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

We describe the molecular etiology of β + ‐thalassemia that is caused by the insertion of the full‐length transposable element LINE ‐1 ( L 1) 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 m RNA 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 m RNA produced by the β ‐ globin L 1 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‐globin L 1 allele were hypermethylated; however, treatment with a demethylating agent did not restore the impaired transcription. A histone deacetylase inhibitor partially reactivated the β‐globin L 1 transcription despite permanent β‐globin L 1 promoter C p G methylation. This result indicates that the decreased rate of transcription from the β ‐ globin L 1 allele is associated with an altered chromatin structure. Therefore, the molecular defect caused by intronic L 1 insertion in the β‐globin gene represents a novel etiology of β‐thalassemia.