Acquired haemoglobin H disease, complicating a myeloproliferative syndrome: a case report

Acquired haemoglobin H disease

Acquired haemoglobin H disease has been described in various premalignant haematological conditions and is most commonly associated with myelodysplastic and myeloproliferative syndromes. The condition is not restricted to any specific population group or geography. Affected individuals have no family or past history of alpha thalassaemia and these subjects usually suffer from severe uncompensated haemolysis. Extensive mapping and sequence analysis of the alpha globin gene cluster have demonstrated intact alpha globin genes, leading workers to conclude that an acquired in trans mechanism is responsible for the disorder. ATRX gene mutations on the X chromosome have been shown to be instrumental in the suppression of alpha globin gene expression. Despite recent advances in the understanding of its pathogenesis, the precise mechanism of acquired haemoglobin H disease remains a mystery.

Acquired alpha-thalassemia in association with myelodysplastic syndrome and other hematologic malignancies

Abnormalities of hemoglobin synthesis are usually inherited but may also arise as a secondary manifestation of another disease, most commonly hematologic neoplasia. Acquired hemoglobin disorders can be seen in any population and are not restricted to areas of the world with high incidences of inherited hemoglobinopathies. In fact, the acquired hemoglobinopathies may be more readily recognized where inherited hemoglobin abnormalities are rare and less likely to cause diagnostic confusion. Acquired alpha-thalassemia is the best characterized of the acquired red blood cell disorders in patients with hematologic malignancy, and it is almost always associated with a myelodysplastic syndrome (MDS). At least 2 molecular mechanisms for acquired alpha-thalassemia are now recognized: acquired deletion of the alpha-globin gene cluster limited to the neoplastic clone and, more commonly, inactivating somatic mutations of the trans-acting chromatin-associated factor ATRX, which cause dramatic down-regulation of alpha-globin gene expression. Here we review the clinical, hematologic, and molecular genetic features of alpha-thalassemia arising in a clonal myeloid disorder, and we discuss howATRX might affect gene expression in normal and abnormal hematopoiesis through epigenetic mechanisms.

Acquired alpha-thalassemia in preleukemia is due to decreased expression of all four alpha-globin genes

A somatic mutation(s), acquired during the evolution of preleukemia in a 75-year-old Caucasian male of North European origin, resulted in a marked decrease in alpha-globin mRNA. The small amount of alpha-globin mRNA present in bone marrow cells was normally processed, had a normal (alpha 1/alpha 2)-globin mRNA ratio, and was translated normally. No detectable zeta-globin mRNA was found. The alpha- and zeta-globin genes were both hypomethylated and restriction endonuclease maps of the alpha- and zeta-globin genes were comparable in the patient's marrow and fibroblast DNA. The data are most consistent with the acquisition of a mutation(s) that resulted in decreased expression of all four alpha-globin genes.

Clinical features and molecular analysis of acquired hemoglobin H disease

The presence of hemoglobin H (beta 4), resulting from a deficiency of alpha-globin chain synthesis, was observed as an acquired characteristic in the red cells of five elderly patients with myeloproliferative disorders or preleukemia. The variability in amount of hemoglobin H and in the alpha/beta globin synthesis ratios in these patients is most likely explained by the relative proportions of normal and abnormal cell populations in the peripheral blood, since some reticulocyte fractions with balanced alpha/beta globin synthesis ratios and others with almost no detectable alpha-chain production could be obtained from these patients. In one patient, the hemoglobin H virtually disappeared despite continuing disease. The amount of cytoplasmic alpha-mRNA matched the proportion of alpha-chain synthesis and, in one patient, this was also true for nuclear RNA. However, extensive analysis of the alpha-globin gene complex by restriction endonuclease mapping revealed no detectable rearrangements of the normal gene organization in any of these patients, suggesting that transcription of each pair of alpha-globin genes on each chromosome 16 is defective. These observations have important implications for both the normal regulation of alpha-globin gene expression and the molecular basis of the underlying defect that is associated with the neoplastic transformation of these cells.