The HbA/HbA2 ratio in newborns and its correlation with fetal maturity

Effects of iron supplements and perinatal factors on fetal hemoglobin disappearance in LBW infants

BACKGROUND

The homeostatic mechanisms of iron metabolism and erythropoiesis in infants are unclear. Infants synthesize both fetal hemoglobin (HbF) and adult hemoglobin (HbA), and it is not known how the hemoglobin switch is regulated. We hypothesized that iron supplements to infants affect the disappearance of HbF.

METHODS

We randomized 285 low-birth-weight infants (2,000-2,500 g) into three intervention groups receiving 0, 1, or 2 mg/kg/d of iron supplements from 6 wk to 6 mo of age. In the present secondary analysis, we analyzed iron status, total hemoglobin (Hb), and HbF fraction at 6 wk, 12 wk, and at 6 mo and calculated absolute levels of HbF.

RESULTS

We observed dose-dependent increased levels of Hb in iron-supplemented groups at 6 mo of age. However, for absolute HbF concentration, there was no similar effect of intervention. Mean (SD) HbF was 81.2 (16.8), 37.0 (13.8), and 8.1 (5.6) g/l at 6 wk, 12 wk, and 6 mo, respectively, similar in all groups. In linear regression analyses, postconceptional age turned out as the major predictor of HbF, independent of gestational age at birth.

CONCLUSION

Our hypothesis was rejected. Instead, we confirmed a close correlation to postconceptional age, supporting a genetically programmed switch, insensitive to most environmental factors including birth.

The genetics of hemoglobin A2 regulation in sickle cell anemia

Hemoglobin A2 , a tetramer of α- and δ-globin chains, comprises less than 3% of total hemoglobin in normal adults. In northern Europeans, single nucleotide polymorphisms (SNPs) in the HBS1L-MYB locus on chromosome 6q and the HBB cluster on chromosome 11p were associated with HbA2 levels. We examined the genetic basis of HbA2 variability in sickle cell anemia using genome-wide association studies. HbA2 levels were associated with SNPs in the HBS1L-MYB interval and SNPs in BCL11A. These effects are mediated by the association of these loci with γ-globin gene expression and fetal hemoglobin (HbF) levels. The association of polymorphisms downstream of the β-globin gene (HBB) cluster on chromosome 11 with HbA2 was not mediated by HbF. In sickle cell anemia, levels of HbA2 appear to be modulated by trans-acting genes that affect HBG expression and perhaps also elements within the β-globin gene cluster. HbA2 is expressed pancellularly and can inhibit HbS polymerization. It remains to be seen if genetic regulators of HbA2 can be exploited for therapeutic purposes.

A four base pair deletion 5' to the A gamma T gene is associated not only with decreased expression of the A gamma T-globin gene, but also of the G gamma-globin gene in cis

A four base pair deletion 5' to A gamma T-globin gene at positions -222 to -225 has been reported to reduce the expression of this gene. To evaluate the prevalence and effect of this deletion, PCR-based methods were employed. The deletion had a gene frequency of 0.06 in a sample of African-American individuals with sickle cell trait, 0.18 in adult African-Americans with normal Hb AA, and 0.36 in caucasians. Seventy cord blood samples from African-American newborns with Hb AA were evaluated by both HPLC and PCR. The frequency of the A gamma T allele was 0.13. The A gamma T-globin chain was always present in a lower proportion than the A gamma I allele (70% of A gamma I), but the percentage of A gamma-globin was the same whether or not A gamma T was present. The total percentage of Hb F, however, was significantly lower in the group with the A gamma T allele (77.1% vs. 87.4%, P < 0.01). These results indicate that the four base pair deletion is not only associated with reduced expression of the A gamma T allele, but also of the G gamma allele in cis, further suggesting a possible role of this region in the modulation of the expression of the linked gamma-globin genes.

Haemoglobin A2 in newborn infants of different maturity

From 70 infants born after gestation periods of 28-41 weeks HbF and HbA2 were determined in the cord blood; HbA was calculated from the two values. HbF decreased with maturity, HbA and HbA2 increased. The percentage of HbA2 calculated from the sum of HbA + HbA2 (non-fetal Hb) increased from 0.57% to 0.83% with maturity. These values are much lower than those in adult controls (1.83%). The results show that during development the synthesis of the delta-globin chains lags behind that of the beta-globin chains. This is remarkable considering the location of the delta-globin gene within the beta-globin like gene cluster.