Expression of G gamma and A gamma globin genes in human adults

Expression of the human fetal G gamma and A gamma globin genes declines shortly after birth, and adults generally have less than 1% fetal hemoglobin or Hb F (alpha 2 gamma 2). However, some adults with hereditary persistence of fetal hemoglobin (HPFH) have elevated expression of either the G gamma or A gamma gene due to a mutation in its upstream promoter. Mutations with strong effects on expression have been found at -175 and -202 of the G gamma gene and at -117, -196, -198 and -202 of the A gamma gene. Mutations at -158 and -161 of G gamma have weaker effects, which are observable primarily as increases in the G gamma:A gamma ratio. Published data are reviewed which suggest that the -158 mutation may lead to observable elevations of Hb F in SS and beta(0)-thal patients and occasionally in normal non-anemic individuals. These data also suggest that additional high Hb F determinants are linked to Benin, Bantu and Asian beta S haplotypes in some instances. A model based on data from SV40 is presented which suggests that specific DNA sequence motifs of the gamma globin gene may bind regulatory proteins. It is proposed that the -158 and -161 mutations have weak effects because they are located on the fringe of regulatory sequence motifs.     

GγAγ(δβ)°-thalassaemia and a new form of γ globin gene triplication identified in the Yugoslavian population

Among several hundred apparently healthy Yugoslavian adults with slightly elevated levels of fetal haemoglobin, we have identified two distinct abnormalities. (a) A G gamma A gamma(delta beta)0-thalassaemia heterozygosity with an approximately 15 kb deletion which involves part of the delta globin gene and the beta globin gene. This deletion is probably the same as that seen among Italians (Ottolenghi et al, 1982; Carè et al, 1984). (b) A nondeletion form of hereditary persistence of Hb F which is caused by a gamma globin gene triplication of the (+)G gamma.(+)G gamma.A gamma type. It is characterized by the presence of some 5% Hb F in the heterozygote containing nearly 100% G gamma chains. The C----T mutation at position--158 5' to the G gamma chain [(+)G gamma], identified through analyses of Xmn I digests, was present at both G gamma globin genes. This mutation is known to be associated with increased G gamma chain production (Gilman & Huisman, 1985), and thus is responsible for the increased G gamma chain production in these heterozygotes. The condition is different from the (+)G gamma.(+)G gamma nondeletion type of HPFH which has been observed in heterozygotes of two Black families, and is associated with the presence of 3-4% Hb F (with mainly G gamma chains) in heterozygotes.     

Concordance of a point mutation 5' to the G gamma globin gene with G gamma beta +. Hereditary persistence of fetal hemoglobin in the black population

Hereditary persistence of fetal hemoglobin (HPFH) is a genetically heterogeneous and clinically benign condition characterized by persistent expression of fetal hemoglobin (Hb F) into adulthood. In the G gamma beta + type, no major deletions in the globin gene cluster occur; adult heterozygotes produce approximately 20% Hb F, which results from overproduction of G gamma chains, with no apparent increase in production from the adjacent A gamma gene. We have recently described a point mutation 202 base pairs 5' to the cap site of the G gamma gene in an individual with G gamma beta + HPFH. This mutation abolishes a normal ApaI restriction endonuclease site, and thus can be detected by blotting of genomic DNA. We present here further data on the ApaI mutation: (1) It occurs in six of seven families with G gamma beta + HPFH. (2) In three families, detailed haplotype analysis using 11 polymorphic restriction sites in the beta globin cluster has been done. The two that carry the missing ApaI site are identical but the third, which has a normal ApaI pattern, differs from the other two in at least two sites, one of which is a new polymorphic Nco I site between the delta and beta globin genes. This suggests the possibility of a different HPFH mutation in the third family. (3) The haplotype of the G gamma beta + HPFH chromosome carrying the ApaI mutation is different from that of 108 beta A chromosomes of black individuals that have been tested. (4) The G gamma ApaI site is normal in 61 beta A and 109 beta S alleles from non-HPFH black individuals, including 22 who share the same haplotype for the intragenic G gamma, A gamma HindIII polymorphisms. These data add support to the possibility that the -202 mutation is actually causative of the G gamma beta + HPFH phenotype.     

