Characterization of a new alpha zero thalassaemia defect in the South African population

A new alpha thalassemia defect has been detected in the South African population. Restriction mapping of the alpha globin gene cluster in affected individuals has established that the defect is associated with the removal of 22.8-23.7 kb of DNA, including the psi zeta 1, psi alpha 1, psi alpha 2, alpha 2 and alpha 1 globin genes. The 5' endpoint of the deletion has been localized between the zeta 2 and psi zeta 1 globin genes, and the 3' endpoint lies 4-5 kb 3' to the alpha 1 globin gene. We have called the deletion - -SA in order to distinguish it from alpha zero thalassaemia defects described in other populations.

The gene coding for tropoelastin is represented as a single copy sequence in the haploid sheep genome

The identity of the primary in vitro translation products of fetal sheep nuchal ligament elastin mRNA was confirmed as two distinct polypeptides of 63 Kdal and 65 Kdal in both rabbit reticulocyte and wheat germ extract cell-free translation systems. Both polypeptides were co-translationally processed by a microsomal membrane signal peptidase, with the removal of 20-25 amino acid residues. A single (3,5 kb) RNA species encodes both tropoelastin polypeptides. Restriction endonuclease mapping of sheep genomic DNA by hydridization with two radiolabelled genomic DNA fragments containing sequences coding for sheep tropoelastin (pSE1-1,3 and pSE1-0.7,) indicated the presence of a single elastin gene. The elastin gene copy number was further quantitated by comparison of hybridisation of pSE1-1.3 and pSE1-0.7 to slot-blots and Southern transfers of sheep genomic DNA and to standard curves constructed with each clone. These results clearly demonstrate that each of these sequences is represented only once per haploid genome, suggesting that the two tropoelastin polypeptides are products of a single elastin gene.

Exclusion of linkage of loci on chromosome 19 with multiple endocrine neoplasia, type 2

Linkage between seven loci on chromosome 19 and multiple endocrine neoplasia type 2a (MEN2A) was examined in a single large Swedish pedigree. A total of 50 cM was excluded from the male genetic map by pairwise analysis and an estimated 63 cM by multipoint analysis. Using existing data on the likelihood of different marker-marker distances and taking into account current exclusions on other chromosomes, the probability that the gene for MEN2A segregating in this pedigree could still be located on chromosome 19 is approximately 0.28%.

Mutations linked to the pro alpha 2(I) collagen gene are responsible for several cases of osteogenesis imperfecta type I

We have analysed six South African families with osteogenesis imperfecta type I using three DNA polymorphisms associated with the pro alpha 2(I) collagen gene. In four of these families linkage of the pro alpha 2(I) gene and the osteogenesis imperfecta phenotype was suggested, whereas in the remaining two families there was a lack of linkage. No distinct correlation could be made between the phenotypic features of the families studied and linkage or lack of linkage to the pro alpha 2(I) gene. Two different haplotypes were found to be associated with the mutant pro alpha 2(I) alleles. These findings suggest that molecular heterogeneity exists within osteogenesis imperfecta type I and that in a significant proportion of cases the defect is linked to the pro alpha 2(I) gene.

Detection of a high frequency RsaI polymorphism in the human pro alpha 2(I) collagen gene which is linked to an autosomal dominant form of osteogenesis imperfecta

Screening of the pro alpha 2(I) collagen genes of Southern African populations for restriction fragment length polymorphisms (RFLPs) has revealed a locus polymorphic for the restriction enzyme RsaI. The frequency of the RFLP was 0.38 in Afrikaners, but much lower in indigenous Southern African populations, which suggests that it is of European origin. The polymorphism was used to study 19 affected and non-affected individuals in a four generation family with the autosomal dominant disorder, osteogenesis imperfecta (OI) type I. Co-inheritance of the loss of the RsaI site and the OI phenotype was observed with a lod score of 3.91 at a recombination fraction (theta) of zero, indicating strong linkage. This suggests that the defect in this family is caused by a structural mutation within or close to the pro alpha 2(I) collagen gene. The use of this high frequency RFLP together with other recently described polymorphisms at this locus will facilitate the analysis of the role of this gene in OI and other inherited disorders of connective tissue.

