MCAD deficiency in Denmark

Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is the most common defect of fatty acid oxidation. Many countries have introduced newborn screening for MCADD, because characteristic acylcarnitines can easily be identified in filter paper blood spot samples by tandem mass spectrometry (MS/MS), because MCADD is a frequent disease, and because of the success of early treatment initiated before clinical symptoms have emerged. In Denmark we have screened 519,350 newborns for MCADD by MS/MS and identified 58 affected babies. The diagnosis of MCADD was confirmed in all 58 newborns by mutation analysis. This gives an incidence of MCADD detected by newborn screening in Denmark of 1/8954. In sharp contrast to this we found that the incidence of clinically presenting MCADD in Denmark in the 10 year period preceding introduction of MS/MS-based screening was only 1 in 39,691. This means that four times more newborns with MCADD are detected by screening than what is expected based on the number of children presenting clinically in an unscreened population. The mutation spectrum in the newborns detected by screening is different from that observed in clinically presenting patients with a much lower proportion of newborns being homozygous for the prevalent disease-causing c.985A>G mutation. A significant number of the newborns have genotypes with mutations that have not been observed in patients detected clinically. Some of these mutations, like c.199T>C and c.127G>A, are always associated with a milder biochemical phenotype and may cause a milder form of MCADD with a relatively low risk of disease manifestation, thereby explaining part of the discrepancy between the frequency of clinically manifested MCADD and the frequency of MCADD determined by screening. In addition, our data suggest that some of this discrepancy can be explained by a reduced penetrance of the c.985A>G mutation, with perhaps only 50% of c.985A>G homozygotes presenting with disease manifestations. Interestingly, we also report that the observed number of newborns identified by screening who are homozygous for the c.985A>G mutation is twice that predicted from the estimated carrier frequency. We therefore redetermined the carrier frequency in a new sample of 1946 blood spots using a new assay, but this only confirmed that the c.985A>G carrier frequency in Denmark is approximately 1/105. We conclude that MCADD is much more frequent than expected, has a reduced penetrance and that rapid genotyping using the initial blood spot sample is important for correct diagnosis and counseling.