Immunoreactive trypsin and a comparison of two ΔF508 mutation analyses in newborn screening for cystic fibrosis: An anonymous pilot study in Denmark

Cystic fibrosis newborn screening in Denmark: Experience from the first 2 years

BACKGROUND

In Denmark, newborn screening (NBS) for cystic fibrosis (CF) was introduced on 1 May 2016. The implementation and results from the first 2 years of the national newborn CF screening program are presented.

METHODS

The screening included immunoreactive trypsinogen (IRT), followed by evaluation for the F508del mutation when a value at or above the 50 ng/mL cutoff was present. In cases with a single F508del mutation or a very high IRT value above 145 ng/mL, next-generation sequencing of the CF transmembrane conductance regulator gene (CFTR) was performed.

RESULTS

Of 126 522 newborn infants 126 338 were tested (99.85%), and 4730 samples (3.7%) were assessed for CFTR mutations. Twenty-six infants were screen-positive and referred for diagnostic follow-up of whom 22 were confirmed to have a CF diagnosis, four had one known and one CFTR allele with unknown pathogenicity, classified as cystic fibrosis screening positive inconclusive diagnosis (CFSPID), PPV 84.6%. One of the four children classified as CFSPID was later found to carry the two identified CFTR variants in cis and was reclassified as a carrier of CF. We found two false negatives; one exhibited an IRT level above the 50 ng/mL cutoff but was below the 145 ng/mL very high cutoff and with no F508del mutation present. The second false-negative fell below the 50 ng/mL IRT cutoff but was diagnosed shortly after birth on the basis of meconium ileus. Screening sensitivity, 91.7%. Two hundred thirty-two children were identified as carriers of CF, which is twofold above the estimated annual number of carriers. All but one carrier were heterozygous for the F508del CFTR mutation. Sixteen percent of the sequenced samples revealed rare CFTR variants, which were classified as nonpathogenic in relation to CF.

CONCLUSIONS

During the first 2 years of NBS CF screening in Denmark, we identified close to the expected number of infants with CF using an algorithm based on IRT, presence of F508del mutation and comprehensive genetic analysis. CFSPID accounted for only a small minority, despite comprehensive CFTR sequencing, whereas more carriers than initially expected were identified.

Storage policies and use of the Danish Newborn Screening Biobank

After routine newborn screening, residual dried blood spot samples (DBSS) are stored at -20 degrees C in the Danish Newborn Screening Biobank (NBS-Biobank), which contains DBSS from virtually all newborns in Denmark since 1982--about 1.8 million samples. The purpose of the storage is: (1) diagnosis and treatment of congenital disorders including documentation, repeat testing, quality assurance, statistics and improvement of screening methods; (2) diagnostic use later in infancy after informed consent; (3) legal use after court order; (4) the possibility of research projects after approval by the Scientific Ethical Committee System in Denmark, The Danish Data Protection Agency and the NBS-Biobank Steering Committee. The operation and use of the NBS-Biobank has until recently been regulated by an executive order of 1993 from the Danish Ministry of Health. The Ethical Council, the Central Scientific Ethical Committee and the National Board of Health were also involved in the regulations. These regulations have now been replaced by detailed general operational guidelines for biobanks in Denmark according to Acts on Processing of Personal Data, Patient's Rights, Health 546/2005 and the Biomedical Research Ethics Committee System. No specific Act on biobanks per se has been made in Denmark, but the new regulations and guidelines make the operations of the Danish NBS-Biobank even more clear-cut and safe. The Danish NBS-Biobank has been used in several research projects for aetiological studies of a number of disorders, recently employing new sensitive multiplex technologies and genetic analyses utilizing whole-genome amplified DNA.

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

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.

A retrospective anonymous pilot study in screening newborns for HFE mutations in Scandinavian populations

We have retrospectively analyzed 837 random anonymized dried blood spot (DBS) samples from neonatal screening programs in Scandinavia for mutations in HFE, the candidate gene for hemochromatosis. We have found C282Y allele frequencies of 2.3% (+2.0%) (-1.3%) in Greenland, 4.5%+/-1.9% in Iceland, 5.1%+/-2.3% in the Faeroe Islands, and 8.2%+/-2.7% in Denmark. The high prevalence of HFE mutations in Denmark suggests that population screening for the C282Y mutation could be highly advantageous in terms of preventive health care. Long-term follow-up evaluation of C282Y homozygotes and H63D/C282Y compound heterozygotes will give an indication of the penetrance of the mutations.

