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Inborn errors of propionate metabolism: Methylmalonic and propionic acidaemias

The field of inborn errors of metabolism is one of the most rapidly advancing areas in medicine, and the rate of progress in our knowledge and understanding of the disorders of propionate metabolism is in proportion to the wider advances in the field. Propionate (propionic acid) is a major substrate for energy production in ruminant animals and, while this role is less relevant in man, it remains an important intermediate in the metabolism of certain fats and of the amino acids valine, isoleucine, threonine and methionine (Fig. 1). Methylmalonic (MMA) and propionic (PA) acidaemias are the most common clinically relevant inborn errors of organic acid metabolism in man with incidences of about 1/50000 and 1 I1 00000 live births respectively. Both typically present with ketoacidosis in the neonatal period after a normal delivery and a normal first 1-3 days. Some patients come to notice later in the first year with failure to thrive, typically with recurrent episodes of vomiting and acidosis. Of clinical interest is the everincreasing range of complications of these conditions. Basal ganglia infarction is now well-recognized and causes a choreoathetoid movement disorder. Presentation with this feature has been described even in adulthood,’ and urinary organic acid analysis in children with unexplained choreoathetosis should be routine. Recurrent pancreatitis’ and renal tubular and glomerular dysfunction3 both appear to be relatively frequent complications of MMA in particular, but so far these have only been described in children in whom the organic acidopathy has previously been well-documented. Cardiomyopathy has also been mentioned as a possible association in MMA.4 Many of the clinical features and complications appear attributable to the recognized ability of propionate to inhibit mitochondria1 function; indeed, the alterations in acid-base balance result from lactate and ketone accumulation secondary to inhibition of mitochondrial energy cycles, rather than through the direct effect of increased propionic (or methylmalonic) acid concentration per se. Diagnosis of these disorders IS established through metabolic screening in symptomatic patients. Most metabolic laboratories now include a simple screening test for increased urinary