Quantitation of three types of gamma chain of HbF by high pressure liquid chromatography; application of this method to the HbF of patients with sickle cell anemia or the S-HPFH condition

A modification of a high pressure liquid chromatographic (HPLC) procedure is described that enables the complete separation and quantitation of the A gamma T, A gamma I, and G gamma chains in human fetal hemoglobin. The method, which is fast and accurate, requires 5 to 2000 micrograms Hb F. The purity of the Hb F is not essential and admixture of up to 70% adult Hb does not interfere with the determination. The method has been applied to the Hb F of 64 Black SS patients and 7 persons with the Hb S-HPFH (G gamma A gamma type) conditions. (A) Both "adult" G gamma to A gamma (2:3) and "newborn" G gamma to A gamma (3:2) ratios were observed in adult SS patients, 8 yr and older. Only 12% of the SS patients had the "newborn" ratio. This high G gamma to A gamma ratio may be due to a modification of the genetic switch mechanism that regulates the change of this ratio after birth. (B) Intermediate G gamma to A gamma ratios were only found in young SS patients, 5 yr of age or less. The results suggest a delayed switch of the newborn leads to adult ratio in sickle cell anemia. (C) The A gamma T chain was present in only 6% of all SS patients. One patient is homozygous for this variant chain. (D) Three of the 7 subjects with Hb S-HPFH were positive for the A gamma T chain. Its percentage was low, which suggests that the A gamma T chain gene is in trans of the HPFH determinant. (E) Quantitation of the three gamma chain types is also possible in the Hb F from Hb S heterozygotes with (nearly) normal Hb F levels. Such an analysis is useful for an evaluation of genetic conditions involving variations in the production of (different types of) Hb F.     

Homozygosity for a new type of G gamma (A gamma delta beta)zero-thalassemia in a Malaysian male

Hematological and clinical data are presented for a young Malay patient with a homozygous (delta beta)zero-thalassemic condition. His red blood cells contained 100% fetal hemoglobin with alpha and G gamma chains only. Detailed gene mapping defined a large deletion with a 5' end between the Aha III and Apa I sites, some 200-400 bp 5' to the A gamma globin gene and a 3' end beyond sequences 17-18 kb 3' to the beta globin gene. This G gamma (A gamma delta beta)zero-type of thalassemia is different from all the other six types described before. Comparison of the hematological data of this patient with those of homozygotes for either the Sicilian or Spanish types of G gamma A gamma (delta beta)zero-thalassemia showed no differences; all homozygotes have a moderate anemia which is accentuated by the relatively high oxygen affinity of the Hb F containing erythrocytes.     

alpha Chain and gamma chain abnormal hemoglobins in newborn babies: structural and genetic aspects

Structural studies and quantitative analyses were conducted on the hemoglobin of 55 newborn babies. Seven alpha chain variants (G-Philadelphia, Montgomery, Inkster, I-Philadelphia, Matsue-Oki, Winnipeg, and O-Indonesia) were present in 26 heterozygous newborns (17 black, eight Caucasian, and one Indonesian). The relative amount of the alpha X containing abnormal Hb F of the Hb G-Philadelphia and Hb Winnipeg babies was less than observed in heterozygous adults, which may indicate a decreased rate of assembly of the alpha X-gamma dimer over that of the alpha X-beta dimer. Of the 29 newborns with gamma chain variants, 16 were Caucasian babies; of these 15 had a Hb A gamma F-Hull heterozygosity and one a Hb G gamma F-Marietta heterozygosity. Six black babies were heterozygous for Hb A gamma F-Texas-I and six for Hb G gamma F-Port Royal. One Japanese baby had a heterozygosity for A gamma F-Iwata and a second was heterozygous for A gamma TF-Yamaguchi. Quantitative analyses of the isolated normal Hb Fo as well as an evaluation of the relative amounts of the Hb Fx in the red cell lysates gave data useful for a speculation of the genetic condition in each of these babies. It was concluded that the babies with the Hbs F-Texas-I, F-Iwata, F-Hull, and F-Marietta were simple heterozygotes with either the G gamma x A gamma/G gamma x A gamma X or the G gamma x A gamma/G gamma X x A gamma genic arrangement. The babies with Hb F-Port Royal had a G gamma x G gamma X/G gamma x A gamma arrangement, which may result from a (to be determined) gene conversion. The newborn baby with Hb F-Yamaguchi has the G gamma x A gamma x/A gamma T-X.-. genic arrangement, suggesting the presence of three distinctly different gamma chain genes of which one, the A gamma T-X gene, produces an A gamma chain (with threonyl at position gamma 75 and an Asn at position gamma 80) at a level usually seen for G gamma rather than A gamma chains. These studies were greatly facilitated by the use of high pressure liquid chromatographic methods.     

