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High levels of fetal hemoglobin (Hb F) protect from many of the complications of sickle cell disease and lead to improved survival. Butyrate and other short chain fatty acids were previously shown to increase Hb F production in erythroid cells in vitro and in animal models in vivo. However, butyrates are also known to inhibit the proliferation of many cell types, including erythroid cells. Experience with the use of butyrate in animal models and in early clinical trials demonstrated that the Hb F response may be lost after prolonged administration of high doses of butyrate. We hypothesized that this loss of response may be a result of the antiproliferative effects of butyrate. We designed a regimen consisting of intermittent or pulse therapy in which butyrate was administered for 4 days followed by 10 to 24 days with no drug exposure. This pulse regimen induced fetal globin gene expression in 9 of 11 patients. The mean Hb F in this group increased from 7.2% to 21.0% (P <.002) after intermittent butyrate therapy for a mean duration of 29.9 weeks. This was associated with a parallel increase in the number of F cells and F reticulocytes. The total hemoglobin levels also increased from a mean of 7.8 g/dL to a mean of 8.8 g/dL (P <.006). The increased levels of Hb F were sustained in all responders, including 1 patient who has been on pulse butyrate therapy for more than 28 months. This regimen, which resulted in a marked and sustained increase in Hb F levels in more than two thirds of the adult sickle cell patients enrolled in this study, was well tolerated without adverse side effects. These encouraging results require confirmation along with an appropriate evaluation of clinical outcomes in a larger number of patients with sickle cell disease.

Hydroxyurea (HU) can increase fetal hemoglobin (HbF) in sickle cell anemia (HbSS). To identify determinants of the HbF response, we studied 150 HU-treated patients grouped by quartiles of change in HbF from baseline to 2 years. Half of the HU-assigned patients had long-term increments in HbF. In the top two quartiles, HbF increased to 18.1% and 8.8%. These patients had the highest baseline neutrophil and reticulocyte counts, and largest treatment-associated decrements in these counts. In the lower two quartiles, 2-year HbF levels (4.2% and 3.9%) and blood counts changed little from baseline. In the highest HbF response quartile, myelosuppression developed in less than 6 months, compliance was best, and final doses of HU were 15 to 22.5 mg/kg. All four quartiles had substantial increases of F cells in the first year. This was maintained for 2 years only in the top three quartiles. Leukocyte and reticulocyte counts decreased initially in all quartiles, but drifted back toward baseline levels in the lowest HbF response quartile. Initial HbF level and phenotype of the F-cell production (FCP) locus were not associated with HbF response, but absence of a Central African Republic (CAR) haplotype was. Bone marrow ability to withstand HU treatment may be important for sustained HbF increases during HU treatment of HbSS.

Five factors have been shown to influence the 20-fold variation of fetal hemoglobin (Hb F) levels in sickle cell anemia (SS): age, sex, the alpha-globin gene number, beta-globin haplotypes, and an X-linked locus that regulates the production of Hb F-containing erythrocytes (F cells), ie, the F-cell production (FCP) locus. To determine the relative importance of these factors, we studied 257 Jamaican SS subjects from a Cohort group identified by newborn screening and from a Sib Pair study. Linear regression analyses showed that each variable, when analyzed alone, had a significant association with Hb F levels (P < .05). Multiple regression analysis, including all variables, showed that the FCP locus is the strongest predictor, accounting for 40% of Hb F variation. beta-Globin haplotypes, alpha-globin genes, and age accounted for less than 10% of the variation. The association between the beta-globin haplotypes and Hb F levels becomes apparent if the influence of the FCP locus is removed by analyzing only individuals with the same FCP phenotype. Thus, the FCP locus is the most important factor identified to date in determining Hb F levels. The variation within each FCP phenotype is modulated by factors associated with the three common beta-globin haplotypes and other as yet unidentified factor(s).

