Neonatal screening in Sweden and disease-causing variants in phenylketonuria, galactosaemia and biotinidase deficiency

Abstract

Phenylketonuria, galactosaemia and biotinidase deficiency were the first three inborn errors of metabolism to be included in the Swedish programme. This thesis describes these conditions with an emphasis on screening performance and disease-causing genetic variants in the patients. Classical galactosaemia (GALT deficiency) is caused by a defect in the conversion of galactose-1-phosphate and UDP-glucose to glucose-1-phosphate and UDP-galactose, resulting in accumulation of toxic levels of galactose-1-phosphate and a deficiency of UDP-galactose. Early treatment prevents severe sequelae and life-threatening infections. The incidence of GALT deficiency in Sweden is 1/100 000, which is lower than the reported incidences for other European countries. The Swedish NBS programme has high sensitivity and specificity, with a positive predictive value (PPV) of approximately 0.5. The increase in PPV was achieved after the decision to adjust the recall level in order to avoid detection of milder variants, which probably do not require treatment.

The genetic studies revealed pathogenic variants on both alleles in all patients with GALT deficiency. Only a few variants were found in more than one patient, with the most cited variant, p.Gln188Arg, occurring most frequently. A high proportion of the variants have not been described before. Women with GALT deficiency who have been fortunate enough to become mothers have been recommended not to breast-feed because of the increase in toxic galactose metabolites seen at the end of pregnancy and after delivery. We observed the same increase, but the levels returned to normal within three weeks after birth in two consecutive pregnancies, in a woman who chose to breast-feed her babies. Our findings have been confirmed by other groups and women with GALT-deficiency are no longer discouraged to breast-feed.

Phenylketonuria (PKU) is caused by a defect in the conversion of the amino acid phenylalanine (Phe) to tyrosine (Tyr). Without treatment, patients develop mental retardation. Inclusion of the Phe/Tyr ratio has decreased the number of false positive screening outcomes to the present PPV of 0.92 without any known missed cases. The recall levels have been lowered several times since the start of screening. An increase in the incidence of patients with milder disease has been observed with time. We were able to show that the impact of the adjusted recall levels was low. Instead, milder genetic variants, which are more common in Southern Europe, are found more often, which is an effect of the large number of non-Nordic immigrants who have come to Sweden during the last 25 years. The immigration has widened the spectrum of detected pathogenic variants.

Biotinidase deficiency (BD) is a rare disorder affecting the recycling of the vitamin biotin. The most common symptoms are unspecific and progressive with eczema, hair loss and delayed psychomotor development. The majority of patients remained unrecognised before the introduction of screening. With NBS, the incidence of BD in Sweden is the same as in other Western countries (1/60 000). With adjustments of initial recall levels, virtually only infants with profound BD are detected in the screening programme. Disease-causing variants were detected in all alleles, with p.Thr532Met occurring most frequently.

In conclusion, the Swedish screening programme for PKU, galactosaemia and BD is well-functioning with an internationally comparatively low rate of false positive outcomes. Future research will tell if attenuated forms of the disorders, that are not targets in the Swedish programme, may benefit from early detection and ought to be included in the programme.