Mutations of the β‐subunit of human NAD‐specific isocitrate dehydrogenase in patients with Retinitis Pigmentosa

Abstract

Mitochondrial NAD‐ and NADP‐isocitrate dehydrogenases (IDH) are crucial in the Krebs cycle. NAD‐IDH is composed of 3 subunits in the ratio 2α:1β:1γ and is allosterically regulated by ADP, by lowering the Km‐isocitrate. Two individuals with autosomal recessive Retinitis Pigmentosa (RP), but no other major symptoms, are homozygous for mutations in the β‐subunit of NAD‐IDH: the missense βL98P, and a 1‐basepair deletion in codon βI163 (ATT to –TT) resulting in a frameshift and premature termination of the β‐subunit. The activities of NAD‐ and NADP‐IDH were evaluated in extracts of lymphoblasts from unaffected individuals, heterozygote carriers of the mutations, and two RP patients. The NADP‐IDH activity was normal in all samples. In contrast, both homozygotes exhibited a Km‐NAD 260–300 times normal, while the Km‐NAD of the heterozygote samples was 4‐8 times that of normal. When ADP is added, Km‐isocitrate decreases only 3‐fold in the RP patient with the I163delA mutation, unlike normals in which ADP decreases Km‐isocitrate 9‐fold. This allosteric effect is completely lost in the homozygote βL98P mutant. The results show that these beta mutations primarily affect the enzyme's NAD affinity and ADP response. NADP‐IDH is the dominant mitochondrial IDH expressed in most tissues, while NAD‐IDH predominates in the eye. This differential expression may explain why the phenotype of NAD‐IDH mutations is confined to the eye.