Black beta-thalassemia homozygotes with specific sequence variations in the 5' hypersensitive site-2 of the locus control region have high levels of fetal hemoglobin.

We have sequenced the 5' hypersensitive-2 (5'HS-2) site of the locus control region (LCR) and the promoters of the two gamma-globin genes located on chromosome 11 of a black patient with mild beta-thalassemia (beta-thal) major due to a homozygosity for the C----T mutation at position -88 of the beta promoter and with a high Hb F level. Sequence variations in the 5'HS-2 were the same as observed for the beta s chromosome with haplotype number 3, while most of the G gamma promoter and the A gamma promoter had sequences similar to that of the beta S chromosome with haplotype number 19. This atypical haplotype (number 19A) is apparently associated with an increased gamma chain production which is particularly evident during periods of severe hematopoietic stress. Additional studies on relatives of the proband and on 10 unrelated black beta-thal homozygotes with either the C----T mutation at -88 or the A----G mutation at -29, confirm the possible importance of the sequence differences in the 5'HS-2, and also suggest that at least two additional factors, namely a C----T mutation at position -158 of the G gamma promoter and a relative deficiency in alpha chain synthesis play a (perhaps less important) role in the increased Hb F synthesis in these patients.

Haplotypes in SS patients from Nigeria; characterization of one atypical beta S haplotype no. 19 (Benin) associated with elevated HB F and high G gamma levels.

We have determined the haplotypes of 669 beta S and 109 beta A chromosomes from numerous members of 297 Nigerian families of various ethnic backgrounds. Among the beta S chromosomes, haplotype 19 was detected in 93.2%, haplotype 17 in 3.4%, and haplotype 20 in 0.1%, while 2.4% represented atypical haplotypes. As many as 60.6% of the beta A chromosomes exhibited haplotype 19 mutations, 8.2% had haplotype 3, and 1.8% had haplotype 20. Two siblings with elevated Hb F and G gamma levels were heterozygous for a beta S chromosome with haplotype 19 and a second chromosome with a hybrid haplotype (termed 19 B). In this hybrid chromosome, haplotype 3-like locus control region (LCR) [hypersensitive site-2 (HS-2)] sequences are in juxtaposition to those of the 5' flanking region of the G gamma promoter of a beta S chromosome with haplotype 19. The presence of this hybrid chromosome is associated with high G gamma values in individuals with both sickle cell anemia (SS) and sickle cell trait (AS); it closely resembles another hybrid beta S chromosome, termed 19 A, observed in a previously reported Turkish SS patient who was homozygous for this chromosome and had high Hb F and high G gamma values. In both instances, it is hypothesized that the haplotype 3-like sequences of the LCR HS-2 contain genetic determinants that can combine with factors produced during hematopoietic stress, resulting in increased gamma-globin gene expression.

A new variant, HB Muscat [alpha 2 beta (2)32(B14)Leu----Val] observed in association with HB S in an Arabian family.

The silent Hb Muscat with a Leu----Val replacement at position beta 32 was discovered by reversed phase high performance liquid chromatography in two members of an Arabian family from Oman; in one person Hb Muscat occurred with Hb S and in the other with Hb A. Hb Muscat is slightly unstable but its presence has no apparent adverse effect on the health of its carriers. Additional hemoglobin abnormalities observed in this family were a common alpha-thalassemia-2 (-3.7 kb) and Hb S. The beta S haplotypes in the heterozygous carriers and the two sickle cell anemia patients were #19 (Benin) and #20 (Bantu); the latter likely originated from an East African population.