The synthesis of fetal hemoglobin types in red blood cells and in BFU-E derived colonies from peripheral blood of patients with sickle cell anemia, beta+ - and delta beta-thalassemia, various forms of hereditary persistence of fetal hemoglobin, normal adults and newborn.

The biosynthesis of two types of human fetal hemoglobin (Hb F), namely Hb F with G gamma chains having glycine in position 136 and Hb F with A gamma chains having alanine in position 136, was studied in blood samples and in cultures of erythroid precursors from blood of patients with different hemoglobinopathies. High pressure liquid chromatography (HPLC) was adapted to allow the separation of the methionyl-containing tryptic peptides G gamma T-15 and A gamma T-15 (which include the Gly leads to Ala polymorphism at position 136) from a digest of microquantitites of 35S-methionyl labelled Hb F. This method was sensitive enough to quantitate the relative production of the G ygamma and A gamma chains by erythroid colonies derived from cloned Burst Forming Units (bfu-e) which were cultured for 16 days on methylcellulose. The production of Hb F in these colonies was generally higher than the level of Hb F in blood except for subjects with the G gamma A gamma-HPFH heterozygosity. The G gamma to A gamma ratio in the Nb F produced in cultures of cells from G gamma delta beta-thalassemia or G gamma-HPFH heterozygotes was lower and that from A gamma-HPFH heterosygotes was higher than the ratios in the Hb F of the corresponding peripheral blood cells. Mixtures of G gamma and A gamma chains were present in cell cultures of SS patients, beta+-thalassemia homozygotes and G gamma A gamma-HPFH heterozygotes in a ratio similar to that in the Hb F of mature red cells. These data suggest that erythroblasts in BFU-E derived colonies reactivate all available gamma chain structural genes, both in cis and in trans to the abnormal determinant. Hb F biosynthesis by adult blood samples concerns primarily the G gamma chains. This was particularly striking for blood samples in which erythroblasts were absent and the biosynthesis took place in fetal reticulocytes. Thus, the F-retuculocytes in blood of A gamma-HPFH heterozygotes with about 5% Hb F of the A gamma type produced primarily Hb F with G gamma chains. Similar differences were observed for G gamma A gamma-HPFH heterozygotes and, less strinkingly, for SS patients. A satisfactory explanation for this observation has not yet been obtained.     

Interaction of two different disorders in the beta-globin gene cluster associated with an increased hemoglobin F production: a novel deletion type of (G) gamma + ((A) gamma delta beta)(0)-thalassemia and a delta(0)-hereditary persistence of fetal hemoglobin determinant.

We report two different disorders of the beta-globin gene cluster segregating in a Belgian family: a novel deletion that results in (G) gamma + ((A) gamma delta beta)(0)-thalassemia (thal) and a heterocellular hereditary persistence of foetal hemoglobin of the Swiss type linked to a delta(0)-thal gene (delta (0)-HPFH). Heterozygosity for the heterocellular HPFH brings about a moderate (3.4% to 8.24%) increase of hemoglobin (Hb) F having a G gamma/A gamma ratio of 4:1, whereas carriers of the G gamma + ((A) gamma delta beta)(0)-thal deletion show in their peripheral blood a considerably higher (15%) percentage of Hb F. Both defects interact in the compound heterozygotes for G gamma + ((A) gamma delta beta)(0)-thal and delta(0)-HPFH producing a further increase (up to 24%) of fetal Hb consisting entirely of G gamma chains. Molecular characterization of the (G) gamma + ((A) gamma delta beta)(0)-thal by means of Southern analysis showed that the deletion spans about 50 kb, removing the 3' end of the A gamma-gene, the psi beta-, delta-, and beta-genes. A number of possible mechanisms leading to the overproduction of Hb F in HPFH and (G) gamma + ((A) gamma delta beta)(0)-thal will be discussed.     

