Thalassemia Deletion Research Articles

Recombinational resolution in primate cells of two homologous human DNA segments with a gradient of sequence divergence.

Human alpha-thalassemia-2 genotype -alpha 4.2 is the result of meiotic recombination between two 1.3 kb long, homologous DNA segments, X(alpha 2) and X(alpha 1), located in the adult alpha globin locus. The two segments can also undergo intramolecular recombination on extrachromosomal vectors transfected into mitotically dividing primate cells (COS 7). The existence of a gradient of sequence divergence between X(alpha 2) and X(alpha 1) makes them an interesting system to study the relationship between efficiencies of homologous DNA recombination and the extent of dispersed and localized base mismatches. By partial restriction mapping and DNA sequencing of plasmids recombined in COS 7 cells and rescued from bacteria HB 101, we have determined the distribution of recombinational resolution sites along the two X blocks. Most, if not all, of the homologous recombination events between the two X blocks appear to be single crossing-over without efficient gene correction or repair of base mismatches. The distribution of the sites of recombinational resolution is inversely correlated with that of the gradient of sequence divergence, with only approximately 7% of the X recombinants resolved within the 3' third of the X blocks where two diverged Alu family repeats reside. The Alu sequence within which one of the X recombinants resolved is homologous to a previously characterized alpha thalassemia deletion point.

The 12.6 kilobase DNA deletion in Dutch β°‐thalassaemia

SummaryThe Dutch β°‐thalassaemia has few clinical symptoms in homozygotes, elevated fetal haemoglobin (4–11%) in heterozygotes, and has a DNA deletion previously estimated as 10 kb which removes the β‐globin gene (Gilman et al, 1984). A DNA fragment containing the breakpoints of the Dutch β°‐thalassaemia deletion has now been cloned. Sequencing across the deletion junction region showed the 3’endpoint to be about 3 kb further 3’than originally thought, so that the deletion covers 12.6 kb. The 3’endpoint lies in a region of Kpn I (LI) repeated sequences, which is also the case for several other deletions. A six bp region of homology (AAATTT) between the 5’and 3’normal sequences at the breakpoint may have contributed to the non‐homologous recombination event that led to the Dutch β°‐thalassaemia deletion. The 12.6 kb Dutch β°‐thalassaemia deletion is now seen to be a member of a 12–13 kb size category of deletions which also includes two δβ‐thalassaemias.

Different hematologic phenotypes are associated with the leftward (-alpha 4.2) and rightward (-alpha 3.7) alpha+-thalassemia deletions.

We have compared the phenotypes of the two common deletion forms of alpha+-thalassemia by analysis of umbilical cord blood samples from Melanesia. Homozygotes for the leftward, 4.2-kilobase, deletion (-alpha 4.2) had significantly higher levels of Hb Bart's at birth than homozygotes for the rightward, 3.7-kilobase, deletion (-alpha 3.7). Compound heterozygotes for each deletion had intermediate values. Although deletion forms of alpha 0 thalassemia were not found in this survey, nondeletion alpha-thalassemia was present at low frequency. Since the predominant rightward deletion in this population, -alpha 3.7III, entirely removes the alpha 1-gene and the 4.2-kilobase deletion deletes the alpha 2-gene, these data indicate that the alpha 2-globin gene has a higher output than the alpha 1-gene, on single alpha-gene chromosomes.