Deficiency of
folate or
vitamin B(12) (
cobalamin) causes
megaloblastic anemia, a disease characterized by
pancytopenia due to the excessive apoptosis of hematopoietic progenitor cells. Clinical and experimental studies of
megaloblastic anemia have demonstrated an impairment of
DNA synthesis and repair in hematopoietic cells that is manifested by an increased percentage of cells in the
DNA synthesis phase (S phase) of the cell cycle, compared with normal hematopoietic cells. Both
folate and
cobalamin are required for normal de novo synthesis of thymidylate and
purines. However, previous studies of impaired
DNA synthesis and repair in
megaloblastic anemia have concerned mainly the decreased intracellular levels of thymidylate and its effects on
nucleotide pools and misincorporation of
uracil into
DNA. An in vitro model of
folate-deficient erythropoiesis was used to study the relationship between the S-phase accumulation and apoptosis in
megaloblastic anemia. The results indicate that
folate-deficient erythroblasts accumulate in and undergo apoptosis in the S phase when compared with control erythroblasts. Both the S-phase accumulation and the apoptosis were induced by
folate deficiency in erythroblasts from p53 null mice. The complete reversal of the S-phase accumulation and apoptosis in
folate-deficient erythroblasts required the exogenous provision of specific
purines or
purine nucleosides as well as
thymidine. These results indicate that decreased de novo synthesis of
purines plays as important a role as decreased de novo synthesis of thymidylate in the pathogenesis of
megaloblastic anemia.