A novel mutation in the HSD17B10 gene of a 10-year-old boy with refractory epilepsy, choreoathetosis and learning disability

Abstract

Hydroxysteroid (17beta) dehydrogenase 10 (HSD10) is a mitochondrial multifunctional enzyme encoded by the HSD17B10 gene. Missense mutations in this gene result in HSD10 deficiency, whereas a silent mutation results in mental retardation, X-linked, syndromic 10 (MRXS10). Here we report a novel missense mutation found in the HSD17B10 gene, namely c.194T>C transition (rs104886492), brought about by the loss of two forked methyl groups of valine 65 in the HSD10 active site. The affected boy, who possesses mutant HSD10 (p.V65A), has a neurological syndrome with metabolic derangements, choreoathetosis, refractory epilepsy and learning disability. He has no history of acute decompensation or metabolic acidosis whereas his urine organic acid profile, showing elevated levels of 2-methyl-3-hydroxybutyrate and tiglylglycine, is characteristic of HSD10 deficiency. His HSD10 activity was much lower than the normal control level, with normal β-ketothiolase activity. The c.194T>C mutation in HSD17B10 can be identified by the restriction fragment polymorphism analysis, thereby facilitating the screening of this novel mutation in individuals with intellectual disability of unknown etiology and their family members much easier. The patient's mother is an asymptomatic carrier, and has a mixed ancestry (Hawaiian, Japanese and Chinese). This demonstrates that HSD10 deficiency patients are not confined to a particular ethnicity although previously reported cases were either Spanish or German descendants.

Figure 1. Electroencephalography of the affected boy at 10 years of age.

Figure 2. Mutation on the HSD17B10 gene of patient with a clinical diagnosis of HSD10 deficiency.

Chromatogram of the forward sequence of the HSD17B10 gene from the patient showing c.194T>C transition. The nucleotide sequence of intron 2 is indicated by lower case. This mutation resulted in mutant HSD10(p.V65A).

Figure 3. Detection of the c.194T>C variant in the HSD17B10 gene by RFLP analysis.

The pGEM-T Easy vectors harboring the HSD17B10 gene cloned from genomic DNA of a normal control [C1–3] (lanes 1–3), the patient's sister [S1–6] (lanes 4–9), the patient [P1–3] (lanes10–12), and the patient's mother [M1–6] (lanes 13–18) were digested by BstEII and then separated on a 1% agarose gel. Amounts of DNA loaded were 1 mg on lanes 1 and 2, 0.75 mg on lanes 3 and 6, and 0.5 mg on all the other lanes. A 2.2 kb fragment (indicated by an arrowhead) results from an allele carrying this variant. For a wild-type allele, this fragment is chopped into two shorter fragments (1.3 kb and 0.9 kb) as indicated by arrows. The vector is in the largest band indicated by an empty arrowhead.

Figure 4. Van der Waals interactions between the adenine ring of NAD⁺ and the side chain of residue 65 of HSD10.

The wild type HSD10 and mutant HSD10 were shown in part (A) and (B), respectively. Different colors represent different atoms: carbon (white), hydrogen (blue), nitrogen (purple), phosphorous (yellow) and oxygen (red). For clarity, all other amino acid residues in the protein, other than the neighboring aspartate 64 have been rendered invisible. Small dots represent the extent of the van der Waals radii for atoms in amino acid residue 65 in the protein and in the NAD⁺.

Figure 5. Comparison of amino acid sequence around valine 65 of HSD10 with those of its othologs in different species.

Residues conserved in all species were bolded. The asterisk * indicates an extremely conserved residue of this NAD⁺-dependent dehydrogenase.

Figure 6. Isoleucine and methylated fatty acid oxidation pathway.

Compounds in dashed line boxes were increasingly excreted from patients with HSD10 deficiency or beta-ketothiolase deficiency (adapted from Ref. 2).