Analysis of the mechanism of action of non-deletion hereditary persistence of fetal hemoglobin mutants in transgenic mice

Transgenic mice carrying an (A)γ gene construct containing a –382 5′ truncation of the (A)γ gene promoter have a phenotype of hereditary persistence of fetal hemoglobin (HPFH) but, when the CACCC box of the –382(A)γ promoter is deleted, there is no γ gene expression in the adult mice. We used this system to investigate the mechanism whereby human HPFH mutations result in γ gene expression in the adult. Introduction of the –198 T→C HPFH mutation into the CACCC-less (A)γ gene construct re-established the HPFH phenotype, indicating that this mutation increases promoter strength, most probably by establishing a novel CACCC box sequence in the –198(A)γ region. The HPFH phenotype was also re-established when the –117 C→T HPFH mutation was introduced into a –141(A)γ promoter with a destroyed CACCC box, indicating that this mutation increases γ promoter strength in the absence of the CACCC motif. The T→A –175 HPFH mutation failed to re-establish the HPFH phenotype when the CACCC box was deleted, indicating that γ gene expression in this mutation is CACCC box dependent. These results provide the first in vivo experimental evidence in support of mechanistic heterogeneity of the non-deletion HPFH mutants.