Characterization of Three Types of β⁰-Thaussemia Resulting from a Partial Deletion of the β-Globin Gene

Abstract

Three patients heterozygous for a partial deletion of the β-globin gene were studied: an American Black with an ≈1.35 kb deletion, a Turkish patient with an ≈300 nucleotide deletion, and a Greek patient with a newly discovered deletion of 44 nucleotides. The DNA was amplified by the polymerase chain reaction procedure and sequenced; only the DNA with the deletion was amplified for the patients with the ≈1.35 kb and ≈300 bp deletion, facilitating the interpretation of the sequencing gels. The amplified DNA fragments from these two chromosomes were also cloned into a plasmid vector and sequenced. The size of the deletion found in the Turkish patient is 290 nucleotides and includes 123, 124 or 125 nucleotides 5′ to the Cap site, the 5′ untranslated region, exon 1, and 25, 24, or 23 nucleotides of the first intron. The total size of the deletion of the Black patient is 1393 nucleotides including 485 (484) bp 5′ to the Cap site, exon 1, intron 1, exon 2, and 413 (414) nucleotides of the second intron. The new deletion in the Greek β-thalassemic patient was detected by direct sequencing of amplified DNA; the 44 bp deletion begins within codon 24 or between codons 24 and 25, and includes the first 26 or 27 nucleotides of intron 1. This deletion was confirmed by hybridization of amplified DNA with a specific oligonucleotide probe and by sequence analysis of amplified DNA cloned in a plasmid. A 7 bp homology sequence (GACAGGT) was found at both sides of the 290 bp deletion, while only 3 nucleotides were repeated at both sides of the 44 nucleotide deletion (66T). No homology was found between the breakpoints of the 1393 nucleotide deletion.