Rapid detection of beta s DNA from Guthrie cards by chromogenic probes

History and current status of newborn screening for hemoglobinopathies

The impact of hemoglobinopathies on healthcare in the United States, particularly sickle cell disease (SCD), has been significant. Enactment of the Sickle Cell Anemia Control Act in 1972 significantly increased the federal interest in the SCDs and other hemoglobinopathies. Only since May 1, 2006, have all states required and provided universal newborn screening for SCD despite a national recommendation to this effect in 1987. In this article, we review the history of screening for SCD and other hemoglobinopathies, along with federal and state activities that have contributed to improved health outcomes for patients with SCD, as well as current newborn screening practices. We also chronicle the federal activities that have helped to shape and to refine laboratory screening and diagnostic proficiency. Finally, we review molecular testing strategies that have evolved and outline their possible future impacts on disease detection and outcome improvement.

Differentiation of homozygous hemoglobin E from compound heterozygous hemoglobin E-beta O-thalassemia by hemoglobin E mutation analysis

OBJECTIVES

To facilitate the differential diagnosis of hemoglobin FE in newborn infants (homozygous hemoglobin E vs hemoglobin E-beta O-thalassemia).

METHODS

The beta-globin gene in DNA from infants found to have hemoglobin FE in the California newborn screening program was amplified by the polymerase chain reaction, and the product was digested with Mnl I, which fails to cut the product when the hemoglobin E mutation is present. When both amplified alleles fail to be cut, homozygous EE is diagnosed. If only one allele is cut, a beta-globin allele without the E mutation is present (non-E), which is most likely a gene with a beta O-thalassemia mutation.

RESULTS

Samples from 18 infants revealed an EE genotype, and from two samples a non-E/E genotype was determined. Clinical examination of these two patients confirmed a diagnosis of hemoglobin E-beta O-thalassemia. An independent clinical diagnosis agreed with DNA analysis for all 17 of the 20 infants for whom follow-up and family studies were available. The DNA results were obtained within a week, but the clinical diagnoses often could not be resolved unequivocally for months.

CONCLUSIONS

The direct analysis of patient DNA samples for the hemoglobin E mutation allowed rapid and accurate diagnosis in this sample of infants with hemoglobin FE on the newborn screen. This rapid discriminatory test should reduce cost and simplify the diagnostic approach for these patients, which currently consists of expensive and lengthy follow-up until clinical data and family studies result in a diagnosis.

Trapped-oligonucleotide nucleotide incorporation (TONI) assay, a simple method for screening point mutations

We present a simple screening method for detecting a known point mutation, using only one 5'-biotinylated oligonucleotide primer, with its 3' end adjacent to the mutation site. In parallel reactions, an amplified DNA template encompassing the biotinylated oligonucleotide and mutation site undergoes 40 step-cycles of single nucleotide incorporation using Taq thermostable DNA polymerase and only one radioactive [alpha-32P]dNTP, specified by either the normal or mutant sequence. The oligonucleotides, now radioactively labelled at the 3' end according to the template sequence, are then trapped by streptavidin-coated magnetic beads, and the percent of radiolabel incorporated is determined directly by the Cerenkov method in a scintillation counter. The trapped-oligonucleotide nucleotide incorporation (TONI) assay has been used for the screening of a mitochondrial polymorphism, and has also been shown to distinguish the genotypes of hemoglobin A/C, A/A, A/S, and S/S. It is reproducible over at least a 100-fold range of radioisotope and a 10-fold range of oligonucleotide primer. This method is particularly useful for diagnosing mutations which do not produce alterations detectable by restriction enzyme analysis, since optimization of conditions is rarely necessary. In addition, it requires only a single oligonucleotide, and no electrophoretic separation of the allele-specific products. It thus represents an improved and simplified modification of the existing allele-specific primer extension methods (Kuppuswamy et al., Proc Natl Acad Sci USA 88:1143-1147, 1991; Sokolov, Nucl Acids Res 18:3671, 1989; Syvanen et al., Genomics 8:684-692, 1990).

Current issues in neonatal screening for cystic fibrosis and implications of the CF gene discovery

Many questions remain regarding the efficacy, toxicity, and costs of CF neonatal screening. It would be premature, in our opinion, to implement mass population screening of newborns for CF until the benefits and risks have been fully defined, and an adequate and logistically feasible testing system developed and/or highly effective therapy for CF lung disease becomes available. In addition, the ethical issues described herein need to be resolved. This pertains not only to the CF patient but also the heterozygote carrier. These reservations notwithstanding, the discovery of the CF gene should have a favorable impact both directly and indirectly on neonatal screening for the disease. Mutation analysis coupled to IRT testing seems most attractive at this time, at least on a research basis, but primary molecular diagnostic procedures might supervene in the future, particularly if they are financially feasible.