CLINICAL AND GENETIC-HETEROGENEITY IN BLACK PATIENTS WITH HOMOZYGOUS BETA-THALASSEMIA FROM THE SOUTHEASTERN UNITED-STATES

  • 1 September 1988
    • journal article
    • research article
    • Vol. 72  (3) , 1007-1014
Abstract
The presence of various substitutions and deletions resulting in .beta.-thalassemia was studied in 19 black patients with homozygous .beta.-thalassemia and in numerous relatives; all patients were from Georgia (USA), South Carolina, and Alabama. Methodology included gene mapping, amplification of genomic DNA with Taq polymerase, identification of known nucleotide substitutions or a single nucleotide deletion through hybridization with synthetic oligonucleotides, cloning and sequencing of a .beta.-globin gene, and sequencing of amplified genomic DNA. Of the 38 chromosomes tested, 21 (55%) had the A .fwdarw. G substitution at nt -29, eight (21%) had the C .fwdarw. T substitution at nt -88, three (8%) had the substitution at codon 24, while one each of the following abnormalities were also detected: frameshift at codon 6, a C .fwdarw. A mutation at nt 848 of the .beta.IVS-II (new), an A .fwdarw. T mutation at codon 61 (new), a deletion of 1.35 kilobases including the 5, end of .beta., a G.gamma.(A.gamma..delta..beta.).degree.-thalassemia, and one thalassemia determinant that remained unidentified. The C .fwdarw. A mutration at nt 848 of IVS-II occurred at a position 3 nucleotides 5'' to the third exon, adjacent to the invariant AG dinucleotide of the acceptor sequence. The A .fwdarw. T mutation in codon 61 (AAG .fwdarw. TAG) resulted in the creation of a stop codon and thus in .beta..degree.-thalassemia. The various mutations occurred on chromosomes with different haplotypes: however, chromosomes with a specific mutation but with different haplotypes belonged to one specific framework, which suggested that crossovers were responsible for these different types. Hemoglobin (Hb) F levels were generally high (55% to 75% with 98.5% in one patients with .beta..degree./.beta..degree.); a few patients with specific haplotypes and an .alpha.-thalassemia-2 heterozygosity had a lower Hb F level. The G.gamma. in the Hb F was consistently high when the C .fwdarw. T mutation occurred at nt -158 to the Cap site of the G.gamma.-globin gene; seven patients with +/+ at this site had an average G.gamma. of 73.8%, eight patients with +/- had 64.8%, and one patient with -/- had 34.2%. Variations in hematologic values and in Hb F, G.gamma. and Hb A2 levels of relatives with a .beta.-thalassemia heterozygosity depended to some extent on the types of mutations or deletions and on the haplotypes of the chromosomes with the .beta.-thalassemia determinant.