Human α2‐globin nonsense‐mediated mRNA decay induced by a novel α‐thalassaemia frameshift mutation at codon 22

Abstract

SummaryWe describe a novel a-thalassaemia determinant in a 3-year-old girl presenting a mild microcytic and hypochromic anaemia, and normal haemoglobin A 2 level. Molecular studies revealed heterozygosity for a novel microdeletion ()C) at codon 22 of the a 2 -globin gene. As the frameshift mutation generates a premature translation termination codon at position 48/49, we investigated the effect of the nonsense codon on the a 2 -globin gene expression. Although it does not affect RNA splicing, the premature nonsense codon induces accelerated mRNA degradation. To our knowledge, this is the first time the nonsense-mediated mRNA decay has been reported to occur in human a-globin mRNA.

Frameshift Mutation in the α2-Globin Gene Causing α+-Thalassemia: Codon 49 (−GC)

“…Three similar mutations have been reported for the α2-globin gene: codon 19 (−G) (6), codon 22 (−C) (7,8), and codons 39/41 (−ACCAAGACC, + TACTTCCC) (9). These mutations create a premature termination codon (TGA) at position 49, which induces NMD (7). Carriers of these mutations had mild α + -thal trait (7)(8)(9), consistent with the phenotype of the patient described in this report.…”

“…The frameshift introduces a premature termination codon (TAA) at position 55, 130 nts upstream of the last exon-exon junction, which likely would result in rapid degradation of the mutant mRNA transcript via nonsense-mediated mRNA decay (NMD) (5). Three similar mutations have been reported for the α2-globin gene: codon 19 (−G) (6), codon 22 (−C) (7,8), and codons 39/41 (−ACCAAGACC, + TACTTCCC) (9). These mutations create a premature termination codon (TGA) at position 49, which induces NMD (7).…”

The canonical UPF1-dependent nonsense-mediated mRNA decay is inhibited in transcripts carrying a short open reading frame independent of sequence context

“…3B). Data from four independent experiments show that the half-life of normal a-globin mRNA is 11.5 h. In contrast, the half-life of the a40 is 2.0 h, consistent with a full sensitivity to NMD, as previously reported (Pereira et al 2006). The half-life of a16 mRNA (7.7 h) is intermediate to the aWT and a40 mRNAs (Fig.…”

“…3A). In contrast, expression of the a40 mRNA was repressed to 25% of aWT levels, compatible with a full NMD response (Pereira et al 2006). Thus, as was the case with the b-globin mRNA, the a-globin mRNA expression levels corresponded to the AUG-proximity of the nonsense codons.…”

“…The 32Met and 76Met are located at 201 nt and 69 nt upstream of the last exon-exon junction, respectively. An additional out-of-frame AUG codon at position 24/25 was not tested because the potential reinitiation at this site would not extend far enough to displace the last EJC (the 24/25Met is in frame with a stop codon at position 48/49) and NMD would thus be triggered, as previously described (Pereira et al 2006). The 32Met and 76Met AUG-to-ACG conversions created aWT-32-76Met/Thr, a14-32-76Met/Thr and a40-32-76Met/Thr genes.…”