Diagnosis of inborn errors of metabolism by tandem mass spectrometry

Incidence and patterns of inborn errors of metabolism in the Eastern Province of Saudi Arabia, 1983-2008

BACKGROUND AND OBJECTIVES

Individual inborn errors of metabolism (IEM) are rare disorders, but may not be that uncommon in our patient population. We report the incidence of IEM in a defined cohort of births at the Saudi Aramco medical facilities in the Eastern Province of Saudi Arabia over 25 years.

METHODS

The records of all patients diagnosed with IEM from 1 January 1983 to 31 December 2008 were reviewed and categorized according to accumulated or deficient metabolites into small-molecule disorders (aminoacidemia, organic acidopathies [OA], urea cycle defects, fatty acid oxidation, and carbohydrate metabolic disorders) and other disorders, including glycogen and lysosomal storage disorders (LSDs), and organelle disorders.

RESULTS

During the study period, 165,530 Saudi Arabian infants were born at Saudi Aramco and 248 were diagnosed with an IEM, corresponding to a cumulative incidence of 150 cases per 100,000 live births. Small-molecule disorders were diagnosed in 134/248 patients (54%). OA were the most common (48/248 patients; 19%), and methylmalonic aciduria was the most frequently observed OA (13/48 patients; 27%). LSDs were diagnosed in 74/248 patients (30%), and mucopolysaccharidosis was the most frequently observed LSD (28/74; 38%).

CONCLUSION

We believe that our data underestimate the true incidence of IEM in the region. Regional and national newborn screening programs will provide a better estimation of the incidence of IEM. We recommend a centralized newborn screening program that employs tandem mass spectrometry.

Homozygosity for a severe novel medium-chain acyl-CoA dehydrogenase (MCAD) mutation IVS3-1G > C that leads to introduction of a premature termination codon by complete missplicing of the MCAD mRNA and is associated with phenotypic diversity ranging from sudden neonatal death to asymptomatic status

Virtually all patients with medium-chain acyl-CoA dehydrogenase deficiency (MCADD) are homozygous or compound heterozygous for the 985A > G mutation, which limits the study of a possible genotype/phenotype correlation. A newborn Palestinian infant died suddenly on the second day of life. A previous sibling had also died in similar circumstances aged 3 weeks. Urine organic acid and bloodspot acylcarnitine analysis were consistent with MCADD. He was homozygous for a novel MCAD splice mutation, IVS3-1G > C. This mutation leads to deletion of 7 bp and introduction of a premature termination codon as a result of complete missplicing of MCAD mRNA. This misspliced MCAD mRNA encodes a non-functional protein and is furthermore reduced in amounts due to nonsense-mediated decay, resulting in total lack of functional MCAD enzyme. This is the first molecular identification of MCADD in an Arab patient and the first reported splice mutation in the MCAD gene that has been functionally characterized. The association of homozygosity for a null mutation with lethal neonatal presentation in the index patient and presumably the previous infant suggested a genotype/phenotype correlation. However, a 6-year-old completely asymptomatic sibling also had the characteristic MCADD biochemical phenotype and was homozygous for the same IVS3-1G > C mutation. As a first candidate to modify the disease presentation, by modulating the overlapping enzyme activity, we tested the entire family for the prevalent SCAD gene 625G > A susceptibility variant. Interestingly, all family members were 625G > A homozygous. Additional genetic and/or environmental factors must play a major role in determining the phenotypic diversity of MCADD.

Pattern of inborn errors of metabolism in an Omani population of the Arabian Peninsula

We analysed all patients < 18 years of age diagnosed with inborn errors of metabolism (IEM) in the Metabolic Division, Sultan Qaboos University Hospital, Muscat, Oman from June 1998 to December 2000. A total of 82 patients from 76 families were studied, of whom 33 (40%) were aged < 1 week at presentation. Disorders identified included different lysosomal storage disorders (22), organic acidurias (9), carbohydrate metabolic disorders (9), congenital lactic acidosis (9), urea cycle disorders (8), amino acidopathies (8), fatty acid oxidation defects (7/82) and various other miscellaneous disorders (10). Tandem mass spectrometry helped in the diagnosis of 26 (32%) cases. Parental consanguinity was twice as frequent in the study patients as in the general population. Duration of follow-up ranged from 1 to 30 months (median 15) during which time 46/56 (82%) patients with disorders amenable to specific dietary and drug therapy available in Oman were free from frequent exacerbation. Our study shows the relevance of identifying patients with IEM in Oman and the need to establish screening for the conditions identified and provide effective management protocols.