Inborn errors of metabolism (IEM) are rare genetic disorders in which a single gene defect causes a clinically significant block in a metabolic pathway. Clinical problems arise due to either accumulation of substrates that are toxic or interfere with normal function, or deficiency of the products that are used to synthesize essential compounds. There is no report of screening results or confirmed cases of IEM in Mongolia. Only pilot study of newborn screening for congenital hypothyroidism was implemented in Mongolia, where the incidence of congenital hypothyroidism is calculated to be 1:3057 in Mongolia.

Two hundred twenty-three Mongolian patients, who had developmental delay, psychomotor retardation with unknown cause, seizures, hypotonia or liver dysfunction, were studied. Urinary organic acid analysis was performed in all cases using gas chromatography mass spectrometric (GC/MS) analysis. Blood amino acids and acylcarnitines were checked in the patients who had abnormal GC/MS analyses. Mutation analysis was done in the patients, who were suspected having specific inborn errors of metabolism by mass spectrometric analysis.

One hundred thirty-nine children had normal urinary organic acid analyses. Thirty one had metabolites of valproic acid, 17 had non- or hypoketotic dicarboxylic aciduria, 14 had tyrosiluria, 12 had ketosis, 4 had elevation of lactate and pyruvate, 3 had increased excretion of urinary glycerol or methylmalonic acids, respectively, and 2 had elevation of phenylacetate and phenyllactate. We checked blood amino acids and acylcarnitines in 38 patients, which revealed phenylketonuria (PKU) in 2 patients, and one with suspected citrin deficiency. Mutation analysis in PAH was done in 2 patients with PKU, and previously reported p.R243Q, p.Y356X, p.V399V, p.A403V mutations were detected.

In conclusion, these were the first genetically confirmed cases of PKU in Mongolia, and the study suggested that the newborn screening program for PKU was significant because it enabled early treatment dietary restriction, specialized formulas and other medical management for prevention of neurological handicaps in these children.