Assay of serum immunoreactive trypsin in dried blood spots and the early detection of cystic fibrosis

Metabolic Signatures of Cystic Fibrosis Identified in Dried Blood Spots For Newborn Screening Without Carrier Identification

Cystic fibrosis (CF) is a complex multiorgan disorder that is among the most common fatal genetic diseases benefiting from therapeutic interventions early in life. Newborn screening (NBS) for presymptomatic detection of CF currently relies on a two-stage immunoreactive trypsinogen (IRT) and cystic fibrosis transmembrane conductance regulator (CFTR) mutation panel algorithm that is sensitive but not specific for identifying affected neonates with a low positive predictive value. For the first time, we report the discovery of a panel of CF-specific metabolites from a single 3.2 mm diameter dried blood spot (DBS) punch when using multisegment injection-capillary electrophoresis-mass spectrometry (MS) as a high-throughput platform for nontargeted metabolite profiling from volume-restricted/biobanked specimens with quality control. This retrospective case-control study design identified 32 metabolites, including a series of N-glycated amino acids, oxidized glutathione disulfide, and nicotinamide that were differentially expressed in normal birth weight CF neonates without meconium ileus ( n = 36) as compared to gestational age/sex-matched screen-negative controls ( n = 44) after a false discovery rate adjustment ( q < 0.05). Also, 16 metabolites from DBS extracts allowed for discrimination of true CF cases from presumptive screen-positive carriers with one identified CFTR mutation and transient neonatal hypertrypsinogenemic neonates ( n = 72), who were later confirmed as unaffected due to a low sweat chloride (<29 mM) test result. Importantly, six CF-specific biomarker candidates satisfying a Bonferroni adjustment ( p < 7.25 × 10^-5) from three independent batches of DBS specimens included several amino acids depleted in circulation (Tyr, Ser, Thr, Pro, Gly) likely reflecting protein maldigestion/malabsorption. Additionally, CF neonates had lower ophthalmic acid as an indicator of oxidative stress due to impaired glutathione efflux from exocrine/epithelial tissue and elevation of an unknown trivalent peptide that was directly correlated with IRT (ρ = 0.332, p = 4.55 × 10^-4). Structural elucidation of unknown metabolites was performed by high-resolution MS/MS, whereas biomarker validation was realized when comparing a subset of metabolites from matching neonatal DBS specimens independently analyzed by direct infusion-MS/MS at an accredited NBS facility. This work sheds new light into the metabolic phenotype of CF early in life, which is required for better functional understanding of CFTR mutations of unknown clinical consequence and the development of more accurate yet cost-effective strategies for CF screening.

Newborn screening for cystic fibrosis: Polish 4 years' experience with CFTR sequencing strategy

Newborn screening for cystic fibrosis (NBS CF) in Poland was started in September 2006. Summary from 4 years' experience is presented in this study. The immunoreactive trypsin/DNA sequencing strategy was implemented. The group of 1,212,487 newborns were screened for cystic fibrosis during the programme. We identified a total of 221 CF cases during this period, including, 4 CF cases were reported to be omitted by NBS CF. Disease incidence in Poland based on the programme results was estimated as 1/4394 and carrier frequency as 1/33. The frequency of the F508del was similar (62%) to population data previously reported. This strategy allowed us to identify 29 affected infants with rare genotypes. The frequency of some mutations (eg, 2184insA, K710X) was assessed in Poland for the first time. Thus, sequencing assay seems to be accurate method for screening programme using blood spots in the Polish population.

Newborn screening for cystic fibrosis using an enzyme linked immunoabsorbent assay (ELISA) technique

Neonatal screening for cystic fibrosis (CF) has been conducted using conventional radioimmunoassay. An alternative immunoassay approach has been developed and applied to screening 35,550 newborns. Seventeen confirmed CF infants were detected by both assays. This monoclonal antibody-based enzyme immunoassay using microtitre plate ELISA technology has proved effective in case finding and offers a number of advantages. A reduced labour component, ease of handling, use of nonradioactive reagents, long reagent shelf-life, greater specificity and a reduction in potential sample handling and transposition errors combine to make this technology appropriate for a large volume neonatal screening laboratory.

Variations in dried blood spot immunoreactive trypsin in relation to gestational age and during the first week of life

Dry blood spot immunoreactive trypsin was measured by radioimmunoassay in 84 preterm babies and in 65 full-term newborns studied daily from the first to the fifth day of life. In a control group of 3858 full-term newborns, trypsin concentrations at days 4-6 of life exhibited a log-normal pattern distribution, the geometric mean being 18 ng/ml serum. Immunoreactive trypsin concentrations did not change significantly between days 1, 2, 3, 4 and 5 after birth. Immunoreactive trypsin was found to be significantly lower (geometric mean 9 ng/ml, P less than 0.01) in preterm newborns before 32 weeks of gestation. In hypotrophic newborns of 34 weeks gestational age, immunoreactive trypsin values were higher than those observed at 31 weeks of gestation in eutrophic newborns, the mean birth weight not being different between both groups. These data suggest that trypsin production by the pancreas is dependent on maturity but does not seem related to intrauterine nutritional status. Immunoreactive trypsin concentrations do not change after 32 weeks gestational age and during the first postnatal week.

Clinical usefulness and methodology of immunological assay of pancreatic enzymes

Radioimmunoassays for plasma pancreatic enzymes, especially cationic trypsin, are good tools to diagnose pancreatic pathology. The trypsin assay is very sensitive. Its detection limit is 300 pg/ml. The plasma level of this enzyme appears to be representative of pancreatic function. This assay was shown to be valuable in the diagnosis of acute pancreatitis, and to follow the course of the disease. In chronic pancreatitis, it enables assessment of residual function of the organ. In the terminal stage of this disease, very low values were found. Used as a screening test, the assay allowed early detection of cystic fibrosis.

Screening for cystic fibrosis

Practicable methods are now available for whole population screening of neonates for cystic fibrosis. Although diagnosis and treatment of the disease from birth has not yet been unequivocally shown to improve prognosis, existing evidence suggests that this is likely. Further ethical reasons are proposed in support of neonatal diagnosis and early treatment. The development of tests for prenatal diagnosis and carrier detection is under active investigation, and raises ethical problems for heterozygotes and their medical advisers. The heavy financial and emotional burden this disease imposes on the patient and the family should not be underestimated when policy decisions are made.