These mild conditions are characterized in the heterozygote by near normal hematology, low levels of Hb A2, and increased levels (15-30%) of Hb F which is pancellularly distributed among the red cells. All seven known types are the result of (partial) deletions involving the delta- and beta-globin genes: two types (1 and 2) are found in Blacks and are characterized by large deletions of 105-106 kb; type 3 is found in Indian families and is ~48-49 kb long; types 4 and 5 are found in a few Italian families; type 6 was discovered in the Vietnamese population, and type 7, the Kenya Ggamma type, has been observed in populations around Lake Victoria in Kenya, Africa. All but type 7 have intact Agamma- and Ggamma-globin genes; the Kenya type is characterized by an intact Ggamma-globin gene and a 3' neighboring Agammabeta hybrid gene. Fig. 13 compares the sizes of the seven different types. All seven are rare conditions, perhaps with the exception of types 1 and 2 which can be found in ~0.1% of Blacks living in the Southeastern United States.
FIG. 13. Comparison of the deletions observed in seven different types of HPFH.
The approach to the characterization of the HPFH deletions consists mainly of gene mapping with multiple restriction enzymes and various probes. More recently, Craig et al (1) have designed a rapid approach to screen for the known HPFH deletions. The technique involves the PCR procedure using primers on either side of the respective deletions. Details about the primers and the sizes of fragments to be expected can be found in the quoted reference. The same paper provides information about a comparable characterization of various types of deltabeta-thal and the Lepore Hbs.
|1.||Craig, J.E., Barnetson, R.A., Prior, J., Raven, J.L., and Thein, S.L.: Blood, 83:1673, 1994.|