Guillaume Lettre

Human Fetal Hemoglobin Expression Is Regulated by the Developmental Stage- Specific Repressor BCL11A

Differences in the amount of fetal hemoglobin (HbF) that persists into adulthood affect the severity of sickle cell disease and the β-thalassemia syndromes. Genetic association studies have identified sequence variants in the gene BCL11A that influence HbF levels. Here, we examine BCL11A as a potential regulator of HbF expression. The high-HbF BCL11A genotype is associated with reduced BCL11A expression. Moreover, abundant expression of full-length forms of BCL11A is developmentally restricted to adult erythroid cells. Down-regulation of BCL11A expression in primary adult erythroid cells leads to robust HbF expression. Consistent with a direct role of BCL11A in globin gene regulation, we find that BCL11A occupies several discrete sites in the β-globin gene cluster. BCL11A emerges as a therapeutic target for reactivation of HbF in β-hemoglobin disorders.

Identification of ten loci associated with height highlights new biological pathways in human growth

Height is a classic polygenic trait, reflecting the combined influence of multiple as-yet-undiscovered genetic factors. We carried out a meta-analysis of genome-wide association study data of height from 15,821 individuals at 2.2 million SNPs, and followed up the strongest findings in >10,000 subjects. Ten newly identified and two previously reported loci were strongly associated with variation in height (P values from 4 × 10−7 to 8 × 10−22). Together, these 12 loci account for ∼2% of the population variation in height. Individuals with ≤8 height-increasing alleles and ≥16 height-increasing alleles differ in height by ∼3.5 cm. The newly identified loci, along with several additional loci with strongly suggestive associations, encompass both strong biological candidates and unexpected genes, and highlight several pathways (let-7 targets, chromatin remodeling proteins and Hedgehog signaling) as important regulators of human stature. These results expand the picture of the biological regulation of human height and of the genetic architecture of this classical complex trait.

Genome-wide association study shows BCL11A associated with persistent fetal hemoglobin and amelioration of the phenotype of -thalassemia

β-Thalassemia and sickle cell disease both display a great deal of phenotypic heterogeneity, despite being generally thought of as simple Mendelian diseases. The reasons for this are not well understood, although the level of fetal hemoglobin (HbF) is one well characterized ameliorating factor in both of these conditions. To better understand the genetic basis of this heterogeneity, we carried out genome-wide scans with 362,129 common SNPs on 4,305 Sardinians to look for genetic linkage and association with HbF levels, as well as other red blood cell-related traits. Among major variants affecting HbF levels, SNP rs11886868 in the BCL11A gene was strongly associated with this trait (P < 10−35). The C allele frequency was significantly higher in Sardinian individuals with elevated HbF levels, detected by screening for β-thalassemia, and patients with attenuated forms of β-thalassemia vs. those with thalassemia major. We also show that the same BCL11A variant is strongly associated with HbF levels in a large cohort of sickle cell patients. These results indicate that BCL11A variants, by modulating HbF levels, act as an important ameliorating factor of the β-thalassemia phenotype, and it is likely they could help ameliorate other hemoglobin disorders. We expect our findings will help to characterize the molecular mechanisms of fetal globin regulation and could eventually contribute to the development of new therapeutic approaches for β-thalassemia and sickle cell anemia.

Common variants in the GDF5-UQCC region are associated with variation in human height

Identifying genetic variants that influence human height will advance our understanding of skeletal growth and development. Several rare genetic variants have been convincingly and reproducibly associated with height in mendelian syndromes, and common variants in the transcription factor gene HMGA2 are associated with variation in height in the general population. Here we report genome-wide association analyses, using genotyped and imputed markers, of 6,669 individuals from Finland and Sardinia, and follow-up analyses in an additional 28,801 individuals. We show that common variants in the osteoarthritis-associated locus GDF5-UQCC contribute to variation in height with an estimated additive effect of 0.44 cm (overall P < 10−15). Our results indicate that there may be a link between the genetic basis of height and osteoarthritis, potentially mediated through alterations in bone growth and development.

