Effect of a group of genetic markers around the 5′ regulatory regions of the β globin gene cluster linked to high HbF on the clinical severity of β thalassemia

The clinical and hematological course of β thalassemia intermedia is influenced by a number of genetic factors which play a role in increasing fetal haemoglobin levels. Several polymorphisms located in the promoters of β and γ globin gene are involved in influencing the disease severity. Our objective was to study the effect of cis-DNA haplotypes, motifs, or polymorphisms (Pre G γ globin gene haplotypes, Aγ–δ intergenic region haplotypes XmnI and (AT)_x(T)_y polymorphisms, β-LCR HS2 and HS3 site motifs) that may contribute to higher HbF levels and a milder clinical course. We found that a combination of T haplotype of the Aγ–δ intergenic region, TAG Pre-Gγ haplotype, presence of the XmnI polymorphism along with the (AT)₉(T)₅ motif constitutes a topography that co-relates with raised HbF levels which may contribute in ameliorating the disease severity.     

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. Am. J. Hematol. 89:1019–1023, 2014. © 2014 Wiley Periodicals, Inc.     

Genetic modifiers of fetal hemoglobin affect the course of sickle cell disease in patients treated with hydroxyurea

The course of sickle cell disease (SCD) is modified by polymorphisms boosting fetal hemoglobin (HbF) synthesis. However, it has remained an open question how these polymorphisms affect patients who are treated with the HbF-inducing drug hydroxyurea/ hydroxycarbamide. The German SCD registry offers the opportunity to answer this question, because >90% of patients are treated according to national guidelines recommending the use of hydroxyurea in all patients above 2 years of age. We analyzed the modifying effect of HbF-related genetic polymorphisms in 417 patients with homozygous SCD >2 years old who received hydroxyurea. HbF levels were correlated with higher total hemoglobin levels, lower rates of hemolysis, a lower frequency of painful crises and of red blood cell transfusions. The minor alleles of the polymorphisms in the γ-globin promoter (rs7482144), BCL11A (rs1427407) and HMIP (rs66650371) were strongly associated with increased HbF levels. However, these associations did not translate into lower frequencies of vaso-occlusive events which did not differ between patients either carrying or not carrying the HMIP and BCL11A polymorphisms. Patients on hydroxyurea carrying the γ-globin promoter polymorphism demonstrated substantially higher hemoglobin levels (P<10^-4) but also higher frequencies of painful crises and hospitalizations (P<0.01) when compared to patients without this polymorphism. Taken together, these data indicate that the γ-globin, HMIP and BCL11A polymorphisms correlate with increased HbF in SCD patients on hydroxyurea. While HbF is negatively correlated with the frequency of painful crises and hospitalizations, this was not observed for the presence of known HbF-boosting alleles.     

Fetal haemoglobin induction in sickle cell disease

Fetal haemoglobin (HbF, α2γ2) induction has long been an area of investigation, as it is known to ameliorate the clinical complications of sickle cell disease (SCD). Progress in identifying novel HbF‐inducing strategies has been stymied by limited understanding of gamma (γ)–globin regulation. Genome‐wide association studies (GWAS) have identified variants in BCL11A and HBS1L‐MYB that are associated with HbF levels. Functional studies have established the roles of BCL11A, MYB, and KLF1 in γ–globin regulation, but this information has not yielded new pharmacological agents. Several drugs are under investigation in clinical trials as HbF‐inducing agents, but hydroxycarbamide remains the only widely used pharmacologic therapy for SCD. Autologous transplant of edited haematopoietic stem cells holds promise as a cure for SCD, either through HbF induction or correction of the causative mutation, but several technical and safety hurdles must be overcome before this therapy can be offered widely, and pharmacological therapies are still needed.     

