyperintensity within the globi pallidi (arrows) accompanied by signal hyperintensity on diffusion weighted image (middle panel) and low signal on ADC map (right panel) in keeping with restricted diffusion which is beginning to pseudonormalise on ADC map. The features are those of a subacute neurometabolic insult. In addition there is some generalised non-specific lack of cerebral volume with prominence of the cerebral sulci and delay in myelin maturation. (b) Patient 3 at 11 months: axial T2 (far left) shows bilateral symmetrical signal hyperintensity and swelling in the globi pallidi (arrows) with restricted diffusion (diffusion weighted image middle panel, ADC map right panel) consistent with cytotoxic oedema. The imaging pattern is suggestive of an acute neurometabolic insult.

Figure 3

Cytochrome oxidase immunocytochemistry in cultured skin fibroblasts. Expression of complex IV subunit 1 (MTCO1) in control and Patient 3 fibroblasts. Cells were cultured in the presence of 5 μM MitoTracker® Red CM-H2-XRos to label mitochondria red, followed by green immunostaining with anti-MTCO1 antibodies and blue nuclear counterstaining with DAPI. (a) Pseudo-coloured fluorescent micrographs. (b) Relative cellular expression levels of MTCO1 in Control (blue bars) and Patient (red bars) cells. The mean grey levels were measured for both the green and red images of 3631 control and 2184 patient cells, and the ratio of the two signals was calculated for each cell. The histogram reveals the frequency of cells with a particular MTCO1/MitoTracker ratio at intervals of 0.025. The total number of cells per culture is 100%.

Figure 4

Homozygous HIBCH mutation in patients 1 and 2. Sequence electropherograms of HIBCH gene. Top panel: Control; Second (from top) panel: Patient 1; Third panel: Patient 2; Fourth panel: Mother; Fifth panel: Father. Both patients are homozygous for the c.950G <A mutation, whilst the parents are heterozygous.