β‐Thalassemia Due to Intronic LINE‐1 Insertion in the β‐Globin Gene (HBB): Molecular Mechanisms Underlying Reduced Transcript Levels of the β‐GlobinL1 Allele

We describe the molecular etiology of β+-thalassemia that is caused by the insertion of the full-length transposable element LINE-1 (L1) into the intron-2 of the β-globin gene (HBB). The transcript level of the affected β-globin gene was severely reduced. The remaining transcripts consisted of full-length, correctly processed β-globin mRNA and a minute amount of three aberrantly spliced transcripts with a decreased half-life due to activation of the nonsense-mediated decay pathway. The lower steady-state amount of mRNA produced by the β-globinL1 allele also resulted from a reduced rate of transcription and decreased production of full-length β-globin primary transcripts. The promoter and enhancer sequences of the β-globinL1 allele were hypermethylated; however, treatment with a demethylating agent did not restore the impaired transcription. A histone deacetylase inhibitor partially reactivated the β-globinL1 transcription despite permanent β-globinL1 promoter CpG methylation. This result indicates that the decreased rate of transcription from the β-globinL1 allele is associated with an altered chromatin structure. Therefore, the molecular defect caused by intronic L1 insertion in the β-globin gene represents a novel etiology of β-thalassemia.