Butyrate analogues have been shown to increase fetal hemoglobin (HbF) production in vitro and in vivo. Sodium phenylbutyrate (SPB), an oral agent used to treat individuals with urea-cycle disorders, has been shown to increase HbF in nonanemic individuals and in individuals with sickle cell disease. We have treated eleven patients with homozygous beta thalassemia (three transfusion dependent) and one sickle-beta-thalassemia patient with 20 g/d (forty 500-mg tablets) of SPB for 41 to 460 days. All patients showed an increase in the percent of F reticulocytes associated with treatment, but only four patients responded by increasing their Hb levels by greater than 1 g/dL (mean increase, 2.1 g/dL; range, 1.2 to 2.8 g/dL). None of the transfusion-dependent thalassemia subjects responded. Increase in Hb was associated with an increase in red blood cell number (mean increase, 0.62 x 10(12)/L), and mean corpuscular volume (mean increase, 6 fL). Changes in percent HbF, absolute HbF levels, or alpha- to non-alpha-globin ratios as measured by levels of mRNA and globin protein in peripheral blood did not correlate with response to treatment. Response to treatment was not associated with the type of beta-globin mutation, but baseline erythropoietin levels of greater than 120 mU/mL was seen in all responders and only two of eight nonresponders to SPB. Compliance with treatment was greater than 90% as measured by pill counts. Side effects of the drug included weight gain and/or edema caused by increase salt load in 2/12, transient epigastric discomfort in 7/12, and abnormal body odor in 3/12 subjects. Two splenectomized patients who were not on prophylactic antibiotics developed sepsis while on treatment. We conclude that SPB increases Hb in some patients with thalassemia, but the precise mechanism of action is unknown.

Intravenous arginine butyrate has been shown to increase fetal hemoglobin (HbF) in sickle cell and thalassemia patients. Recently, we observed that sodium 4-phenylbutyrate, a drug administered orally to treat urea cycle disorders, increases HbF production in nonanemic children and adults. We treated six subjects with sickle cell disease over a period of 14 to 179 days. All subjects received their initial therapy of 9 to 13 g/m2/day as 0.5-g tablets of sodium 4-phenylbutyrate as inpatients. All subjects showed a rapid increase in the percentage of F-reticulocytes (pretreatment, 1% to 20%; posttreatment, 10% to 44%). Four subjects were treated only 11 to 25 days as inpatients. Two of these four subjects failed to respond to the outpatient component because of their inability to maintain an intake of 30 to 40 tablets per day. One subject (C) developed a rash at day 10 and discontinued treatment at day 14. Another subject (B) was transfused for a painful crisis on day 25. Subject A, treated for 179 days, has an increased percentage of F cells, from 54% to 77%, and increased HbF levels, from 10.6% to 18%. Subject F, treated for 154 days, has an increased percentage of F cells, from 59% to 73%, and an increased percentage of HbF, from 10.4% to 16%. All subjects showed some increase in weight. Subject A developed mild transient ankle edema. Myelotoxicity was not seen in any treated patient. Oral administration of sodium 4-phenylbutyrate rapidly increases F-cell production in sickle cell disease.

Fetal hemoglobin (Hb F) production in sickle cell (SS) disease and in normal individuals varies over a 20-fold range and is under genetic control. Previous studies suggested that variant Hb F levels might be controlled by genetic loci separate from the beta-globin complex on chromosome 11. Using microscopic radial immunodiffusion and flow cytometric immunofluorescent assays to determine the percentage of F reticulocytes and F cells in SS and nonanemic individuals, we observed that F-cell levels were significantly higher in nonanemic females than males (mean +/- SD, 3.8% +/- 3.2% v 2.7% +/- 2.3%). F-cell production as determined by F reticulocyte levels in SS females was also higher than in SS males (17% +/- 10% v 13% +/- 8%). We tested the hypothesis that F-cell production in both normal and anemic SS individuals was controlled by an X-linked locus with two alleles, high (H) and low (L). Using an algorithm to determine the 99.8% confidence interval of a normal distribution in nonanemic individuals, we estimated that males and females with at least one H allele had greater than 3.3% F cells. Comparisons of male-male or female-female SS sib pairs with discordant F reticulocyte levels distinguished two phenotypes in SS males (L, less than 12%; H, greater than 12%) and three phenotypes in SS females (LL, less than 12%; HL, 12% to 24%, HH greater than 24%). Linkage analysis using polymorphic restriction sites along the X chromosome in eight SS and one AA family localized the F-cell production (FCP) locus to Xp22.2, with a maximum lod score (logarithm of odds of linkage v independent assortment) of 4.6 at a recombination fraction of 0.04.

