Molecular Genetics and Metabolism 84 (2005) 317–325 www.elsevier.com/locate/ymgme 1096-7192/$ - see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.ymgme.2004.11.011 Genetic analysis of three genes causing isolated methylmalonic acidemia: identiWcation of 21 novel allelic variants Maria Angeles Martínez 1 , Ana Rincón 1 , Lourdes R. Desviat, Begoña Merinero, Magdalena Ugarte ¤ , Belén Pérez Centro de Biología Molecular “Severo Ochoa”, CSIC-UAM,Universidad Autónoma, Madrid, Spain Received 5 August 2004; received in revised form 23 November 2004; accepted 29 November 2004 Available online 22 January 2005 Abstract Isolated methylmalonic aciduria (MMA) is an inborn error of metabolism due to the impaired isomerization of L-methylmalonyl- CoA to succinyl-CoA. This reaction is catalyzed by the mitochondrial protein methylmalonyl-CoA mutase (MCM, EC 5.4.99.2), an adenosylcobalamin-dependent enzyme. Four diVerent forms of isolated MMA have been described: mut MMA associated with defects in the MCM apoenzyme, and phenotypically divided into two subtypes mut ¡ and mut 0 MMA, and three diVerent defects involved in the synthesis of the active form of the cofactor adenosylcobalamin, termed cbl MMA, and classiWed into three diVerent complementation groups cblA, cblB, and cblH associated with defects in the MMAA and MMAB genes and with an unidentiWed protein, respectively. In this work we describe the genetic analysis of 25 MMA patients, mainly from Spain. Using biochemical and cellular approaches our patients have been classiWed, identifying 13 mut MMA, 7 cblA, 2 cblB, and 3 noncblA, noncblB deWcient patients. cDNA and genomic DNA sequence analysis of the MUT, MMAA, and MMAB genes have allowed us to identify 27 diVer- ent changes, 21 novel ones. Among the missense mutations identiWed in the MUT gene only one, the c.970G > A (p.A324T) variant located in the substrate binding domain is likely a mut ¡ mutation. The remaining missense mutations c.326A > G (p.Q109R), c.983T > C (p.L328P), c.1846C > T (p.R616C), and c.1850T > G (p.L617R) are probably mut 0 . In the MMAA patients analyzed, frameshift mutations are prevalent. We have explored the genotype–phenotype correlation for this clinically heterogeneous disease.  2004 Elsevier Inc. All rights reserved. Keywords: Methylmalonic aciduria; MMA; Mutations; Cobalamin; cblA; cblB; MethylmalonylCoA mutase; MCM Introduction Methylmalonic aciduria (MMA) is an inborn error of metabolism due to the impaired isomerization of L-met- hymalonyl-CoA to succinyl CoA during the oxidation of propionate towards the Krebs cycle. The intramolecular rearrangement is catalyzed by a cobalamin-dependent mitochondrial enzyme named L-methylmalonyl-CoA mutase (MCM, EC 5.4.99.2) using adenosylcobalamin (Adocbl) as cofactor. In mammals the enzyme is required for the degradation of odd-chain fatty acids, the amino acids valine, isoleucine, methionine, threo- nine, and cholesterol. Biochemical and somatic cell stud- ies have delineated four diVerent forms associated with isolated MMA deWciency. Disorders associated with defects in the MCM apoenzyme are designed mut MMA (OMIM 251000) and the defects in enzymes required for mitochondrial synthesis of the active form of Adocbl are termed cbl MMA. Three diVerent complementation groups, namely cblA (OMIM 607,481), cblB (OMIM 607,568), and cblH (OMIM 606,169) have been identi- Wed, in all of them the synthesis of AdoCbl is blocked. Patients with mut MMA have been divided into two sub- types, mut 0 , with no MCM activity in Wbroblasts and * Corresponding author. Fax: +34 917347797. E-mail address: mugarte@cbm.uam.es (M. Ugarte). 1 These contributed equally to this work.