Four categories of gamma-globin gene triplications: DNA sequence comparison of low G gamma and high G gamma triplications

The human fetal gamma chains are produced by closely linked G gamma and A gamma genes, and unequal crossing over between them leads to gamma gene deletions and triplications. Nine gamma gene triplications from seven ethnic groups were analyzed for G gamma and hemoglobin F (Hb F) values of heterozygotes and for the presence of polymorphic XmnI restriction sites 5' to the gamma genes. Four categories of triplication were found: I had low G gamma and low Hb F values and lacked XmnI sites 5' to the three gamma genes [---]. II had high G gamma and slightly elevated Hb F values but was also [---]. III was similar to II, except that XmnI was [+--]. IV had very high G gamma and slightly elevated Hb F values, and XmnI was [++-]. One case each of triplications I and IV were cloned into Charon 35. For both, the two 5' gamma gene code for G gamma chain, while the 3' gamma gene codes for A gamma chain. DNA sequencing showed that the unequal crossover occurred between 472 and 398 base pairs (bp) 5' to the gamma gene Cap sites (- 472 and -398) for the type IV triplication and between -271 and codon 136 for the type I triplication. In addition, type I had a 4-bp deletion of AGCA from -225 to -222. The high G gamma values of the type IV triplication are explained by its -G gamma-G gamma-A gamma-gene arrangement and the XmnI sites 5' to the G gamma genes. We hypothesize that the low G gamma value of the type I triplication, which is also -G gamma-G gamma-A gamma-, is due to inactivation of the middle G gamma gene by the AGCA deletion at -225 to -222.     

Haplotypes of beta S chromosomes among patients with sickle cell anemia from Georgia

Fetal hemoglobin and G gamma levels have been correlated with the presence or absence of eight restriction sites within the beta globin gene cluster (haplotypes) for numerous sickle cell anemia patients from Georgia. The most common haplotypes were #19 (Benin) and #20 (CAR); all patients with haplotype combinations 19/19, 20/20, and 19/20 were severely affected with low Hb F and low G gamma levels. A modified #19 beta S chromosome with a -G gamma-G gamma- globin gene arrangement, instead of -G gamma-A gamma-, was present in SS and SC newborn babies with G gamma values above 80%. Haplotype #3 (Senegal) was present among 15% of the beta S chromosomes; the two adult patients with the 3/3 combination were mildly affected with high Hb F and G gamma values. The haplotype AT with the variant A gamma T chain was a rarity. A new haplotype was found in one 17-year-old SS patient and five of his Hb S heterozygous relatives. This haplotype is associated with an increased production of Hb F in heterozygous and homozygous Hb S individuals; this Hb F contained primarily A gamma chains. A comparison was made between the different haplotypes among SS patients and normal Black individuals, and a remarkable similarity was noted in the fetal hemoglobin data for subjects with these different chromosomes.     

Upstream promoter mutation associated with a modest elevation of fetal hemoglobin expression in human adults

In hereditary persistence of fetal hemoglobin, Hb F (alpha 2 gamma 2) is elevated after birth. Screening of sickle cell patients has revealed a family with elevated Hb F and high A gamma values. The propositus was a sickle cell patient with approximately 25% Hb F and 68.4% A gamma. He was heterozygous for the Benin (#19) and Mor beta S haplotypes. Five AS relatives with the Mor haplotype had 2.5% +/- 0.9% fetal hemoglobin and 92.8% +/- 2.8% A gamma, whereas two with the Benin haplotype had normal fetal hemoglobin (0.5%). The Mor haplotype is thus associated with the elevated Hb F in this family. The 13-kilobase (kb) Bg/II fragment containing the G gamma and A gamma genes of the Mor haplotype was cloned, and the G gamma and A gamma promoters sequenced from -383 to beyond the Cap sites. The Mor G gamma gene was normal, but the A gamma gene had a unique C----T mutation at -202. A different mutation at -202 of G gamma (C----G) was previously detected by other researchers in association with considerably higher Hb F in AS cases (15% to 25%). These data suggest either that -202 mutations affect the G gamma and A gamma promoters differently or that different nucleotide substitutions at -202 have divergent effects. Alternatively, additional unknown mutations could cause the differences in gene expression.     

