Estimation of the total number of disease-causing mutations in ornithine transcarbamylase (OTC) deficiency.: Value of the OTC structure in predicting a mutation pathogenic potential

Ornithine transcarbamylase deficiency (OTCD), the X-linked, most frequent urea cycle error, results from mutations in the OTC gene, encoding a 354-residue polypeptide. To date 341 OTCD clinical mutations, including 222 missense single nucleotide changes (mSNCs), have been compiled (Hum Mutat 2006;27:626). OTCD mutation detection might be simplified if the entire repertoire of OTCD-causing mutations were known. We estimate the size of this repertoire from 23 new OTCD patients exhibiting 22 different mutations, of which 9, including 4 mSNCs, are novel. The complete repertoire of OTCD-causing mutations is estimated as 560 mutations (95% confidence interval, 422-833 mutations), including 290 mSNCs (95% confidence interval, 230394 mSNCs). Thus, OTCD diagnosis based on the screening for known mutations might attain similar to 90% sensitivity in < 5 years. Since disease-causing mSNCs represent < 20% of the 2064 possible OTC mSNCs, simple approaches are essential for discrimination between causative and trivial mSNCs. Observation of the OTC structure appears a simple approach for such discrimination, comparing favourably in our sample with three formalized structure-based and/or sequence-based in silico,assessment methods, and supporting the causation of complete deficiency by the mutations p.Pro305Arg and p.Ser96Phe, and of partial deficiency by p.Asp41Gly, p.Glu122Gly, p.Leu179Phe, p.Pro220Thr and p.Glu273del. Five non-mSNC novel mutations (p.Gly71X, a 7-nucleotide and a 10-nucleotide duplication and deletion in exon 5, G > A transit ions at bases +1 and +5 of introns 4 and 9, respectively) are obviously pathogenic. The previously reported mSNCs p.Arg26Gln, p.Arg40His, p.Glu52Lys, pLys88Asn, p.Arg129His, P-Asn161Ser, p.Thr178Met, p.His202Tyr, p.Ala208Thr and p.His302Arg, found in our cohort, are also discussed.