Patients with sickle cell anemia were treated with daily doses of hydroxyurea, to assess pharmacokinetics, toxicity, and increase in fetal hemoglobin (Hb) production in response to the drug. Plasma hydroxyurea clearances were not a useful guide to maximum tolerated doses of the drug. The mean daily single oral dose that could be maintained for at least 16 weeks was 21 mg/kg (range, 10 to 35 mg/kg). Among 32 patients, last HbF levels were 1.9% to 26.3% (mean, 14.9%) with increases in HbF over initial values of 1.4% to 20.2% (mean, 11.2%). The most significant predictors of last HbF were last plasma hydroxyurea level, initial white blood count and initial HbF concentration. Last HbF was not related to beta globin haplotype or alpha globin gene number. No serious toxicity was encountered. Clinically significant bone marrow depression was avoided, and chromosome abnormalities after 2 years of treatment were no greater than those observed before treatment. The period of observation has been too short to evaluate the risk of carcinogenesis. Patient's red cells developed striking macrocytosis. Median red cell Hb concentrations did not change. Hb concentrations increased, on average 1.2 g/dL, but serum erythropoietin levels increased. Patients' body weights increased, and some returned to work or school, but no conclusions regarding therapeutic efficacy could be drawn from this uncontrolled open-label study.

A rationale for clinical trials of hydroxyurea (HU) treatment in sickle cell disease is that the agent increases red blood cell (RBC) fetal hemoglobin content. However, an additional effect of HU is to raise the mean corpuscular volume (MCV). To investigate the action of HU in a species that makes no electrophoretically distinguishable fetal hemoglobin, we treated dogs with the drug and compared their response to that of five patients with sickle cell anemia. Both dogs and patients had an increase in MCV, but the effect of HU treatment on the mean corpuscular hemoglobin concentration (MCHC), density, and water content of the RBCs differed in the two species. The dog RBCs became low in MCHC, high in ion and water content, and low in mean density. Thus, HU can raise MCV and lower MCHC without influencing fetal hemoglobin synthesis. A different pattern was seen in the sickle cell patients during HU treatment. Although the MCV of their RBCs increased, there was no change in MCHC, ion content, or mean density. A notable change in the sickle cell patients' blood was that two subpopulations of cells were nearly eliminated during HU treatment; the hypodense reticulocyte fraction and the hyperdense fraction that contains irreversibly sickled cells. These findings lead us to suggest that trials of HU in sickle cell disease must recognize the possibility that any beneficial effect of this agent might be due not only to an increase in hemoglobin F alone, but perhaps also to the associated increase in MCV or the altered RBC density profile.

Single nucleotide substitutions in the promoter regions of the A gamma- and G gamma-globin genes have been associated with increased fetal hemoglobin (HbF) production. We wished to determine whether these or other unrecognized substitutions in the gamma promoter regions are responsible for the 20-fold variation in HbF production in sickle cell patients or normal adults. From a random sampling of 250 sickle cell (SS) patients and 125 normal adults, 17 individuals representing the highest and lowest HbF producers were selected for study. All three common restriction fragment length polymorphism beta-globin region haplotypes (Benin, Central African Republic, and Senegal) were found in both the highest and lowest HbF producers with SS disease. Using the polymerase chain reaction amplification and direct sequencing of the amplified DNA product, we examined the promoter regions of both the A gamma and G gamma genes from -350 bp to +50 bp of the CAP site. No mutations were found in either gamma gene promoter region. We conclude that nucleotide substitutions in the promoter regions (-350 to +50 bp) of both gamma genes are not responsible for the marked variation in HbF production among SS or normal individuals.

