PDH E₁β deficiency with novel mutations in two patients with Leigh syndrome

Most cases of pyruvate dehydrogenase complex (PDHc) deficiency are attributable to mutations in the PDHA1 gene which encodes the E₁α subunit, with few cases of mutations in the genes for E₃, E3BP (E₃ binding protein), E₂ and E₁-phosphatase being reported. Only seven patients with deficiency of the E₁β subunit have been described, with mutations in the PDHB gene in six of them. Clinically they presented with a non-specific encephalomyopathy. We report two patients with new mutations in PDHB and Leigh syndrome. Patient 1 was a boy with neonatal onset of hyperlactataemia, corpus callosum hypoplasia and a convulsive encephalopathy. After neurological deterioration, he died at age 5 months. Autopsy revealed the characteristic features of Leigh syndrome. Patient 2, also a boy, presented a milder clinical course. First symptoms were noticed at age 16 months with muscular hypotonia, lactic acidosis and recurrent episodes of somnolence and transient tetraparesis. MRI revealed bilateral signal hyperintensities in the globus pallidus, midbrain and crura cerebri. PDHc and E₁ activities were deficient in fibroblasts in patient 1; in patient 2 PDHc deficiency was found in skeletal muscle. Mutations in PDHA1 were excluded. Sequencing of PDHB revealed a homozygous point mutation (c.302T>C), causing a predicted amino acid change (p.M101T) in patient 1. Patient 2 is compound heterozygote for mutations c.301A>G (p.M101V) and c.313G>A (p.R105Q). All three mutations appear to destabilize the E₁ enzyme with a decrease of both E₁α and E₁β subunits in immunoblot analysis. To our knowledge, these patients with novel PDHB mutations are the first reported with Leigh syndrome.