Alpha-thalassemia due to novel deletions and complex rearrangements in the subtelomeric region of chromosome 16p

Abstract

Introduction: Inherited deletions removing the α-globin genes and/or their upstream regulatory elements (MCSs) give rise to alpha-thalassemia, one of the most common genetic recessive disorders worldwide. The pathology is characterized by microcytic hypochromic anemia due to reduction of the α-globin chain synthesis, which are essential for hemoglobin tetramerization. Material and Methods: In order to clarify the suggestive α-thalassemia phenotype in eleven patients, we performed Multiplex Ligation-dependent Probe Amplification with commercial and synthetic engineered probes, gap-PCR, and Sanger sequencing to search for deletions in the subtelomeric region of chromosome 16p. Results: We have identified five distinct large deletions, two of them novel, and one indel. The deletions range from approximately 3.3 to 323 kb, and i) remove the whole α-globin cluster; or ii) remove exclusively the upstream regulatory elements leaving the α-globin genes structurally intact. The indel consists in the loss of MCS-R2 (HS-40), which is the most important distal regulatory element for the α-globin gene expression, and the insertion of 39 bp, seemingly resulting from a complex rearrangement involving two DNA segments (probably from chromosome 3q) bridging the deletion breakpoints with a CC-bp orphan sequence in between. Finally, in one patient no α-globin deletion or point mutation were found. This patient revealed to be a very unusual case of acquired alpha-thalassemia associated with a myelodysplastic syndrome. Conclusions: Our study widens the spectrum of molecular lesions by which α-thalassemia may occur and emphasizes the importance of diagnosing large α-zero-deletions to provide patients with appropriate genetic counseling.