Biological specimen banks in neonatal screening

The Danish neonatal screening program analyses dried blood spot samples (DBSS) from close to 70,000 newborns annually from Denmark, Greenland and the Faroe Islands. Since 1982, all DBSS have been stored in a biological specimen bank at -20 degrees C as a routine procedure after analysis. Before sampling, parents are given written information about the screening tests, the biobank and its use, and can choose to opt out. Since 1993 the biobank has been regulated by specific legislation, and thus assumes a unique position among biological specimen banks. Its purposes are: (i) diagnosis and treatment of diseases screened for, including repeat testing, quality assurance and group statistics; (ii) other diagnostic uses during infancy; and (iii) research projects. The stored samples have been used successfully to diagnose a range of genetic diseases using biochemical and molecular genetic assays, and to diagnose congenital CMV and toxoplasmosis infections using assays for specific IgM antibodies and pathogen nucleic acids. The unbiased nature and comprehensive coverage of the samples in the biobank make them attractive for research purposes. Our studies have focused on the epidemiology of genetic disease alleles and other molecular disease markers and on retrospective screening projects, which have allowed rapid appraisal of the performance of novel screening modalities, saving years of prospective screening trials. Storage of neonatal screening samples is thus beneficial not only to the individual testees, but also to future generations of newborns.

The 20th century Danish facial cleft population--epidemiological and genetic-epidemiological studies

Since Dr. Fogh-Andersen's legendary 1942 thesis, the Danish facial cleft population has been one of the most extensively studied in terms of epidemiology and genetic-epidemiology. The etiology of cleft lip and/or palate (CLP) is still largely an enigma, and different results concerning environmental and genetic risk factors are obtained in different countries and regions. This may be due to etiological heterogeneity between settings. Therefore, an in-depth studied area with an ethnically homogeneous population, such as Denmark, has provided one of the best opportunities for progress in CLP etiological research. The present review summarizes epidemiological and genetic-epidemiological studies conducted in the 20th century Danish facial cleft population. Furthermore, analyses of sex differences, time trends and seasonality for more than 7000 CLP cases born in Denmark in the period 1936 to 1987 are presented. The review also points toward the excellent opportunities for continued etiological CLP research in Denmark in the 21st century using already established resources and an on-going prospective cohort study of 100,000 pregnant women.

The frequency of an inactivating point mutation (566C-->T) of the human follicle-stimulating hormone receptor gene in four populations using allele-specific hybridization and time-resolved fluorometry

We have described previously in the Finnish population an inactivating point mutation (566C-->T) in the human FSH receptor (FSHR) gene. In women, this mutation causes hypergonadotropic ovarian failure with arrest of follicular maturation and infertility, whereas in men, there is variable suppression of spermatogenesis, but no absolute infertility. To determine whether the same FSHR mutation occurs in other populations, its frequency was determined in Finland, Switzerland, Denmark, and the Chinese population of Singapore. The mutation was screened for using genomic DNA extracted from whole blood or dried blood spots. Exon 7 of the FSHR gene was first amplified using a pair of biotinylated primers. The PCR products were then immobilized on streptavidin-coated microtitration wells and hybridized using short allele-specific oligonucleotide probes labeled with europium. Time-resolved fluorometry was used for europium signal detection. To test the reliability of this method, 40 isolated DNA samples and 35 dried blood spot samples were blindly tested for the 566C-->T FSHR mutation. The analyses yielded identical results with denaturing gradient gel electrophoresis and allele-specific restriction enzyme digestion of the same samples, thus demonstrating the reliability of the tested method. Automation of this procedure allows the screening of large numbers of samples, which was subsequently carried out to investigate the frequency of the 566C-->T mutation in the study populations. A total of 4981 samples from the above-mentioned 4 countries were analyzed. The frequency of the 566C-->T mutation was 0.96% for all Finnish samples (n=1976), with a strong enrichment of the mutant allele in the northeastern part of the country. Only 1 mutation carrier was identified in the samples from Switzerland (n=1162), whereas none was found in samples from Denmark (n=1094) and the Singapore Chinese (n=540). These results suggest that the 566C-->T mutation of the FSHR gene is enriched in Finland, but is uncommon in other populations.

Simple triple-label detection of seven cystic fibrosis mutations by time-resolved fluorometry

We describe a simple hybridization assay performed in microtitration wells with use of DNA probes labeled with three different lanthanide chelates for detection of seven mutations that cause cystic fibrosis. The assay is based on DNA amplification of four fragments containing the mutations (ΔF508, G1717→A, G542X, R553X, 3905 insertion T, W1282X, and N1303K) by PCR, followed by hybridization with short, allele-specific oligonucleotide probes labeled with europium, terbium, or samarium chelates. Because the technology makes it possible to hybridize three DNA probes simultaneously in one reaction, all 14 mutation-related alleles were detected in a total of five reaction wells. Blood spot specimens, obtained from children with cystic fibrosis, their parents, and their siblings, have been assayed, and for all the probes the positive signal-to-noise ratios are &gt;10. Solution hybridization utilizing triple-label time-resolved fluorometry combined with PCR is a suitable procedure for large-scale screening and automation.