Adult erythroid progenitors produce significantly higher fetal hemoglobin (HbF) levels in cultures containing fetal calf serum (FCS) and erythropoietin (Ep) than in vivo. The precise mechanisms for this increased HbF production in culture have not been elucidated. We examined HbF biosynthesis by enriched human progenitors in serum-free (SF) culture. We measured globin chain biosynthesis by combination of isoelectric focusing and autoradiography and examined percent nucleated erythrocytes containing HbF (%FNRBC) using microscopic immunodiffusion. CD34 (My10)-positive marrow cells from a normal subject yielded an almost negligible amount of gamma-globin in SF culture stimulated by 100 U/mL interleukin-3 (IL-3) and 2 U/mL Ep, while corresponding FCS culture revealed significant gamma-globin biosynthesis. The %FNRBC of the erythroid bursts in SF cultures derived from nine normal adults (2.0% +/- 0.9% F cells) was 3.0% +/- 3.4%, while in FCS culture, it was 25% +/- 12% (mean +/- SD). Dosages of IL-3 between 10 and 10,000 U/mL did not increase %FNRBC in FCS of SF conditions. Mean Hb contents of nucleated erythrocytes (NRBC) assayed by microdensitometry of pericellular immunoprecipitate were similar in FCS and SF cultures. The number of erythroid bursts per 2 x 10(3) CD34-positive marrow cells was 48 +/- 20 in FCS and 36 +/- 12 in SF cultures. In two experiments, progenitors grown for 7 days under SF conditions were isolated and recultured in either SF or FCS conditions for 7 days, and the resulting erythroid bursts were analyzed for FNRBC. The bursts that had been returned to FCS cultures yielded values of %FNRBC intermediate between those obtained from progenitors grown entirely in SF or FCS cultures, indicating that serum effect is not solely due to growth selection for certain subpopulations of erythroid burst-forming units. This experiment also demonstrated that the factors present in serum responsible for HbF augmentation act at both early and late stages during erythroid burst development. SF culture of peripheral blood progenitors of one subject with heterocellular hereditary persistence of fetal hemoglobin (HPFH) yielded elevated levels of FNRBC (19% +/- 5%) that accurately reflected the F cell (18%) of the circulating blood. Similarly, FNRBC in cultures of progenitors from one umbilical cord blood sample (86% F reticulocytes) was 87 +/- 3% FNRBC. The SF culture for enriched human progenitors, which nearly reflects the physiologic HbF programs of the donor, should facilitate studies of the exact mechanisms of postnatal reactivation of HbF production.

Single-cell microscopic immunodiffusion assays were used to determine the cellular mechanisms that regulate fetal hemoglobin (HbF) levels in cultures of primitive and late erythroid precursors obtained from human adult bone marrow. Two variables--the percentage of cells containing HbF (F cells) and the picograms (pg) of HbF/F cell--were assayed in cells derived from erythroid colony-forming units (CFU-E) and from erythroid burst-forming units (BFU-E) at 7 and 14 days in culture, respectively. The percentage of F cells among all nucleated cells from CFU-E-derived colonies (29.4% +/- 12.5%, mean +/- SD) was not significantly different (p = 0.2) from the percentage of F cells from BFU-E-derived bursts (37.3% +/- 10.1%). Serial daily assays of all cells in cultures on days 3 through 7 and on day 14 revealed a marked increase in F cells between days 4 and 6 in culture. The average amount of HbF/F cell was less in CFU-E-derived F cells than in BFU-E-derived cells (3.5 +/- 0.3 pg versus 6.2 +/- 3.3 pg; p less than 0.01), while adult hemoglobin (HbA) levels in CFU-E-and BFU-E-derived cells remained comparable (19.9 +/- 2.2 pg versus 21.9 +/- 5.3 pg, p = 0.3). These findings indicate that F cell number in culture is not significantly influenced by the relative maturity of the erythroid precursors from which the cells are derived. Differences in the levels of HbF between CFU-E-and BFU-E-derived cells are due to differences in the amount of HbF per F cell, not F cell number.

Varying doses of 5-azacytidine (5-aza) were given to four sickle cell individuals for 500, 200, 100, and 30 days. The percentage of fetal hemoglobin (HbF) containing reticulocytes (F reticulocytes) increased two- to five-fold within five days of 5-aza therapy in all patients, with a two- to three-fold rapid response (less than 48 hours after initial dose) in three patients. Reticulocyte suppression was not observed prior to, during, or after therapy in those patients who responded within 48 hours. Subcutaneous 5-aza was given in 35-day courses consisting of every day, every other day, or three consecutive days a week. No marrow toxicity was observed on any of the regimens. For three patients, the highest average F reticulocyte level was observed on the three consecutive day a week regimen. Oral 5-aza, given with tetrahydrouridine, produced comparable F reticulocyte response. In the two patients treated for more than 100 days, Hb levels increased to 11 to 12 and 9 g/dL, MCV and MCH increased by 25%, and lysate HbF levels peaked at 12% and 20%. Fetal erythroid characteristics (i-antigen, galactokinase activity, and G gamma/A gamma ratios) did not correlate with maximal HbF production. The frequency of vasoocclusive crises appeared to decrease in both patients followed for more than 100 days.

