The title of this chapter is quite ambitious, and in this respect we beg the reader's indulgence. As hematologists, we must have the ambition to explain anemia at the molecular level; this has been done successfully in some cases but not yet in others. One major reason is that anemia is not a disease, but a vast collection of diseases, extremely heterogeneous in terms of etiology, pathophysiology and clinico-hematological manifestations, as well as in our ability to treat them effectively. Since this book focusses on molecular pathophysiology, it is particularly pertinent to blood diseases that have a genetic basis. There are three main groups of anemias that qualify in this respect: (1) the hemoglobinopathies, covered in Chapters 1 and 14; (2) red cell membrane cytoskeleton disorders, which are not covered in this book; and (3) inherited hemolytic anemia due to enzyme abnormalities, covered in this chapter. In addition, considering the space allocated to this chapter, we have included several other types of inherited and acquired anemias: our main inclusion criterion was that, based on current knowledge, we could offer at least some meaningful discussion of their molecular basis. In this area there have been at least three breakthrough additions since the first edition of this book: (1) autosomal dominant dyskeratosis congenita has been found to be due to mutations of telomerase; (2) the gene mutated in congenital dyserythropoietic anemia type I has been identified; and (3) the gene encoding red cell 5' nucleo-tidase has been identified, enabling at long last the molecular diagnosis of one of the commonest red cell enzymopathies.

Megaloblastic anemia is defined by a highly characteristic set of morphological changes which affect cells of the erythroid, myeloid and megakaryocytic lineages in the peripheral blood and bone marrow. These changes include macrocytosis, Howell-Jolly bodies, hypersegmented neutrophils, giant metamyelocytes and giant platelets. Despite the multitude of these signs, the one pathognomonic feature which we regard as a sine qua non for the diagnosis of megaloblastic anemia is the peculiarly finely stippled chromatin of erythroid cells, combined with derangement of the normally precisely ordered parallel pattern of maturation of the nucleus and of the cytoplasm. This asynchrony, whereby the maturation of the nucleus lags behind that of the cytoplasm, is the morphological hallmark of what we call megaloblastic erythropoiesis.

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