Polymorphism of DNA sequence in the human pro alpha 2(I) collagen gene

The human pro alpha 2(I) collagen gene was analysed for the presence of restriction fragment length polymorphisms. DNA from randomly selected unrelated persons of three Southern African populations was cleaved with one of eight different restriction enzymes, electrophoresed, blotted, and hybridised with cDNA and genomic probes specific for the pro alpha 2(I) gene. An MspI polymorphism was detected which results from the loss of a cleavage site within the 3' half of the gene. In two of the populations studied, the polymorphism occurred at significant frequencies, and should therefore prove useful as a genetic marker for the study of inherited disorders of connective tissue involving collagen structure or biosynthesis.

Two new polymorphic markers in the human pro alpha 2(1) collagen gene

Structural defects in the human type 1 collagen genes are known to be the cause of several inherited disorders of connective tissue, such as osteogenesis imperfecta. The analysis and prenatal diagnosis of these disorders would be facilitated by establishing a set of polymorphic markers at these gene loci. We have previously reported the presence of an Msp 1 restriction fragment length polymorphism in the pro alpha 2 (1) collagen genes of several Southern African populations (Grobler-Rabie et al., in press). This report describes the detection of a Bgl II and an EcoRI polymorphism in the pro alpha 2 gene of South African Blacks.

The isolation of high molecular weight eukaryotic DNA

The isolation of high molecular weight eukaryotic DNA in good yield is an important prerequisite for the analysis of specific sequences by Southern blotting ( Chapter 9 ), or for molecular cloning in phage or cosmid vectors ( Chapter 49 ).

Incidence of Hb Barts and alpha-thalassaemia genotypes in a South African population

The cord blood of 1,207 randomly selected neonates from the Cape Coloured population of South Africa was analysed for the presence of Hb Barts. 40 individuals (3.3%) had detectable Hb Barts levels with values ranging from 1.1 to 7.3%. Restriction enzyme analysis of DNA from subjects with Hb Barts showed that 85% of the cases studied had the genotypes -alpha 3.7/alpha alpha or -alpha 3.7/-alpha 3.7. The observed frequency of the -alpha/alpha alpha genotype was much lower than the expected frequency which suggests that this genotype is often not associated with detectable levels of Hb Barts. Mapping of the sigma-globin locus in three subjects with HbH disease revealed the presence of the--SEA/alpha-thalassaemia determinant in this population.

Haemoglobin E variants: a clinical, haematological and biosynthetic study of 4 South African families

The clinical, haematological and biosynthetic features of subjects with Hb E variants are described. An association with red cell hypochromia and microcytosis was confirmed, although this was not invariable in Hb E trait. Imbalanced globin chain synthesis was found in the majority of Hb E carriers. A patient doubly heterozygous for Hb E and Hb S, a condition we have not previously seen reported, had a benign clinical course with minor haematological changes, despite a relatively large amount of Hb S (67%).

Restriction endonuclease mapping of globin genes in beta-thalassaemia

We have used the restriction endonuclease mapping technique to analyse the globin genes in local cases of beta-thalassaemia major. No gross deletions or rearrangements of the beta-globin gene locus were detected. In one individual the point mutation within the beta-globin gene which gives rise to a beta(0)-thalassaemia allele was detected by use of a restriction enzyme which cuts the gene at this site. Analysis of restriction fragment length polymorphisms at the beta-globin locus showed that antenatal diagnosis of the disorder could be accomplished in an indigenous family with thalassaemia. The application of the gene-mapping technique to the diagnosis of sickle cell anaemia and alpha-thalassaemia is also discussed.