We outline a method for estimating either HbF or HbA content in single erythrocytes and their precursors. Our method depends on microphotometric assay of darkfield reflectance arising from individual pericellular immunoprecipitates developed with anti-HbF or anti-HbA. When uniform-diameter latex microspheres were used to normalize comparisons between preparations, mean coefficient of variation for HbF reflectance among separate preparations of the same sample was < 3%. Reflectance is a faithful (r = 0.99) linear function of the logarithm of picograms per cell in samples with known HbF or HbA content. The following features emerged from such analyses. First, despite the use of antigenically-specific antihemoglobins from different sources, the least detectable quantity of HbF (3.2 pg) and HbA (14.8 pg) remained invariant. Second, these detection thresholds depends on antihemoglobin affinity constants but are little influenced by antibody concentration. Third, our procedure is equally valid for persons with normal HbF constant (mean +/- SD = 4.4 +/- 0.3 pg per cell, 15 subjects) and for those with much higher levels. Fourth, like the percentage of HbF-bearing cells, HbF content is usually unchanging in serial samples. Fifth, the utility of the method is evidenced in bone marrow analyses of five hematologically normal persons in whom HbF content, unlike HbA content, remained constant throughout maturation from erythroblasts to erythrocytes. In vivo HbF biosynthesis is thus normally completed long before HbA production.

We have developed methodology that allows comparison of the mean corpuscular hemoglobin (MCH) of fetal hemoglobin (HbF)-containing red cells (F cells) with the MCH of non-F cells from the same individual. To do this, suspensions of peripheral blood erythrocytes and their internal contents are fixed with an imidodiester, dimethyl-3,3'-dithiobispropionimidate dihydrochloride (DTBP). Thereafter fixed cells are made permeable to antisera by treatment with Triton X-100 and isopropanol, reacted with a mouse monoclonal antibody (MoAb) against HbF, and then with fluorescein-conjugated antimouse IgG. No appreciable hemoglobin is lost during such manipulation. Red cells from a diversity of subjects were thus treated and examined microscopically, first by transmitted light and then by epifluorescence. A direct correlation between Coulter-derived MCH and mean absorbance of 415 nm transmitted light was found for 100 unfixed (r = 0.96) and for 100 antibody-treated fixed-permeabilized red cells (r = 0.99) among individuals selected so as to provide a range of Coulter MCH values between 20 and 35. Comparisons of microscopically derived MCH of F cells and non-F cells were statistically nondistinguishable (P greater than 0.05) in all subjects. Such comparisons included normal individuals (less than 1% F cells), SS patients (7% to 48% F cells), subjects with congenital anemia (22% to 65% F cells), individuals with heterocellular hereditary persistence of HbF (HPFH) (12% to 21% F cells), and heterozygotes for beta + thalassemia (11% to 31% F cells). We conclude that gamma- and beta-globin production within F cells is regulated in a reciprocal fashion both among normal individuals and among individuals with elevated HbF production.

To determine whether hemoglobin regulation is normal in diseases affecting beta-globin gene expression, globin synthesis was examined in members of a family of a patient with hereditary persistence of fetal hemoglobin/beta o-thalassemia (HPFH/beta o-thal). The HPFH defect is the Ghanian type II, with a deletion from psi beta 1 to at least 20 kb 3' to beta. The beta o-thal gene has the haplotype II restriction enzyme pattern and has the beta 39 nonsense mutation. Erythroid colonies from blood BFU-E were radiolabeled, and globin chains were separated by gel electrophoresis. Colonies from the beta o-thal heterozygote had non-alpha/alpha ratios more balanced than in the reticulocytes. Gamma synthesis was 11% of non-alpha, which is higher than in reticulocytes, but within the range seen in normal adult colonies. Both HPFH heterozygotes produced 20%-30% gamma in erythroid colonies as well as reticulocytes, although non-alpha/alpha was more balanced in the colonies. The HPFH/beta o-thal patient produced 100% gamma in reticulocytes and in colonies. G gamma and gamma-synthetic proportions were not correlated at the individual colony level in the heterozygotes, suggesting that they had "adult" and not "fetal" progenitor cells. The Hb expression of these adult progenitors is presumably modulated normally in vivo in beta o-thal, but the normal decrease in HbF production does not occur in gene deletion HPFH.

