Editorials and Perspectives Beta-thalassemia: from genotype to phenotype Fabrice Danjou, Franco Anni, Renzo Galanello Dipartimento di Scienze Biomediche e Biotecnologie, Università di Cagliari, Ospedale Regionale Microcitemie ASL8, Cagliari, Italy E-mail: [email protected] doi:10.3324/haematol.2011.055962 (Related Original Article on page 1712) eta-thalassemias are heterogeneous autosomal a chain imbalance as a consequence of reduced a chain recessive hereditary anemias characterized by output. 4 Breduced or absent b globin chain synthesis. The A substantial decrease in a/non- a chain imbalance can resulting relative excess of unbound a globin chains pre - also be obtained through the coinheritance of genetic cipitate in erythroid precursors in the bone marrow, lead - determinants able to sustain a continuous production of ing to their premature death and, hence, to ineffective gamma chains which, binding the excess a chains, result erythropoiesis. b-thalassemia phenotypes are variable, in a persistent fetal hemoglobin (Hb F) production meas - ranging from the severe transfusion dependent tha - urable in adult life. In delta- b0 thalassemia, this ability is lassemia major to the mild form of thalassemia interme - due to deletions of variable extent within the b globin dia. Patients with the major form of the disease have cluster, 5 while in other cases it depends on the co-trans - severe anemia, microcytic and hypochromic anemia, mission of point mutations at A-gamma or G-gamma hepatosplenomegaly, and usually come to medical atten - promoters (−196 C →T A-gamma; −158 C →T G- tion within the first two years of life. Without treatment, gamma). 6,7 A mild phenotype may also be determined by affected children have severely compromised growth and coinheritance of genetic determinants associated with development and shortened life expectancy. Treatment increased gamma chain production mapping outside the with a regular transfusion program and chelation therapy, b globin cluster. Different polymorphisms at the BCL11A aimed at reducing transfusional iron overload, allows for gene on 2p16.1 8 and HBS1L-MYB intergenic region on normal growth and development and extends life 6q23.3 have been described. 9,10 Several polymorphisms expectancy into the third to fifth decade. 1 Individuals within intron 2 of the BCL11A gene have been strongly with thalassemia intermedia present later in life, have associated with Hb F levels: rs766432, rs4671393, milder anemia (that never or only rarely requires transfu - rs1427407 and rs11886868, all in high linkage disequilib - sion), liver and spleen enlargement, typical bone modifi - rium (LD) with each other. 11-15 Other independent signals cations, and mild to moderate jaundice. 2 Occasionally in the same area were also identified with rs10189857 patients with thalassemia intermedia are completely and rs7599488, in high LD, as well as rs7606173 and asymptomatic until adult life with only mild anemia. The rs6706648, 13 also in high LD with each other. 12,13 In the major and intermedia forms of the disease are the two HBS1L-MYB intergenic region, different SNPs have been extremes of a wide range of clinical variability. Each described as being associated with Hb F variations in dif - group includes a continuous scale of severity, as demon - ferent studies: rs9399137, as well as rs4895441, strated by the variability in age at which thalassemia rs9402686 and rs28384513. 10,12 However, evidence has major patients need transfusion; from months to years of been reported of other contributing loci that have not life. been validated in recent genome-wide association stud - Beta -thalassemias are also very heterogeneous at the ies, such as the 8q26-28 and Xp22.2–22.3 loci. 16,17 molecular level, with more than 200 disease-causing Together with these discoveries, the interest for predic - mutations so far identified; a complete updated list is tion of Hb F levels and b-thalassemia phenotype has nat - available at the Globin Gene Server Web Site - http://glo - urally grown in recent years, and the three loci previously bin.cse.psu.edu/. In most cases, mutations are single mentioned have now been reported to be responsible for nucleotide substitutions, deletions or insertions of single 20 to 50% of the Hb F trait variance in patients with b- nucleotides or small oligonucleotides leading to thalassemia or sickle cell disease, and in healthy frameshift. Their diversity and the consequent variable Europeans. 18 Meanwhile, Galanello et al. reported the degree of globin chain imbalance are the main determi - impact of variants in the BCL11A and HBS1L-MYB loci nants for milder phenotypes, the coinheritance of together with a gene defects on the clinical severity of homozygosity or compound heterozygosity for mild b- b0-39-thalassemia, quantifying their overall contribution thalassemia alleles being responsible for a consistent resid - to 75% of the variation differences between b0-39- ual output of b chains from the affected b globin locus. thalassemia major and intermedia phenotypes. 