DNA diagnosis for the detection of sickle hemoglobinopathies

Multiplex allele-specific PCR assay for differential diagnosis of Hb S, Hb D-Punjab and Hb Tak

BACKGROUND

Apart from hemoglobin (Hb) E, Hb D-Punjab [beta121(GH4)Glu-Gln] and Hb Tak [beta147Term-Thr] are the two most common beta-chain variants among the Asian population. These two Hb variants have similar alkaline electrophoretic mobilities and HPLC profiles as those of the Hb S [beta6(A3)Glu-Val]. Differential diagnosis of these clinically relevant hemoglobinopathies is therefore problematic. Direct detection of the beta-globin gene mutations would be another diagnostic alternative.

METHODS

A simultaneous DNA diagnosis of the three Hb variants was developed based on the multiplex allele-specific polymerase chain reaction (PCR) approach. The method was validated on 10 carriers of Hb D-Punjab, 5 carriers of Hb Tak, 2 carriers of Hb S and 50 normal individuals of Thai origin.

RESULTS

The three abnormal Hbs could be correctly diagnosed with the simultaneous PCR approach, and a complete concordance with results using other established methods was obtained.

CONCLUSIONS

The multiplex allele-specific PCR approach developed should prove useful in complementing routine Hb analysis for differential diagnosis of these three common Hb variants and should facilitate a program of hemoglobinopathy screening in the region.

Multivariant confirmation of sickle cell disease using a non-radioactive minisequencing reaction

A non-radioactive solid-phase minisequencing method for confirmation of abnormal hemoglobin variants causing sickle cell disease has been developed. In this method amplified 5'-biotinylated target sequences containing normal and mutation sites are immobilized onto streptavidin-coated microplates. Detection primers corresponding to target sequences are annealed immediately adjacent to the mutation site and single-step, hapten-labeled nucleotide primer extension reactions are performed. The incorporation of the labeled nucleotide is detected through immunological reaction with an enzyme-labeled anti-hapten conjugate and a substrate. The method enables confirmation of mutations of the beta-globin gene variants (Hbs S, C, E D-Punjab, O-Arab) and the alpha-globin gene variant (Hb G-Philadelphia). The test was evaluated using characterized dried blood spot specimens (n = 100) The advantages of the procedure are easy performance and objectiveness. The non-radioactive minisequencing assay will prove helpful for genotyping in neonatal screening for hemoglobinopathies and in prenatal and pre-implantational diagnostics.

Pitfalls in the interpretation of molecular diagnostic tests

Application of the tools of molecular genetics has vastly increased the power of genetic diagnosis, but also raises significant challenges in interpretation. Molecular tests can be used for diagnosis of an affected individual or determination of carrier status. Either direct mutation analysis or linkage analysis is used in various disorders. Major pitfalls in interpretation of direct tests include mistaking polymorphisms for mutations, inaccurate prognostication based on identification of a mutation, and misinterpretation of the implications of not finding a mutation. Interpretation of linkage tests must take account of the accuracy of proband diagnosis, interpretation of family relationships, and genetic recombination. Cognizance of these issues is vital to avoidance of clinical errors and must be incorporated into genetic counseling.

Diagnosis of sickle-cell disease with a universal heteroduplex generator

A new diagnostic technique, universal heteroduplex generator screening, has been developed to detect homozygosity and heterozygosity for sickle-cell disease. Since it is a polymerase chain reaction-based technique, it may be used to detect haemoglobin S and haemoglobin C genotypes in adults, neonates, or from coelocentesis during the first 10 weeks of pregnancy. beta-globin gene nucleotide sequences are amplified by the polymerase chain reaction, and are heteroduplexed with a beta-globulin universal heteroduplex generator. Haemoglobins S and C mutations are identified by characteristic polyacrylamide minigel banding patterns. The technique is simple and rapid to do, even by nonspecialist laboratories, and is applicable to large-scale screening for haemoglobin S and C mutations as well as prenatal diagnosis of sickle cell disease.