A candidate transacting modulator of fetal hemoglobin gene expression in the Arab—Indian haplotype of sickle cell anemia

Fetal hemoglobin (HbF) levels are higher in the Arab–Indian (AI) β‐globin gene haplotype of sickle cell anemia compared with African‐origin haplotypes. To study genetic elements that effect HbF expression in the AI haplotype we completed whole genome sequencing in 14 Saudi AI haplotype sickle hemoglobin homozygotes—seven selected for low HbF (8.2% ± 1.3%) and seven selected for high HbF (23.5% ± 2.6%). An intronic single nucleotide polymorphism (SNP) in ANTXR1, an anthrax toxin receptor (chromosome 2p13), was associated with HbF. These results were replicated in two independent Saudi AI haplotype cohorts of 120 and 139 patients, but not in 76 Saudi Benin haplotype, 894 African origin haplotype and 44 AI haplotype patients of Indian origin, suggesting that this association is effective only in the Saudi AI haplotype background. ANTXR1 variants explained 10% of the HbF variability compared with 8% for BCL11A. These two genes had independent, additive effects on HbF and together explained about 15% of HbF variability in Saudi AI sickle cell anemia patients. ANTXR1 was expressed at mRNA and protein levels in erythroid progenitors derived from induced pluripotent stem cells (iPSCs) and CD34⁺ cells. As CD34⁺ cells matured and their HbF decreased ANTXR1 expression increased; as iPSCs differentiated and their HbF increased, ANTXR1 expression decreased. Along with elements in cis to the HbF genes, ANTXR1 contributes to the variation in HbF in Saudi AI haplotype sickle cell anemia and is the first gene in trans to HBB that is associated with HbF only in carriers of the Saudi AI haplotype. Am. J. Hematol. 91:1118–1122, 2016. © 2016 Wiley Periodicals, Inc.     

The influence of uridine diphosphate glucuronosyl transferase 1A promoter polymorphisms, beta-globin gene haplotype, co-inherited alpha-thalassemia trait and Hb F on steady-state serum bilirubin levels in sickle cell anemia

PURPOSE: Homozygosity for the (AT)7 allele of uridine diphosphate glucuronosyl transferase 1A (UGT1A1) gene polymorphism is associated with increased bilirubin levels in sickle cell anemia (SCA). In the present study, in addition to UGT1A1 promoter genotype, serum bilirubin level was related to other genetic modifiers -beta(S)-globin gene haplotype, Hb F, co-inherited alpha-thal trait, age and gender. METHODS: The patients were randomly selected from the sickle cell clinic, Medical College of Georgia. UGT1A1 promoter polymorphisms were determined using automated sequencing. Other investigations were with standard techniques. RESULTS: There were 67 SCA patients (41 males and 26 females), aged 2-44 yr (mean of 20.6 +/- 10.7). Ten (14.9%) patients were homozygous for the (AT)6 UGT1A1 allele, 35 (52.2%) were heterozygous for (AT)6 and (AT)7 alleles while 22 (32.8%) were homozygous for (AT)7. Serum bilirubin was significantly higher in the homozygous (AT)7 group (3.7 +/- 1.5, 3.8 +/- 2.3 and 5.6 +/- 2.4 mg/dL, respectively). It was also significantly higher in males than females and in patients aged >10 yr. There was a significant negative linear correlation (r = -0.304, P = 0.016) of serum bilirubin with Hb F. The beta-globin haplotype and co-existing alpha-thal trait did not have any significant influence on serum bilirubin levels. Patients on hydroxyurea were older, had lower Hb F, but higher mean serum bilirubin. The latter also was signifcantly higher among those with UGT1A1 (AT)7 homozygosity. CONCLUSIONS: Apart from UGT1A1 (AT)7 homozygosity, Hb F, age and gender are the other factors that significantly influence serum bilirubin level in SCA.     

Genetic variants at HbF‐modifier loci moderate anemia and leukocytosis in sickle cell disease in Tanzania

