Two novel C-terminal frameshift mutations in the β-globin gene lead to rapid mRNA decay

Abstract

BackgroundThe thalassemia syndromes are classified according to the globin chain or chains whose production is affected. β-thalassemias are caused by point mutations or, more rarely, deletions or insertions of a few nucleotides in the β-globin gene or its immediate flanking sequences. These mutations interfere with the gene function either at the transcriptional, translational or posttranslational level.MethodsTwo cases of Polish patients with hereditary hemolytic anemia suspected of thalassemia were studied. DNA sequencing and mRNA quantification were performed. Stable human cell lines which express wild-type HBB and mutated versions were used to verify that detected mutation are responsible for mRNA degradation.ResultsWe identified two different frameshift mutations positioned in the third exon of HBB. Both patients harboring these mutations present the clinical phenotype of thalassemia intermedia and showed dominant pattern of inheritance. In both cases the mutations do not generate premature stop codon. Instead, slightly longer protein with unnatural C-terminus could be produced. Interestingly, although detected mutations are not expected to induce NMD, the mutant version of mRNA is not detectable. Restoring of the open reading frame brought back the RNA to that of the wild-type level.ConclusionOur results show that a lack of natural stop codon due to the frameshift in exon 3 of β-globin gene causes rapid degradation of its mRNA and indicate existence of novel surveillance pathway.Electronic supplementary materialThe online version of this article (doi:10.1186/s12881-017-0428-1) contains supplementary material, which is available to authorized users.

Four Novel PAX9 Variants and the PAX9-Related Non-Syndromic Tooth Agenesis Patterns

“…In the last few years, it has become evident that mRNA stability/turnover provides an important mechanism for post-transcriptional control of gene expression [ 25 ]. Many studies have reported that frameshift variants can lead to disease by affecting the stability of mRNA [ 26 , 27 , 28 ]. Therefore, the p.S119Pfs*2 variant may lead to tooth agenesis by affecting the stability of mRNA, but the specific mechanism needs more experiments to verify.…”

Dominant β-thalassaemia with unusually high Hb A2and Hb F caused by βCD121(−G)(HBB:c.364delG) in exon 3 of β-globin gene

“…It is noteworthy that the majority of them are located between codons 109 and 141, and no corresponding Hb variants could be detected in peripheral blood of the patients. In fact, the dominant form of β-thalassaemia intermedia could be related to messenger RNA (mRNA) stability mechanisms 8. If the premature termination is located so early, for example, within exon 2, the corresponding transcript is targeted for nonsense-mediated mRNA decay (NMD), no protein is synthesised and heterozygotes are asymptomatic.…”

Three Mexican Families with β thalassemia intermedia with different molecular basis

“…This deletion causes the loss of the third exon, therefore may produce a short unstable mRNA that could undergo early degradation. This early degradation mechanism is typical for mutations inducing a premature termination codon in coding sequences (Grosso et al, 2008;Rawa et al, 2017). It is important to notice the unusual increased HbF in the mother, who is a simple heterozygote for this deletion.…”

“…Although single nucleotide mutations represent the vast majority of b-thalassemias, deletions involving HBB gene, or its regulating sequences have also been associated to this pathology (Patrinos GP et al, 2004). Despite being an autosomal recessive inherited condition, some forms of b-thal are inherited dominantly (b D alleles), so is the case of nonsense or frameshift mutations in exon three that result in the formation of a premature termination codon and therefore the production of unstable b-globin chains that precipitate along with the free a-globin chains in the erythroid precursor causing premature death and ineffective erythropoiesis (Thein, 2018;Rawa et al, 2017). b-thal is classified according to severity as minor, major and intermedia (Thein and Wood et al, 2009;Thein, 2018).…”