title: Megaloblastosis: from morphos to molecules.
abstract: OBJECTIVE
  Megaloblastosis (i.e., megaloblastic transformation of erythroid precursor cells in the bone marrow) is the cytomorphological hallmark of megaloblastic anemia resulting from vitamin B12 and folate deficiency. It is characterized by a finely stippled lacy pattern of nuclear chromatin, which is believed to be an expression of deranged cellular DNA synthesis. However, the molecular basis of these cytomorphological aberrations still remains obscure. The current presentation describes the results of our studies on some molecular events associated with the development of megaloblastosis.
  
  
  METHODS
  Transmission electron microscopy was used to study megaloblasts as well as DNA fibers extracted from megaloblastic and normoblastic bone marrows with and without treatment with proteinase K during the extraction procedure; cellular DNA synthesis in bone marrow cultures was studied by incorporation of 3H-thymidine and deoxyuridine suppression test, while histone biosynthesis in bone marrow cells was studied by in vitro incorporation of 3H-tryptophan, 3H-lysine and 3H-arginine into histones.
  
  
  RESULTS
  Derangement of DNA synthesis occurred due to an impaired de novo pathway of thymidylate synthesis in both vitamin-B12- and folate-deficient human megaloblastic bone marrows as well as in the bone marrows of rhesus monkeys and rats with experimentally induced folate deficiency. Interestingly, folate-deficient monkeys developed frank megaloblastic bone marrows, but folate-deficient rats did not. On the other hand, megaloblastic changes in the bone marrow of human patients with myelodysplastic syndrome and erythroleukemia were not associated with this DNA synthetic abnormality. Biosynthesis of predominantly arginine-rich histones in megaloblastic bone marrows was markedly reduced as compared to normoblastic bone marrows, which was consistently associated with elongation and despiralization of chromosomes and finely stippled nuclear chromatin in megaloblasts.
  
  
  CONCLUSIONS
  The impaired biosynthesis of predominantly arginine-rich nuclear histones appeared to be a common molecular event (a denominator) underlying the development of megaloblastosis with or without abnormal DNA synthesis.
authors.affiliation: Hematology Unit, Department of Pathology, Faculty of Medicine, Kuwait University, Kuwait. drkcdas@hotmail.com
authors.forename: K C
authors.initials: KC
authors.lastname: Das
authors.forename: Monisha
authors.initials: M
authors.lastname: Das
authors.forename: D
authors.initials: D
authors.lastname: Mohanty
authors.forename: M M
authors.initials: MM
authors.lastname: Jadaon
authors.forename: A
authors.initials: A
authors.lastname: Gupta
authors.forename: R
authors.initials: R
authors.lastname: Marouf
authors.forename: S K
authors.initials: SK
authors.lastname: Easow
mesh_terms: D000293:Adolescent; D000328:Adult; D000749:Anemia, Megaloblastic / Q000097:blood / Q000209:etiology / Q000235:genetics* / Q000473:pathology*; D000818:Animals; D016022:Case-Control Studies; D002648:Child; D020732:Cytogenetic Analysis; D004247:DNA / Q000096:biosynthesis; D005260:Female; D005494:Folic Acid Deficiency / Q000503:physiopathology; D005819:Genetic Markers; D042421:Histone Code; D006657:Histones / Q000097:blood / Q000235:genetics*; D006801:Humans; D008253:Macaca mulatta; D008297:Male; D008534:Megaloblasts / Q000473:pathology*; D046529:Microscopy, Electron, Transmission; D051381:Rats; D017208:Rats, Wistar; D013489:Suppression, Genetic; D014806:Vitamin B 12 Deficiency / Q000503:physiopathology
chemical_list: D005819:Genetic Markers; D006657:Histones; D004247:DNA