Fetal hemoglobin (HbF) is a recognized modulator of sickle cell disease (SCD) severity. HbF levels are strongly influenced by genetic variants at three major genetic loci, Xmn1‐HBG2, HMIP‐2, and BCL11A, but the effect of these loci on the hematological phenotype in SCD, has so far not been investigated. In a cohort of individuals with SCD in Tanzania (HbSS and HbS/β° thalassemia, n = 726, aged 5 or older), HbF levels were positively correlated with hemoglobin, red blood cell (RBC) indices, mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH), and negatively with white blood cell (WBC) and platelet counts (all P < 0.0001). We subsequently assessed the contribution of the three HbF modifier loci and detected diverse effects, including a reduction in anemia, leukocytosis, and thrombocytosis associated with certain HbF‐promoting alleles. The presence of the ‘T’ allele at Xmn1‐HBG2 led to a significant increase in hemoglobin (P = 9.8 × 10^?3) but no changes in cellular hemoglobin content. Xmn1‐HBG2 ‘T’ also has a weak effect decreasing WBC (P = 0.06) and platelet (P = 0.06) counts. The BCL11A variant (rs11886868‐‘C’) increases hemoglobin (P = 2 × 10^?3) and one of the HBS1L‐MYB variants decreases WBC values selectively (P = 2.3 × 10^?4). The distinct pattern of effects of each variant suggests that both, disease alleviation through increased HbF production, and ‘pleiotropic’ effects on blood cells, are involved, affecting a variety of pathways. Am. J. Hematol. 90:E1–E4, 2015. © 2014 Wiley Periodicals, Inc.     

Pharmacokinetics, pharmacodynamics, and pharmacogenetics of hydroxyurea treatment for children with sickle cell anemia

Hydroxyurea therapy has proven laboratory and clinical efficacies for children with sickle cell anemia (SCA). When administered at maximum tolerated dose (MTD), hydroxyurea increases fetal hemoglobin (HbF) to levels ranging from 10% to 40%. However, interpatient variability of percentage of HbF (%HbF) response is high, MTD itself is variable, and accurate predictors of hydroxyurea responses do not currently exist. HUSTLE (NCT00305175) was designed to provide first-dose pharmacokinetics (PK) data for children with SCA initiating hydroxyurea therapy, to investigate pharmacodynamics (PD) parameters, including HbF response and MTD after standardized dose escalation, and to evaluate pharmacogenetics influences on PK and PD parameters. For 87 children with first-dose PK studies, substantial interpatient variability was observed, plus a novel oral absorption phenotype (rapid or slow) that influenced serum hydroxyurea levels and total hydroxyurea exposure. PD responses in 174 subjects were robust and similar to previous cohorts; %HbF at MTD was best predicted by 5 variables, including baseline %HbF, whereas MTD was best predicted by 5 variables, including serum creatinine. Pharmacogenetics analysis showed single nucleotide polymorphisms influencing baseline %HbF, including 5 within BCL11A, but none influencing MTD %HbF or dose. Accurate prediction of hydroxyurea treatment responses for SCA remains a worthy but elusive goal.     

Bilirubin concentrations in thalassemia heterozygotes in university students

Objectives: To investigate the difference of bilirubin concentrations between α‐ and β‐thalassemia carriers and the role of variation status in the UDP‐glucuronosyltransferase (UGT) 1A1 gene on such a difference. Methods: A total of 2713 university freshmen who attended a regular physical examination were enrolled in underwent screenings for thalassemias. Finally, 123 subjects whose mean corpuscular volume was ≤80 fL and who had no iron deficiency anemia were tested by PCR and PCR–restriction fragment length polymorphism (RFLP) for α‐ and β‐thalassemias, respectively, and tested by PCR–RFLP for the five known variations of the UGT1A1 gene. Results: Among the 123 subjects, 76 and 47 were diagnosed with heterozygous α‐thalassemia and with heterozygous β‐thalassemia, respectively. Between the α‐ and β‐thalassemia heterozygotes, variation status of the UGT1A1 gene was not statistically different (P = 0.898), while hemoglobin and bilirubin concentrations differed significantly (P = 0.005 and 0.001, respectively). Bilirubin concentrations were significantly higher among individuals with compound heterozygous variations/homozygous variation in the UGT1A1 gene than in those possessing the wild type and heterozygous variation (P < 0.001 for both α‐ and β‐thalassemia heterozygotes). Compound heterozygous variations/homozygous variation in the UGT1A1 gene and anemia were the main causes of hyperbilirubinemia in α‐ and β‐thalassemia heterozygotes, respectively. Conclusions: The difference in bilirubin concentrations between α‐ and β‐thalassemia heterozygotes may be attributable to more bilirubin being produced in β‐thalassemia heterozygotes than in α‐thalassemia heterozygotes, while variation status of the UGT1A1 gene affects bilirubin concentrations in both α‐ and β‐thalassemia heterozygotes.     

