An initiation codon mutation (AUG----GUG) of the human alpha 1-globin gene. Structural characterization and evidence for a mild thalassemic phenotype.

Abstract

a-globin is encoded by two adjacent genes, al and a2. Recent evidence suggests that these genes are not equally expressed and that the a2-globin gene encodes the majority of a-globin. This finding would predict that a thalassemic mutation of the a2-globin gene would result in a more severe loss of a-chain synthesis than a similar mutation in the al-globin gene. In a previous study we described a nondeletion a-thalassemia defect in the a2-globin gene resulting from an AUG --ACG initiation codon mutation. In the present study we describe a different initiation codon mutation, AUG -GUG, present in the al-globin gene. The al-and a2-globin gene initiation codon mutations result in similarly lowered levels of encoded mRNA. Despite the similarity of these two mutations, the a2 mutant results in a more severe loss of a-globin synthesis and a more severe clinical a-thalassemia phenotype than the corresponding al-globin gene mutation. This difference reflects the dominant role of a2-globin gene in overall a-globin synthesis.

Homozygous 4.1(-) hereditary elliptocytosis associated with a point mutation in the downstream initiation codon of protein 4.1 gene.

“…It looks as if the spliceoforms (-135 kD), arising from the use of the upstream initiation codon, compensate for spliceoforms (-80 kD), arising from the use of the downstream initiation codon (40,41 The present mutation correlates with a decrease of approximatively 60% in the level ofmRNA. Similarly, initiation codon mutations in al-and a2-globin genes have been found associated with a three-to fourfold decrease in the level of corresponding mRNA (44,45). Nonsense mutations in j3-globin gene affect mRNA metabolism to a comparable extent (46).…”

Inactivation of human alpha-globin gene expression by a de novo deletion located upstream of the alpha-globin gene cluster.

“…Total reticulocyte RNA was extracted from acid-precipitated polysomes (29). In vitro translations were carried out in the presence of [35S]methionine (1400 Ci/mmol; 1 Ci = 37 GBq; Amersham), analyzed on a Triton X-100/ acid/urea slab gel, and quantitated by soft-laser densitometry as described (30,31). As the absolute a-globin to 8-globin synthetic ratio in control samples can demonstrate interassay variation, all comparisons were made intra-assay, and each study was repeated at least three times.…”

“…As the absolute a-globin to 8-globin synthetic ratio in control samples can demonstrate interassay variation, all comparisons were made intra-assay, and each study was repeated at least three times. Analysis of relative a2-and al-globin mRNA levels was carried out by a reverse transcription mapping assay exactly as described (30). (32).…”

An initiator codon mutation in ornithine-delta-aminotransferase causing gyrate atrophy of the choroid and retina.

“…We know of two other examples of initiation codon mutations in man. Pirastu and colleagues (25) found an AUG -ACG transition in the a 2-globin gene to be a common cause of a-thalassemia in Sardinia; and Liebhaber and co-workers have discovered an AUG -GUG transition in the a'-globin gene of a patient with mild a-thalassemia (26).…”