Medical genetics in Singapore.

This paper is a brief review of the scope of research and clinical work in human genetics in Singapore. Clinical genetics and karyotyping were established in the early sixties. G6PD deficiency was discovered then as the commonest cause of kernicterus in the newborn. Screening of all newborns was instituted. The measures taken have been very successful and kernicterus is virtually unknown since the early 1970s. Numerous G6PD variants have been discovered and characterized. During the 1980s the emphasis shifted to molecular genetics. Work on the molecular genetics of alpha- and beta-thalassemias, Duchenne muscular dystrophy, hemophilia and retinoblastoma have been established, and good progress on diseases such as neurofibromatosis, leukemias, and lymphoid malignancies. The diagnosis of tuberculosis by DNA amplification (PCR) has been successfully implemented. Numerous papers have been published on the molecular genetics of coronary artery disease, as well as in population genetics.

Molecular analysis of Hb Q-H disease and Hb Q-Hb E in a Singaporean family.

Hb Q (alpha 74Asp-His) results from a mutation in the alpha-gene such that abnormal alpha Q-chains are synthesized. The alpha Q-chains combine with the normal Beta A-chains to form abnormal Hb alpha 2Q beta 2A (Hb Q). Hb Q-H disease is rare, and has been reported only in the Chinese. We report here a Chinese family, were the mother diagnosed with Hb Q-H disease and the father with Hb E heterozygosity and a child with Hb Q-E-thalassemia. Thalassemia screening of the mother's blood revealed a Hb level of 6.8g/dl with low MCV and MCH. Her blood film was indicative of thalassemia. Cellulose acetate electrophoresis showed Hb H and Hb Q with the absence of Hb A. Globin chain biosynthesis was carried out and alpha Q- and beta-chains were detected. Normal alpha- chains were absent. Digestion of the mother's DNA with Bam HI and Bgl II followed by hybridization with the 1.5 kb alpha-Pst probe showed a two alpha-gene deletion on one chromosome and the -alpha Q chain mutant with the -alpha 4.2 defect on the other chromosome. DNA amplification studies indicated the two-gene deletion to be of the -SEA/ defect. The patient was concluded to possess Hb Q-H disease (--SEA/-alpha 4.2Q). Cellulose acetate electrophoresis of the father's blood showed the presence of Hb A, F and E. Molecular analysis of the father's DNA confirmed an intact set of alpha-genes (alpha alpha/alpha alpha). Globin chain biosynthesis of fetal blood of their child showed gamma, beta A, beta E, alpha A and alpha Q-chains. Molecular analysis of the child's DNA showed one alpha-gene deletion, thus giving a genotype of alpha alpha/-alpha 4.2Q beta beta E.

Usefulness of dinucleotide polymorphism markers in genetic analysis of Duchenne's muscular dystrophy cases in Singapore.

Dinucleotide polymorphisms are short tandem repeat sequences that can be used as probes for haplotype analysis in Duchenne's muscular dystrophy (DMD). There are approximately a total of 50,000 to 100,000 such loci in the human genome, and they are highly informative due to the variability of allele lengths at these loci. Primers can be designed to amplify across such repeats located in the dystrophin gene to provide diagnostic information when RFLP analysis is uninformative. We report the usefulness of three such loci for analysis of DMD families in Singapore. The STR50 marker consists of (CA)n repeats located in intron 50 of the dystrophin gene while DYS1 marker is located upstream to the transcriptional start site for the brain dystrophin promoter and BSTRH marker is identified in the 3' untranslated region of the gene. End-labeled PCR products were resolved on 6% denaturing polyacrylamide sequencing gel. Alleles were identified by comparison with sequencing markers. PCR product typically ranged between 174 bp to 255 bp with five to six alleles observed. The heterozygosity rates estimated from 50 X chromosomes of unrelated individuals were 76.0% (BSTRH), 86.6% (DYS1) and 93.3% (STR50). In 38 DMD families studied, the results obtained show that these markers were highly informative and reveal Mendelian mode of inheritance. They were useful for linkage analysis, identification of deletion mutations, confirmation of paternity and mapping of gene recombination.