The pathogenic mechanisms of severe intellectual disabiIity caused by PLEKHA5 or SLC19A3 mutations studied using mouse models of the diseases.

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We identified mutations in candidate genes associated with 2 different diseases characterized by severe intellectual disability. Patient 1 had a balanced translocation t(6;12)(ql6;pl2). We analyzed the brains of homozygous Plekha5 knockout mice produced using exon trap method at the Kumamoto University. The mice survived more than 1 year, and typical pathological findings were not noted by hematoxylin-eosin staining. The results suggested that haploinsufficiency of PLEKHA5 and a fusion protein (c-terminal PLEKHA5 expression driven by SFRS18 promoter) caused by the translocation were involved in the pathogenesis of the disease in patient 1. Analysis of Plekha5 knockout mice produced by conventional or Cre/loxP system is necessary to confirm the phenotype of Plekha5 deficient mouse. Patient 2 had severe intellectual disability and specific magnetic resonance imaging findings, including abnormal density of basal ganglions and severe brain atrophy. We identified a missense mutation (E320Q) in SLC19A3 encoding thiamine (vitamin B1) transporter in the patient and generated a knock-in (NI) mouse that had the equivalent mutation as in the patient. The NI mice survived more than 1 year when fed standard mouse chow, CE-2 (Clea Japan Inc., Tokyo, Japan). However, when the homozygous NI mice were provided special feed CE-2 containing 35% vitamin B1, they died after 24 days; however, the feed did not affect the survival of wild-type and heterozygous NI mice. Theses results suggest that the patients harboring E320Q mutation in SLC19A3 have specific sensitivity for vitamin B1, and intake of high dose vitamin B1 is the possible treatment for the patients.