Population screening for the common G985 mutation causing medium-chain acyl-CoA dehydrogenase deficiency with Eu-labeled oligonucleotides and the DELFIA system

We have screened 10 171 neonatal blood spots from the Trent and West Midlands regions of the UK for the common G985 mutation to more accurately establish the incidence of medium-chain acyl coenzyme (Co)A dehydrogenase (MCAD) deficiency. We have used a technique involving PCR and Eu-labeled allele-specific oligonucleotides detected by using time-resolved fluorometry on the dissociation-enhanced fluorescence immunoassay (DELFIA) system for the detection of the G985 mutation. We have also evaluated the feasibility of neonatal screening with this technique. We identified 158 G985 heterozygotes and no G985 homozygotes. The calculated incidence of MCAD deficiency in the population studied (all mutations, assuming 90% of MCAD mutations are G985) is 1 in 13 426 (95% confidence limits 1 in 10 070–1 in 18 791). At the optimum cutoff criteria, the technique has a sensitivity of 97.5%, specificity of 99.6%, and positive predictive value of 80.2%. We conclude that this study confirms that MCAD deficiency is a common inherited metabolic disease and is a candidate for neonatal screening. The methodology used is robust and suitable for large-scale population studies such as this. The technique is also potentially suitable for screening.

Neonatal screening for cystic fibrosis in the Trent region (UK): two-stage immunoreactive trypsin screening compared with a three-stage protocol with DNA analysis as an intermediate step

OBJECTIVES

To assess neonatal screening for cystic fibrosis using immunoreactive trypsin, either alone or in conjunction with DNA analysis for the delta F508 mutation. A novel three-stage screening protocol was compared with the previously introduced two-stage immunoreactive trypsin-DNA protocol.

DESIGN

(a) Collection of data from a 4 1/2 year period (phase 1) of two-stage immunoreactive trypsin screening. The initial dried blood samples were obtained at 6 days of age and repeat samples at 27 days of age from babies with results above the 99.5th centile. Babies with persistent hypertrypsinaemia were referred for a diagnostic sweat test. (b) Retrospective DNA analysis: patients with cystic fibrosis diagnosed in phase 1 were genotyped and most samples from babies with increased initial immunoreactive trypsin but normal results in the second sample were analysed for the delta F508 mutation. (c) Phase 2, a prospective study of a three-stage neonatal screening protocol, in which only babies heterozygous for the delta F508 cystic fibrosis mutation progressed to the second immunoreactive trypsin test.

SETTING

The Trent neonatal screening programme.

SUBJECTS

437 859 babies born between August 1989 and March 1996.

MAIN OUTCOME MEASURES

Proportions of unaffected babies requiring a second blood sample or a sweat test. Overall sensitivity for the detection of cystic fibrosis.

RESULTS

The two-stage screen failed to identify six out of 94 cases of cystic fibrosis (without meconium ileus). The introduction of the DNA analysis step would have resulted in one additional case being missed. With the three-stage screen there was a 92% reduction in babies requiring a second blood sample and an 80% reduction in negative sweat tests, results close to the predictions of the retrospective study.

CONCLUSIONS

The three-stage screening protocol is a marked improvement on the two-stage immunoreactive trypsin strategy and on the two-stage immunoreactive trypsin-DNA strategy recently introduced in some other screening programmes.

High incidence of cystic fibrosis on the Faroe Islands: a molecular and genealogical study

We have studied the genetics of cystic fibrosis (CF) in The Faroe Islands. Based on the number of affected children born during the period 1954-1993, the incidence of CF at birth is 1:1775, which is more than twice the incidence in the rest of Denmark. We have tested all known CF patients and/or their parents for the presence of delta F508 and found it to be the only CF mutation in this population. Based on testing 881 unrelated control individuals, the carrier frequency was estimated to be 1:24, given a calculated incidence of 1:2300. Genealogical studies enabled us to trace several of the families over seven generations. Haplotype investigations within the families suggest that delta F508 was introduced by two founders, probably from the Celtic population in Brittany, Ireland, Wales or the North West of Scotland.

A specific immunoradiometric assay of cationic trypsin(ogen) that does not recognize trypsin-alpha-1-proteinase inhibitor complex

An immunoradiometric assay using two monoclonal antibodies directed to human trypsin 1 was developed for measuring trypsin(ogen) in biological fluids. The assay is different from other assays in that it is specific for cationic trypsinogen and does not recognize the alpha-1-proteinase inhibitor-trypsin complex. It can be used as a complement to classical immunoassays to characterize trypsinogen activation in pathological cases. The evaluation and the specificity of the assay are presented.

Laboratory screening for genetic disorders and birth defects

Screening for inherited disease is a preventative health measure that started in the 1960s with the development of programs for the detection of PKU in newborns and that has had a major impact on reducing the burden of disease. Developments in technology have led to the availability of large scale testing for an increasing number of both acquired and genetic disorders. Laboratory testing is only one facet of a screening program and consideration should be given to availability of testing to all individuals, education regarding the program, effectiveness of treatment, long-term benefits both for individuals and society, ethical issues, and cost benefits. In this review, newborn, prenatal, and heterozygote screening are discussed.