14 However, much of the phenotypic variability is also The work by Badens et al. , presented in this issue of the explained by other genetic determinants capable of reduc - Journal, extends previous studies by integrating the −158 ing the a/non- a chain imbalance thereby resulting in a C→T G-gamma polymorphism and b0/ b+ status, in addi - lesser degree of a chain precipitation. 3 tion to rs11886868 in the BCL11A gene, rs9389268 in the One of the first discovered mechanisms able to reduce HBS1L-MYB intergenic region and a globin genes this imbalance is the coinheritance with homozygous b- defects, to define b-thalassemia severity. 19 Multivariate thalassemia of a-thalassemia determinant. In this case, analysis including these five genetic modifiers was car - the severity of the clinical phenotype correlates with the ried out and an accurate prediction has been made regard - a globin chain deficiency and with the improved a/non- ing major/intermedia status in more than 80% of haematologica | 2011; 96(11) 1573 Editorials and Perspectives patients. The heterogeneity of this 106 patient cohort, Pediatric Clinic II of the Thalassemia Unit at Ospedale with thirty different b globin gene mutations, might Regionale per le Microcitemie (a WHO Collaborating Center introduce variability not accountable for in the model, but for Community Control of Hereditary Disease) and of the it certainly provides a large amount of information on the Department Biomedical Sciences and Biotechnologies, Cagliari, prediction ability of the b0/ b+ status. Also, while it is Italy. His main research interests are the clinical aspects and likely that future studies will better define the genetic molecular genetics of thalassemias and other hemoglo - polymorphisms that modulate the effect of the BCL11A binopathies, glucose-6-phosphate dehydrogenase deficiency, and HBS1L-MYB intergenic region loci and eventually and Gilbert syndrome. Over the last ten years he has been the uncover causal variants, results from Badens et al. already principal investigator of several clinical trials of new oral iron provide clinically relevant information to practitioners, chelators. He is author of over 210 peer reviewed publications clarifying the impact of genetic modifiers on the clinical and is co-author of the books “Prevention of thalassemias and severity of the disease. Furthermore, future studies will other haemoglobin disorders” (vols. 1 and 2), edited by the probably expand this predictive ability, including the Thalassemia International Federation, and of the chapter “The effect of the different strongest independent predictors Thalassemias” in Wintrobe’s Clinical Hematology book (11th known to date for each gene (or even identified causal edition). variants), and will eventually relate genetic modifiers to a more detailed measurement of clinical severity. Financial and other disclosures provided by the author using Very few other complex disease phenotypes can be the ICMJE (www.icmje.org) Uniform Format for Disclosure of explained in such depth, and prediction of patient risk as Competing Interests are available with the full text of this paper a function of their personal genetic background already at www.haematologica.org. offers support in clinical settings for b-thalassemia as opposed to most complex diseases. Extended molecular References diagnosis can be carried out in patients affected by 1. Borgna-Pignatti C, Rugolotto S, De Stefano P, Zhao H, Cappellini homozygous b-thalassemia to define their genotype for MD, Del Vecchio GC, et al. Survival and complications in patients different modifiers and to better understand their pheno - with thalassemia major treated with transfusion and deferoxamine. typic modulation abilities. Before long, b-thalassemia Haematologica. 2004;89(10):1187-93. modifiers should allow us to redefine, on a genetic basis, 2. Weatherall DJ, Clegg JB. The thalassemia syndromes. 4th Ed, Blackwell Science Ltd. 2001. the phenotypic definition actually in use. 3. Cao A, Galanello R, Rosatelli MC. Genotype-phenotype correlations Linkage analysis and genome-wide association studies in beta-thalassemias. Blood Rev. 1994;8(1):1-12. have greatly contributed to such results, and next genera - 4. Galanello R, Cao A. Relationship between genotype and phenotype. Thalassemia intermedia. Ann N Y Acad Sci. 1998;850:325-33. tion sequencing might further improve prediction ability 5. Loudianos G, Cao A, Ristaldi MS, Pirastu M, Tzeti M, Kannavakis E, and eventually guide the development of new therapies. et al. Molecular basis of delta beta-thalassemia with normal fetal At present, recent studies on Hb F modifier genes have hemoglobin level. Blood. 1990;75(2):526-8. 6. Tasiopoulou M, Boussiou M, Sinopoulou K, Moraitis G, Loutradi- produced mixed results: while a 3bp deletion associated Anagnostou A, Karababa P. G gamma-196 C-->T, A gamma-201 C-- with Hb F levels has been recently identified in the >T: two novel mutations in the promoter region of the gamma-globin HBS1L-MYB intergenic region, 20 no such results have genes associated with nondeletional hereditary persistence of fetal been obtained with the sequencing of the BCL11A gene.