The molecular basis of alpha thalassaemia in a South African population

We have investigated the molecular basis of alpha thalassaemia in the so-called 'Cape Coloured' population of Cape Town. DNA from 17 cases was analysed by Southern blotting and hybridization with an alpha globin complementary DNA probe. Three types of alpha thalassaemia genetic determinants were detected: the 3.5 kb deletion which inactivates one alpha globin gene per chromosome (-alpha/haplotype), a much larger deletion which removes both alpha globin genes (- -/haplotype), and a non-deletion determinant which leaves both alpha globin genes intact. The interaction of these determinants with each other or with the normal chromosome (alpha alpha/) produced the phenotypes alpha thalassaemia silent carrier, alpha thalassaemia trait and Hb H disease. All cases of the -alpha/haplotype result from the rightward deletion which removes the Bgl II site between the duplicated alpha globin genes. The predominance of the -alpha/haplotype (21 out of the 28 alpha thalassaemia determinants) over the - -/haplotype is consistent with the low incidence of Hb H disease and the apparent absence of Hb Bart's hydrops fetalis in this population group.

The high mobility group proteins and transcribed nucleosomes

Monomer nucleosomes released from nuclei during brief micrococcal nuclease digestions are enriched in transcribed sequences (Bloom and Anderson, 1978). These nucleosomes are depleted in H1 and enriched in three high mobility group proteins HMG14, HMG17 and another HMG-like protein. Analysis of such nucleosomes by polyacrylamide gel electrophoresis reveal that they are heterogenous. Similarly, monomer nucleosomes soluble in 0.1 M NaCl separate on polyacrylamide gels into mainly two types of particle, one of which has HMG14 and HMG17 bound. However, the DNA of the HMG-nucleosomes from chick erythrocytes is not enriched in globin sequences, suggesting that protein rearrangement may have occurred.

Analysis of the high mobility group proteins associated with salt-soluble nucleosomes

Two methods have recently been described for the isolation of monomer nucleosomes enriched in transcribed sequences which depend on their solubility in 0.1 M NaCl (Levy, W.B. and Dixon (1978), Nucleic Acid Res., 5, 4155-4163) or solutions containing divalent metal ions (Bloom, K.S. and Anderson, J.N. (1978), Cell, 15, 141-150). Using these procedures the proteins associated with such nucleosomes from rabbit thymus, calf liver and hen oviduct nuclei were isolated and analysed. Increased amounts of proteins HMG14 AND HMG17 and small amounts of HMG1 and HMG2 were found associated with the four core histones H2A, H2B, H3 and H4 in these nucleosomes. HMG14 and HMG17 were found to be enriched 2 - 7 fold, suggesting an involvement of these two proteins with transcribed sequences. 0.1 M NaCl-soluble monomer nucleosomes prepared by the method of Levy and Dixon were analysed by polyacrylamide gel electrophoresis and found to be composed of principally two types of particle: 1. Core particles of 145 base pairs of DNA associated with the four core histones only. 2. Nucleosomes with 160 base pairs of DNA associated with the four core histones, increased amounts of HMG14 and 17, and no H1. Small amounts of HMG1 and HMG2 are also detected. These results suggest that HMG14 and HMG17 might be interacting with the 15 base pair linker DNA. A model is presented for the structure of transcriptionally active chromatin.

Studies on the association of the high mobility group non-histone chromatin proteins with isolated nucleosomes

Nucleosomes have been isolated from rabbit thymus by sucrose gradient centrifugation, and their high mobility group (HMG) protein content analysed by electrophoresis on polyacrylamide gels. The results suggest that proteins HMG 14 and HMG 17 are associated with the core particle of the nucleosome, and that there are two or more sub-populations of both HMG 1 and HMG 2 molecules. One sub-population appears to be fairly tightly bound to the nucleosome, while another is rapidly released from the chromatin by digestion with micrococcal nuclease. The latter fraction may participate in a higher order folding of the nucleosomes.