Realizing effectiveness across continents with hydroxyurea: Enrollment and baseline characteristics of the multicenter REACH study in Sub‐Saharan Africa

Despite its well‐described safety and efficacy in the treatment of sickle cell anemia (SCA) in high‐income settings, hydroxyurea remains largely unavailable in sub‐Saharan Africa, where more than 75% of annual SCA births occur and many comorbidities exist. Realizing Effectiveness Across Continents with Hydroxyurea (REACH, ClinicalTrials.gov NCT01966731) is a prospective, Phase I/II open‐label trial of hydroxyurea designed to evaluate the feasibility, safety, and benefits of hydroxyurea treatment for children with SCA in four sub‐Saharan African countries. Following comprehensive training of local research teams, REACH was approved by local Ethics Committees and achieved full enrollment ahead of projections with 635 participants enrolled over a 30‐month period, despite half of families living >12 km from their clinical site. At enrollment, study participants (age 5.4 ± 2.4 years) had substantial morbidity, including a history of vaso‐occlusive pain (98%), transfusion (68%), malaria (85%), and stroke (6%). Significant differences in laboratory characteristics were noted across sites, with lower hemoglobin concentrations (P < .01) in Angola (7.2 ± 1.0 g/dL) and the DRC (7.0 ± 0.9 g/dL) compared to Kenya (7.4 ± 1.1 g/dL) and Uganda (7.5 ± 1.1 g/dL). Analysis of known genetic modifiers of SCA demonstrated a high frequency of α‐thalassemia (58.4% with at least a single α‐globin gene deletion) and G6PD deficiency (19.7% of males and 2.4% of females) across sites. The CAR β‐globin haplotype was present in 99% of participants. The full enrollment to REACH confirms the feasibility of conducting high‐quality SCA research in Africa; this study will provide vital information to guide safe and effective dosing of hydroxyurea for children with SCA living in Africa.     

rs11886868 and rs4671393 of BCL11A associated with HbF level variation and modulate clinical events among sickle cell anemia patients

Aims: Fetal hemoglobin (HbF) modulates the phenotype of sickle cell anemia (SCA) by inhibiting deoxy sickle hemoglobin (HbS) polymerization. HbF genes are genetically regulated, and the level of HbF and its distribution among sickle erythrocytes is highly variable. Herein, we aimed to determine whether two functional polymorphisms of BCL11A are implicated in the variation of HbF and clinical events in SCA Tunisian patients.

Material and methods: The studied population consisted of 148 SCA patients with SS phenotype. The group of patients was divided into two subgroups according to the threshold point of %HbF which is 15%. Genotyping of rs11886868 and rs4671393 was performed using PCR/Sequencing. To test for trait association with the candidate SNPs, genotype and allele frequencies between ‘group who had %HbF?<?15’ and ‘group who had %HbF >15’ (controls) were compared using Pearson's chi-square test (compare 2, version 1.02). The association of each genotype and the combined genotype with complications was performed by logistic regression test.

Results: Our findings showed that the majority of patients carried genotype CT of rs11886868 and genotypes AG and GG of rs4671393 present HbF level?<?15%. RR?=?0.08, RR?=?0.176, and RR?=?0.189, respectively. The results showed a significant association between the alleles T of rs11886868 and G of rs4671393 and %HbF?<?15% with P?=?0.016; RR?=?0.39 and P?=?8.9?×?10^?3: RR?=?0.567, respectively. Interestingly, the C allele of the rs11886868 and the A allele of the rs46713939 were associated with an ameliorated phenotype in patient's SCA. The combination of the genotypes GG and CT explains more phenotypic variance than the sum of the two BCL11A SNPs taken individually.     