The gamma globin chain heterogeneity of the Sardinian newborn baby

The hemoglobin of 2048 newborn babies from Sardinia was analyzed by isoelectric focusing and polyacrylamide gel electrophoresis in order to determine the level of Hb F-Sardinia (with A gamma T) and the G gamma chain. Hb F-Sardinia values of 15.5 +/- 2.6% were present in the A gamma T heterozygote whereas 30.7 +/- 5.2% were present in the homozygote. The A gamma T gene frequency was 0.17. Most of the babies tested showed the normal G gamma level either in the absence of the A gamma T anomaly (69.6 +/- 4.1%), or in the presence of the anomaly in both the heterozygous state (70.9 +/- 4.8%) and the homozygous state (71.1 +/- 3.4%). Similar values were shown in nine homozygotes for beta-thalassemia discovered during the screening. Nine newborn babies (0.44%) showed particularly low G gamma levels (38.3 +/- 6.8%) whereas 18 newborn babies showed high G gamma levels (83.9 +/- 2.6%). The frequencies of the anomalies (0.0022 for the low G gamma and 0.0044 for the high G gamma) were the lowest observed in Caucasian and other ethnic groups. Data suggest the presence of abnormal gamma globin gene arrangements in the Sardinian population.     

DNA sequence variation associated with elevated fetal G gamma globin production

The percentage of G gamma chains in the Hb F of SS patients and beta-thalassemia heterozygotes is generally 40%, but some have 60% to 70% G gamma. To test the hypothesis that DNA sequence variation 158 base pairs 5' of the G gamma gene is associated with this variation in G gamma values, DNA was analyzed using the restriction endonuclease Xmn I (gamma IVS-II probe). Xmn I recognizes the sequence from -157 to -166 only if T is at position -158. Individuals from five families had T at -158 for G gamma genes in both chromosomes, and the mean G gamma value was 69.7% +/- 4.6% (SD). For 13 families, individuals with T at -158 for the G gamma gene of one chromosome had a G gamma value of 60.6% +/- 5.7%. With one exception, lack of T at -158 was associated with low G gamma values (39.6% +/- 4.0%). In low Hb F G gamma-beta+-HPFH, the Xmn I site was seen 5' to both G gamma and A gamma genes, which suggests that T at -158 is associated with elevated Hb F; Pst I digestion showed that the A gamma gene T producer G gamma globin, which accounts for high levels of G gamma (87-88%). Calculations show that T at -158 is associated with a three- to 11-fold increase in production per G gamma gene, which is an order of magnitude less than that associated with the previously identified -202 C----G substitution of high Hb F G gamma-beta+-HPFH.     

Molecular and hematological characterization of HPFH-6/Indian deletion-inversion Ggamma(Agammadeltabeta)0-thalassemia and Ggamma(Agammadeltabeta)0-thalassemia/HbE in Thai patients

The hereditary persistence of fetal hemoglobin (HPFH)-6 is sporadically found in Thailand whereas the deletion-inversion type (G)gamma((A)gamma delta beta)(0)-thalassemia is described among Indians. We report a hitherto un-described case in which these two defects co-segregate. He was a 3-year-old Thai boy who had a feature of thalassemia intermedia phenotype with the following hematologic data; Hb 8.8 g/dL, Hct 29.2%, MCV 66.9 fL, MCH 20 pg, and MCHC 30.1 g/dL. Hemoglobin analysis revealed 100% Hb F with only (G)gamma-globin chain. Globin gene analyses demonstrated that he carried the HPFH-6 deletion in trans to the Indian deletion-inversion (G)gamma((A)gamma delta beta)(0)-thalassemia. Hematologic data of the patient was compared to those of the HPFH-6 heterozygote found in his father, to (G)gamma((A)gamma delta beta)(0)-thalassemia heterozygotes detected in his mother and sister, and to that of an unrelated Thai patient who was a compound heterozygote for the deletion-inversion (G)gamma((A)gamma delta beta)(0)-thalassemia and HbE.     

Black beta-thalassemia homozygotes with specific sequence variations in the 5' hypersensitive site-2 of the locus control region have high levels of fetal hemoglobin.

