The application of PCR amplification and the polymorphic marker KM.19 to dried blood spots: comparison with deletion 508 for the confirmation of the neonatal screening test for cystic fibrosis.

Cystic fibrosis (CF) screening by measurement of immunoreactive trypsin (IRT) lacks specificity: only 9% of hypertrypsinemic neonates have CF. We have studied retrospectively 114 hypertrypsinemic samples (including 37 CF) for KM.19 polymorphic DNA marker and made risk calculations. If the neonate is homozygous for KM.19 allele 2, the risk of CF rises to 55%; if homozygous for allele 1, the risk is very low (less than 1%) and if heterozygous, the risk is intermediate (4%). In a prospective study including 28,000 IRT tests, 76 neonates with IRT greater than 800 micrograms/L have been identified: 16 were homozygous for allele 2 (8 CF), 30 for allele 1 (1 CF), and 30 were heterozygotes (no CF). Deletion 508 was present in 10 neonates: 4 homozygotes (4 CF) and 6 heterozygotes (3 CF). Two CF did not carry any copy of deletion 508. We have studied 181 (presumably non-CF) neonates with IRT greater than 600 micrograms/L. The KM.19 genotypes distribution is significantly different from the one expected in the French population: homozygotes for allele 2 are more numerous. Furthermore, heterozygotes for deletion 508 are 1 in 15 (expected: 1 in 42). In conclusion, molecular biology in dried blood spots can enhance the specificity of CF neonatal screening, but IRT and genotype may not be independent.