Prevalence of CFTR mutations in hypertrypsinaemia detected through neonatal screening for cystic fibrosis

Abstract

Nowadays, most of the neonatal screening programs for cystic fibrosis (CF) combine the assay of immunoreactive trypsinogen (IRT) with the analysis of the most common mutations of the CFTR gene. The efficiency of this strategy is now well established, but the identification of heterozygotes among neonates with increased IRT is perceived as a drawback. We proposed to assess the heterozygosity frequency among the children with hypertrypsinaemia detected through the CF screening program implemented in Brittany (France) 10 years ago, to describe the CFTR mutations detected in them and to determine the frequency of the IVS8-5T variant. The molecular analysis relies, in our protocol, on the systematic analysis of three exons of the gene (7-10-11). A total of 160,019 babies were screened for CF in western Brittany between 1992 and 1998. Of the 1964 newborns with increased IRT (1.2%), 60 were CF and 213 were carriers. Heterozygosity frequency was 12.8%), i.e. 3 times greater than in the general population (3.9%; p < 10(-6)), Variability of mutations detected in carriers was greater than in CF children (21 mutations versus 10) and a high proportion of mild mutations or variants (A349V, R297Q, R347H, V317A, G544S, R553G, etc) was observed in carriers. The allelic frequency of the 5T (5.6%) was not significantly increased in this cohort. This study is consistent with previous ones in finding a significantly higher rate of heterozygotes than expected among neonates with hypertrypsinaemia. The strategy of screening used here allows to highlight the variability of mutations detected in heterozygotes and to show that severe mutations, as well as mild mutations, have been observed in neonates with hypertrypsinaemia. If there is no doubt that neonatal hypertrypsinaemia is associated with an elevated frequency of carriers, the underlying mechanisms remain obscure.

Risk calculations for cystic fibrosis in neonatal screening by immunoreactive trypsinogen and CFTR mutation tests

“…The number of CF newborns among newborns with IRT levels within normal limits was not determined. 14 However, the number of CF newborns with normal IRT levels should be small, considering that the CF incidence of 60/160,019 (ϭ 1/2,667) in this study was so close to the 1/2,500 incidence widely used as the incidence of CF among non-Hispanic Caucasians. 1 Therefore, the probability that a CF affected, carrier, or noncarrier newborn has hypertrypsinogenemia is estimated to be 1 (ϭ 60/60), 0.041 (ϭ 250/6,077), or 0.011 [ϭ 1654/(160,019 Ϫ 60 Ϫ 6,077)], respectively ( Fig.…”

“…[11][12][13]16 Figure 1 illustrates the numbers derived from the data of Scotet et al 14 to calculate each conditional probability that a newborn has hypertrypsinogenemia if the newborn is affected by CF, a carrier, or a noncarrier. In the study by Scotet et al, 14 a cut-off level of IRT was set as 600 g/L, and 60 CF affected newborns were detected among 160,019 newborns screened, leading to a disease allele frequency of 0.0194 (q 2 ϭ 60/160,019) assuming Hardy-Weinberg equilibrium. Using the 2pq (ϭ 0.0380) carrier frequency, approximately 6,077 newborns were carriers among 160,019 newborns.…”

“…Neonatal screening is relatively easy to perform, and has been shown to be an effective way to prevent malnutrition and improve long-term growth of neonates affected with CF. 23 The assay characteristics of the IRT test that we used here were derived from a single large study on newborn screening by the IRT test, 14 because this study is the only study available that allows calculations of the probability of a positive IRT test if a neonate is affected, a carrier, or a noncarrier. However, normal IRT value ranges may be different in different ethnic groups.…”

“…This is because there are considerable differences in CFTR allele frequencies among different ethnic groups, 9,10 and published data regarding IRT assay characteristics are complex. [11][12][13][14] In this article, we evaluated published data to calculate the assay characteristics of the IRT test, and provide Bayesian methods for accurate CF risk calculations using concrete scenarios. Our Bayesian methods allow CF disease probabilities to be calculated accurately, taking into account all relevant information.…”

Neugeborenenscreening auf Mukoviszidose in Deutschland: Vergleich des neuen Screening-Protokolls mit einem Alternativprotokoll

“…Dabei wurde zunächst der Anteil für die gesamte Studienpopulation berechnet. Für die IRT-positiv getesteten NG, wurde der dafür berechnete Anteil mit 2,5 multipliziert, da aus der Literatur bekannt ist, dass die Zahl der heterozygoten NG unter IRT-positiven Kindern 2-bis 3-mal höher ist als der der Gesamtpopulation [15,20]. Die so erhaltene Zahl heterozygoter NG wurde dann für die nach den einzelnen Protokollschritten jeweils positiv getesteten NG heruntergebrochen.…”