Five patients with sickle cell anemia were treated with hydroxyurea (HU), in hopes of augmenting their production of fetal hemoglobin. Laboratory responses in two patients treated for more than 2 years were encouraging and there were suggestions of clinical improvement. Long-term HU therapy should be considered for severely affected adults with sickle cell anemia who are willing to accept what is probably a small risk of carcinogenesis. Preliminary chromosomal analysis and knowledge of the clastogenic properties of HU suggest that conception and pregnancy should be avoided. Pharmacokinetic studies will probably be necessary to adjust individual dosage schedules so that cytotoxicity is avoided. F cell responses can be seen in 2 to 3 weeks if the HU dose is optimal, but establishment of a large number of F cells in the circulation may take a month or longer.

Initial alterations in fetal hemoglobin (HbF) production among eight sickle cell anemia subjects treated with hydroxyurea (Hu) are summarized. Four of these subjects had been previously treated with 5-azacytidine (5-aza). All subjects treated with Hu (50 mg/kg/d for three to five days) had suppression of their total reticulocyte counts by seven days, whereas the four subjects previously treated with 5-aza (2 mg/kg/d for three to five days) had increased reticulocyte counts at day 7. The effect of Hu on increasing the number of HbF-containing reticulocytes (F reticulocytes) is extremely variable, ranging from ten-to less than onefold differences in maximal posttherapy v pretherapy levels. Recovery from marrow suppression did not result in greater than twofold increases in F reticulocyte counts. Mean day 7 F reticulocyte levels in the four subjects treated with both Hu and 5-aza were 4.1 X 10/microL and 15.4 X 10(4)/microL, respectively. Among Hu-treated subjects, increased F reticulocyte production was correlated with low serum creatinine levels and rapid removal of Hu from the plasma. Furthermore, suppression of CFU-E colony formation on day 2 of therapy with Hu was inversely correlated with maximal F reticulocyte response. We conclude that where Hu treatment results in marrow toxicity (decreased reticulocyte counts, decreased CFU-E colony formation) HbF production is less likely to increase. Those sickle cell anemia subjects with minimal renal dysfunction (serum creatinine level, greater than 1.0 mg/dL) exhibit the most cytotoxicity and least F reticulocyte response to Hu.

Fetal hemoglobin (HbF)-bearing reticulocytes (F reticulocytes) can be detected in peripheral blood by a modification of the microscopic single-cell radial immunodiffusion method. Thereby otherwise inappreciable changes in HbF production can readily be recognized. F reticulocyte frequencies are reporducibly measurable whenever the product of whole blood HbF-bearing red cell (F cell) frequency and reticulocyte frequency is approximately 5 X 10(-4) or greater. Serial analyses of F reticulocytes and nonreticulocyte F cells (F erythrocytes) illustrate that (1) levels of F reticulocytes and F erythrocytes are persistently similar in normal adults with more than 6% F cells and thus cell survival times of F and non-F cells must be essentially the same, (2) changing levels of F reticulocytes can be sensitive predictors of later changes in mature F cell frequencies during infancy and in adults recovering from aplastic anemia, and (3) alterations in F reticulocyte frequency and the amount of HbF per F reticulocyte are discordant in some settings but concordant in others.

