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. Conclusion: 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.

Folate deficiency in human peripheral blood lymphocytes induces chromosome 8 aneuploidy but this effect is not modified by riboflavin

“…Folate is required as a methyl donor to convert (a) homocysteine to methionine from which Sadenosylmethione, the common methyl donor, is derived and (b) dUMP to dTMP [Fenech, 2001]). Folate deficiency can cause hypomethylation of DNA at CpG sequences and incorporation of uracil into DNA which lead to altered gene expression, despiralisation of chromosomes and increased chromosomal fragility [Das et al, 2005;Kim, 2004]. These pathological events are thought to be the mechanisms which might cause defective chromosome segregation during mitosis possibly by disabling appropriate kinetochore assembly at centromeres.…”

Modulation of One-Carbon Metabolism by B Vitamins: Implications for Transformation and Progression of Prostate Cancer

“…GNMT modulation by Testosterone, CH3-THFG5 polyglutamate and vitamin A, effects of folates and B12 on polyamines and spermidine and spermine on methionine synthetase. appearance of chromatin within basophilic megaloblasts (Das et al, 2005). Though megaloblastic change is characteristic of blood cells, similar morphology is noted in other cells undergoing cell division during B12 and or folate insufficiency.…”

Fragile sites, chromosomal lesions, tandem repeats, and disease

“…Deficiencies of vitamin B12 and/or folate, due to malnourishment or genetic defects in folate absorption/metabolism, lead to striking chromosomal aberrations in both direct marrow peripheral blood preparations, observations dating back to the 1950s ( Cingam et al, 2017 ; Green and Datta Mitra, 2017 ). Chromosomal lesions include fragile sites (gaps and breaks), centromere spreading, and chromosome elongation/contraction ( Heath, 1966 ; Jensen and Friis-Moller, 1967 ; Das et al, 2005 , 1986 ). Numerical (ploidy) was unaltered.…”