Inactivation of HDAC1 or HDAC2 induces gamma globin expression without altering cell cycle or proliferation

Other than hydroxyurea, no pharmacologic agents are clinically available for fetal hemoglobin (HbF) induction in sickle cell disease (SCD). An optimal candidate would induce HbF without causing cell cycle inhibition and would act independently of hydroxyurea in order to yield additional HbF induction when combined. We explored whether inhibition of histone deacetylase (HDAC) 1 or HDAC2 could achieve these goals. In human erythroid progenitor cells, shRNA knockdown of the HDAC1 or HDAC2 genes induced gamma globin, without altering cellular proliferation in vitro, and without altering cell cycle phase. Treatment with hydroxyurea in combination with HDAC2 knockdown yielded a further increase in gamma globin expression. Additionally, when CD34+ cells were treated with both hydroxyurea and MS‐275 (an inhibitor of HDAC 1, 2, and 3), an additive induction of relative gamma globin expression was achieved. Our findings support further clinical investigation of HDAC inhibitors in combination with hydroxyurea in SCD patients. Am. J. Hematol. 90:624–628, 2015. © 2015 Wiley Periodicals, Inc.     

Alpha thalassemia and the hematology of homozygous sickle cell disease in childhood

alpha Thalassemia modifies the hematologic expression of homozygous sickle cell (SS) disease, resulting in increased total hemoglobin and HbA2 and decreased HbF, mean cell volume, reticulocytes, irreversibly sickled cells, and bilirubin levels. The age at which these changes develop in children with SS disease is unknown. Ascertainment of globin gene status in a large representative sample of children with SS disease has afforded an opportunity to study the hematologic indices in nine children homozygous for alpha thalassemia 2 (two-gene group), 90 children heterozygous for alpha thalassemia 2 (three-gene group), and 167 children with a normal alpha globin gene complement (four-gene group). The two-gene group had significantly lower mean cell volumes from birth, higher red cell counts from one month, lower reticulocytes from three months, and higher HbA2 levels from one year, as compared with the four-gene group. Children with three genes had intermediate indices but resembled more closely the four-gene group. Differences in total hemoglobin or in fetal hemoglobin between the groups were not apparent by eight years of age. The most characteristic differences of the two-gene group were the raised proportional HbA2 level and low mean cell volume, the latter having some predictive value for alpha thalassemia status at birth.     

Accumulation of γ-globin mRNA in human erythroid cells treated with angelicin

The aim of the present study was to determine whether angelicin is able to increase the expression of γ‐globin genes in human erythroid cells. Angelicin is structurally related to psoralens, a well‐known chemical class of photosensitizers used for their antiproliferative activity in treatment of different skin diseases (i.e., psoriasis and vitiligo). To verify the activity of angelicin, we employed two experimental cell systems, the human leukemic K562 cell line and the two‐phase liquid culture of human erythroid progenitors isolated from normal donors. The results of our investigation suggest that angelicin, compared with cytosine arabinoside, mithramycin and cisplatin, is a powerful inducer of erythroid differentiation and γ‐globin mRNA accumulation of human leukemia K562 cells. In addition, when normal human erythroid precursors were cultured in the presence of angelicin, increases of γ‐globin mRNA accumulation and fetal hemoglobin (HbF) production, even higher than those obtained using hydroxyurea, were detected. These results could have practical relevance, as pharmacologically‐mediated regulation of the expression of human γ‐globin genes, leading to HbF induction, is considered a potential therapeutic approach in hematological disorders, including β‐thalassemia and sickle cell anemia.     

BCL11A is a major HbF quantitative trait locus in three different populations with beta-hemoglobinopathies

Increased HbF levels or F-cell (HbF containing erythrocyte) numbers can ameliorate the disease severity of β-thalassemia major and sickle cell anemia. Recent genome-wide association studies reported that single nucleotide polymorphisms (SNPs) in BCL11A gene on chromosome 2p16.1 were correlated with F-cells among healthy northern Europeans, and HbF among Sardinians with β-thalassemias. In this study, we showed that SNPs in BCL11A were associated with F-cell numbers in Chinese with β-thalassemia trait, and with HbF levels in Thais with either β-thalassemia or HbE trait and in African Americans with sickle cell anemia. Taken together, the data suggest that the functional motifs responsible for modulating F-cells and HbF levels reside within a 3 kb region in the second intron of BCL11A.     