We have sequenced the 5' hypersensitive-2 (5'HS-2) site of the locus control region (LCR) and the promoters of the two gamma-globin genes located on chromosome 11 of a black patient with mild beta-thalassemia (beta-thal) major due to a homozygosity for the C----T mutation at position -88 of the beta promoter and with a high Hb F level. Sequence variations in the 5'HS-2 were the same as observed for the beta s chromosome with haplotype number 3, while most of the G gamma promoter and the A gamma promoter had sequences similar to that of the beta S chromosome with haplotype number 19. This atypical haplotype (number 19A) is apparently associated with an increased gamma chain production which is particularly evident during periods of severe hematopoietic stress. Additional studies on relatives of the proband and on 10 unrelated black beta-thal homozygotes with either the C----T mutation at -88 or the A----G mutation at -29, confirm the possible importance of the sequence differences in the 5'HS-2, and also suggest that at least two additional factors, namely a C----T mutation at position -158 of the G gamma promoter and a relative deficiency in alpha chain synthesis play a (perhaps less important) role in the increased Hb F synthesis in these patients.     

Chinese A gamma fetal hemoglobin: C to T substitution at position-196 of the A gamma gene promoter

The molecular basis for the hereditary persistence of fetal hemoglobin (HPFH) phenotype was studied in a Chinese individual who was heterozygous for a nondeletion form of A gamma-HPFH. Both allelic A gamma-globin genes were isolated by molecular cloning and subjected to nucleotide sequence analysis. One A gamma gene promoter showed a cytosine to thymine transition at position -196, whereas the other promoter was normal. This mutation at position -196 has now ben found in unrelated individuals with the A gamma-HPFH phenotype from Italy, Sardinia, and China, suggesting that it may have arisen independently. The implications of this mutation for models of fetal globin gene switching are discussed.     

The Greek A gamma beta+-HPFH observed in a large black family

Several members of a Black family with a heterozygosity for an A gamma beta+-HPFH, shown in 1969 to have relatively low levels of Hb F and a low glycine to alanine ratio in the gamma chain of this Hb F, were reinvestigated. Thirteen of 30 available family members in two generations had the heterozygous form of this condition, which was characterized by a decreased level of Hb A2, an average Hb FAD value of 13.3%, an equal distribution of Hb F over the red cells, and normal hematological values. The gamma chain composition of isolated Hb F was determined by reversed phase high performance liquid chromatography for all 13 heterozygotes and showed an average A gamma value of 84.5%. Hybridization with synthetic oligonucleotides, specific for normal and mutant sequences at positions 111-129 5' to the A gamma globin gene, identified a G----A base substitution at position 117, similar to that seen in subjects with the Greek A gamma-HPFH. Our data support conclusions by others that this replacement is causative of the increased A gamma chain synthesis in this condition. Haplotype analysis supported the suggestion that the G----A substitution occurred as an independent event in this Black family.     

Hb F-Bonaire-Ga or alpha 2 A gamma 2 39(C5) Gln replaced by Arg, characterized by high pressure liquid chromatographic and microsequencing procedures

A slowly moving gamma chain variant was discovered in the cord blood of a baby of English-Vietnamese descent. The abnormality concerned the substitution of Gln residue in position 39(C5) of the A gamma chain by an Arg residue resulting in an -Arg-Arg- sequence at positions 39 and 40. The quantity of the A gamma chain variant was nearly 10% of the total Hb F with 15% of the Hb F having normal A gamma chains and 75% of Hb F having G gamma chains. High pressure liquid chromatographic and microsequencing methods greatly facilitated the structural analyses.     

Association of the level of G gamma chain in the fetal hemoglobin of normal adults with specific haplotypes

The levels of G gamma chain in the fetal hemoglobin of more than 40 Black and Caucasian females were determined with a sensitive high performance liquid chromatography procedure and were correlated with their haplotypes, defined by the presence or absence of 10 different restriction sites. Blood was collected during the 16th and 31st week of pregnancy because of a slightly elevated level of Hb F which facilitated the isolation of this protein from a relatively small sample. Four distinct G gamma levels were observed, each being associated with a specific haplotype. Homozygosity for sub-haplotype A [- + + - + +] is associated with high G gamma values (60-70%); that for sub-haplotype B [- - - - - +] with low levels (25-30%); and that for sub-haplotype C [+ - - - - -] with very low levels (10-15%) (restriction sites listed are Hinc II at epsilon; Xmn I 5' to G gamma; Hind III at G gamma and A gamma; Hinc II at psi beta and 3' to it). Sub-haplotype D [(14)- + - - +] with a rare polymorphism 5' to epsilon is associated with extremely high G gamma values. Hb F levels were low (less than 2.5%) and were independent of the haplotype. It is speculated that, yet unknown, variations in the DNA of gene activity controlling regions are responsible for the differences in G gamma value.