Fetal and adult erythrocyte characteristics were studied serially in a 30-mo-old female with juvenile chronic myelocytic leukemia. On presentation the erythrocytes exhibited predominantly fetal characteristics as indicated by 69% hemoglobin F (HbF), 1.1% hemoglobin A2 (HbA2), absent I antigen, and fetal levels of the erythrocyte enzymes, carbonic anhydrase I and II, glucose-6-phosphate dehydrogenase, hexokinase, pyruvate kinase, and lactate dehydrogenase; 100% of the erythrocytes present contained HbF. However, Orskov-Jacobs-Stewart hemolysis demonstrated that at least one adult characteristic was present. Seven months later HbF was 17%; I antigen and carbonic anhydrase I had increased to adult levels. The number of cells containing HbF had decreased to 30%. Further studies indicated that at least three new populations of red cells were present after 7 mo which had not previously been detected. Two of these populations exhibited a mixture of both fetal and adult characteristics. Such findings suggested that an ongoing disturbance of regulatory mechanisms was responsible for the variable expression of fetal versus adult erythrocyte characteristics.

Levels of fetal hemoglobin (HbF) bearing reticulocytes (F reticulocytes) range from 2% to 50% in patients with sickle cell (SS) anemia. To learn whether any portion of such variation in F cell production is regulated by loci genetically separable from the beta-globin gene cluster, percentages of F reticulocytes were compared in 59 sib pairs composed solely of SS members, including 40 pairs from Jamaica and 19 from the United States. We reasoned that differences in F reticulocyte levels might arise (1) from any of several kinds of artifact, (2) via half-sib status, or (3) because one or more genes regulating F cell production segregate separately from beta S. We minimized the role of artifact by assay of fresh samples from 84 SS individuals, including both members of 38 sib pairs. In 78 of the 84 subjects, serial values for percent F reticulocytes fell within 99.9% confidence limits or were alike by t test (P greater than or equal to .05). This left 32 sib pairs for which F reticulocyte levels in each member were reproducible. When sib-sib comparisons were limited to these 32 pairs, percentages of F reticulocytes were grossly dissimilar within 12 Jamaican and 3 American sibships. Within them, the probability that sibs were alike was always less than or equal to .005 and usually less than or equal to 10(-4). We next minimized the contribution of half-sibs among Jamaicans by a combination of paternity testing and sib-sib comparison of beta-globin region DNA restriction fragment length polymorphisms, especially among discordant pairs. We thereafter concluded that at least seven to eight Jamaican pairs were composed of reproducibly discordant full sibs. There is thus little doubt that there are genes regulating between-patient differences in F cell production that are separate from the beta-globin gene cluster. Still unanswered is (1) whether or not these genes are actually linked to beta S, (2) why F reticulocyte levels in Americans tend to be lower than in Jamaicans, and (3) whether or not differences in F cell production among SS patients are regulated by several major loci or by only one.

Fetal hemoglobin (HbF) levels vary widely among individuals with sickle cell anemia (SS). Previous studies have suggested that HbF levels in SS individuals with alpha-thalassemia (two or three functional alpha-globin genes) are lower than HbF levels in SS individuals with four normal alpha-globin genes. Using immunocytochemical techniques, we studied F cell production as measured by % F reticulocytes, the amount of HbF per F cell, and the preferential survival of F cells versus non-F cells in 51 subjects with four alpha genes, 32 subjects with three alpha genes, and 18 subjects with two alpha genes. Comparison between alpha-globin gene groups was performed for the total sample as well as for a subset of 82 individuals who had replicate samples and a further subset of 39 age-matched individuals. %HbF levels were 6.8, 4.9, and 4.5 percent for the total four-, three-, and two-alpha-globin-gene groups, respectively. The percentage of F reticulocytes, percentage HbF per F cell, and the enrichment ratio (% F cell/% F reticulocytes) did not change significantly with alpha-globin gene number. Moreover, no correlation existed between alpha-globin gene number and the absolute number of F cells in any group studied. However, there was a strong inverse correlation (r = -0.407, P = .0001) between non-F cell levels (1.7 +/- 2, 2.2 +/- 5, 3.0 +/- 1.0 X 10(12)/L) and decreasing alpha-globin gene number. These data suggest that falling HbF levels among SS individuals with lessened numbers of alpha-globin genes reflect prolonged survival of non-F cells and are not due to intrinsic differences in F cell production or in the amount of HbF per F cell. The improved survival of non-F cells in SS alpha-thalassemia is presumed to be due to the lower MCHC observed in such individuals.