Fetal compensation of the hemolytic anemia in mice homozygous for the normoblastosis (nb) mutation.

The mouse autosomal recessive mutation nb causes a deficiency of erythroid ankyrin and generates a life-threatening hemolytic anemia in adult mice; however, at birth, nb/nb mice appear to be robust and show no pallor. In our study, the time of disease onset was sought by comparison of nb/nb and +/? mice both in utero and postnatally. Erythroid ankyrin messenger RNA (mRNA) is expressed in fetal erythroid progenitors from normal mice, but is reduced to 10% of normal levels in mutant fetuses. Despite the deficiency of erythroid ankyrin mRNA, 16 and 18 day nb/nb fetuses have normal levels of red blood cells (RBCs) and the RBCs are morphologically normal by scanning electron microscopy. The earliest signs of any clinical anomaly are an increase in the number of circulating reticulocytes and the deposition of minor amounts of iron just before birth in the 18 day fetal nb/nb liver, suggesting that RBCs are being destroyed. Within 24 hours after birth, nb/nb neonates have a slight but significant decrease of their RBC counts. During the next 5 days, the nb/nb RBC counts decrease markedly, the reticulocyte counts assume the mutant adult levels of 60%, the erythrocytes become microcytic and fragmented, and iron deposits accumulate in the liver. The rapid onset of clinical disease postnatally, coupled with our findings that the erythroid ankyrin gene is transcribed in fetal erythroid cell precursors from normal mice, suggest that mechanisms exist in the nb/nb fetus to compensate for the erythroid ankyrin deficiency.