[1] Univ Cambridge, Addenbrookes Hosp, Dept Pediat, Cambridge CB2 2QQ, England
[2] Univ Cambridge, Addenbrookes Hosp, Dept Clin Biochem, Cambridge CB2 2QQ, England
[3] Univ Cambridge, Addenbrookes Hosp, Dept Med Genet, Cambridge CB2 2QQ, England
[4] Univ Tokyo, Grad Sch Agr & Life Sci, Clin & Mol Genet Unit, Tokyo 1138657, Japan
[5] Univ Tokyo, Grad Sch Agr & Life Sci, Dept Anim Resource Sci & Appl Biol Chem, Tokyo 1138657, Japan
[6] Inst Child Hlth, Biochem Endocrinol & Metab Unit, London WC1N 1EH, England
[7] Great Ormond St Hosp Sick Children, London WC1N 1EH, England