The linear effects of alpha-thalassaemia, the UGT1A1 and HMOX1 polymorphisms on cholelithiasis in sickle cell disease

Serum bilirubin levels and predisposition to gallstones in sickle cell disease (SCD) are influenced by genetic variation in the hepatic uridine diphosphate (UDP)-glucuronosyltransferase (UGT1A1) gene, but the association is not consistent. This study investigated whether variation in the gene encoding haem oxygenase (HMOX1), a rate-limiting enzyme upstream of UGT1A in the haem catabolic pathway, and alpha-thalassaemia could explain some of the inconsistent effects. The UGT1A1 [TA](n) and HMOX1 [GT](n) promoter polymorphisms and alpha globin genotypes were determined in 263 SCD patients (199 HbSS, 5 HbS/beta(0), 59 HbSC). Detection of gallstones was based on ultrasound of the liver/biliary tree. Regression analysis showed that serum bilirubin levels and the incidence of gallstones were strongly associated with the number of UGT1A1 [TA] repeats in all subjects (P < 0.0001 and P < 0.01, respectively). While HMOX1 genotype had no effect, co-inheritance of alpha-thalassaemia reduced serum bilirubin levels in all SCD patients independently of the number of UGT1A1 [TA] repeats. Each additional [TA] repeat is associated with an increase in mean serum bilirubin levels of 21% and cholelithiasis risk of 87% in SCD.     

A clinically meaningful fetal hemoglobin threshold for children with sickle cell anemia during hydroxyurea therapy

Hydroxyurea has proven clinical benefits and is recommended to be offered to all children with sickle cell anemia (SCA), but the optimal dosing regimen remains controversial. Induction of red blood cell fetal hemoglobin (HbF) by hydroxyurea appears to be dose‐dependent. However, it is unknown whether maximizing HbF% improves clinical outcomes. HUSTLE (NCT00305175) is a prospective observational study with a primary goal of describing the long‐term clinical effects of hydroxyurea escalated to maximal tolerated dose (MTD) in children with SCA. In 230 children, providing 610 patient‐years of follow up, the mean attained HbF% at MTD was >20% for up to 4 years of follow‐up. When HbF% values were ≤20%, children had twice the odds of hospitalization for any reason (P < .0001), including vaso‐occlusive pain (P < .01) and acute chest syndrome (ACS) (P < .01), and more than four times the odds of admission for fever (P < .001). Thirty day readmission rates were not affected by HbF%. Neutropenia (ANC <1000 × 10⁶/L) was rare (2.3% of all laboratory monitoring), transient, and benign. Therefore, attaining HbF >20% was associated with fewer hospitalizations without significant toxicity. These data support the use of hydroxyurea in children, and suggest that the preferred dosing strategy is one that targets a HbF endpoint >20%.     

Influence of Xmn 1Gγ (HBG2 c.-211 C → T) Globin Gene Polymorphism on Phenotype of Thalassemia Patients of North India

Beta (β) thalassemia is the most common single gene disorder in India. It has been reported that in patients with β-thalassemia in the presence of Xmn 1^Gγ polymorphic site the level of fetal hemoglobin (HbF) is increased thereby reducing the severity of disease. To determine the prevalence of Xmn 1^Gγ polymorphic site and its effect on the clinical phenotype and HbF level in 39 β-thalassemia major and 62 thalassemia intermedia patients, along with response to hydroxyurea therapy in thalassemia intermedia cases. Status of Xmn 1^Gγ polymorphism was determined by polymerase chain reaction-restricted fragment length polymorphism procedure. The HbF level was determined using high performance liquid chromatography. Genotypes and allele frequencies of the Xmn 1^Gγ polymorphism did not vary significantly between the various thalassemia groups. HbF levels were observed to be significantly increased and age at presentation was significantly greater in presence of Xmn 1^Gγ polymorphic site on both alleles as compared to its absence in thalassemia major but not in thalassemia intermedia cases. The response of hydroxyurea in thalassemia intermedia was found only in a few patients irrespective of their Xmn 1^Gγ status. Xmn 1^Gγ polymorphisms appear to significantly influence HbF levels and age at presentation in thalassemia major but not in thalassemia intermedia patients. Small numbers precluded a definitive correlation of the polymorphism with response to hydroxyurea therapy.