Autoimmune diseases: insights from genome-wide association studies

Autoimmune diseases occur when an individuals own immune system attacks and destroys his or her healthy cells and tissues. Although it is clear that environmental stimuli can predispose someone to develop autoimmune diseases, twin- and family-based studies have shown that genetic factors also play an important role in modifying disease risk. Because many of these diseases are relatively common (prevalence in European-derived populations: 0.01-1%) and exhibit a complex mode of inheritance, many DNA sequence variants with modest effect on disease risk contribute to the genetic burden. Recently, the completion of the HapMap project, together with the development of new genotyping technologies, has given human geneticists the tools necessary to comprehensively, and in an unbiased manner, search our genome for DNA polymorphisms associated with many autoimmune diseases. Here we review recent progress made in the identification of genetic risk factors for celiac disease, Crohns disease, multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus and type-1 diabetes using genome-wide association studies (GWAS). Strikingly, GWAS have increased the number of genetic risk variants associated with these autoimmune diseases from 15 before 2006 to 68 now. We summarize what this new genetic landscape teaches us in terms of the pathogenesis of these diseases, and highlight some of the outstanding challenges ahead. Finally, we open a discussion on ways to best maximize the impact of these genetic discoveries where it matters the most, that is for autoimmune disease patients.

Candidate gene association resource (CARe): design, methods, and proof of concept.

Background— The National Heart, Lung, and Blood Institutes Candidate Gene Association Resource (CARe), a planned cross-cohort analysis of genetic variation in cardiovascular, pulmonary, hematologic, and sleep-related traits, comprises >40 000 participants representing 4 ethnic groups in 9 community-based cohorts. The goals of CARe include the discovery of new variants associated with traits using a candidate gene approach and the discovery of new variants using the genome-wide association mapping approach specifically in African Americans.nnMethods and Results— CARe has assembled DNA samples for >40 000 individuals self-identified as European American, African American, Hispanic, or Chinese American, with accompanying data on hundreds of phenotypes that have been standardized and deposited in the CARe Phenotype Database. All participants were genotyped for 7 single-nucleotide polymorphisms (SNPs) selected based on prior association evidence. We performed association analyses relating each of these SNPs to lipid traits, stratified by sex and ethnicity, and adjusted for age and age squared. In at least 2 of the ethnic groups, SNPs near CETP , LIPC , and LPL strongly replicated for association with high-density lipoprotein cholesterol concentrations, PCSK9 with low-density lipoprotein cholesterol levels, and LPL and APOA5 with serum triglycerides. Notably, some SNPs showed varying effect sizes and significance of association in different ethnic groups.nnConclusions— The CARe Pilot Study validates the operational framework for phenotype collection, SNP genotyping, and analytic pipeline of the CARe project and validates the planned candidate gene study of ≈2000 biological candidate loci in all participants and genome-wide association study in ≈8000 African American participants. CARe will serve as a valuable resource for the scientific community.Background—The National Heart, Lung, and Blood Institute’s Candidate Gene Association Resource (CARe), a planned cross-cohort analysis of genetic variation in cardiovascular, pulmonary, hematologic, and sleep-related traits, comprises >40 000 participants representing 4 ethnic groups in 9 community-based cohorts. The goals of CARe include the discovery of new variants associated with traits using a candidate gene approach and the discovery of new variants using the genome-wide association mapping approach specifically in African Americans. Methods and Results—CARe has assembled DNA samples for >40 000 individuals self-identified as European American, African American, Hispanic, or Chinese American, with accompanying data on hundreds of phenotypes that have been standardized and deposited in the CARe Phenotype Database. All participants were genotyped for 7 single-nucleotide polymorphisms (SNPs) selected based on prior association evidence. We performed association analyses relating each of these SNPs to lipid traits, stratified by sex and ethnicity, and adjusted for age and age squared. In at least 2 of the ethnic groups, SNPs near CETP, LIPC, and LPL strongly replicated for association with high-density lipoprotein cholesterol concentrations, PCSK9 with low-density lipoprotein cholesterol levels, and LPL and APOA5 with serum triglycerides. Notably, some SNPs showed varying effect sizes and significance of association in different ethnic groups. Conclusions—The CARe Pilot Study validates the operational framework for phenotype collection, SNP genotyping, and analytic pipeline of the CARe project and validates the planned candidate gene study of ≈2000 biological candidate loci in all participants and genome-wide association study in ≈8000 African American participants. CARe will serve as a valuable resource for the scientific community.

Developmental cell biology: Developmental apoptosis in C. elegans : a complex CEDnario

Apoptosis, an evolutionarily conserved programme of cellular self-destruction, is essential for the development and survival of most multicellular animals. It is required to ensure functional organ architecture and to maintain tissue homeostasis. During development of the simple nematode Caenorhabditis elegans, apoptosis claims over 10% of the somatic cells that are generated ? these cells were healthy but unnecessary. Exciting insights into the regulation and execution of apoptosis in C. elegans have recently been made. These new findings will undoubtedly influence our perception of developmental apoptosis in more complex species, including humans.

Common genetic variation in eight genes of the GH/IGF1 axis does not contribute to adult height variation.

Stature (adult height) is one of the most heritable human traits, yet few genes, if any, have been convincingly associated with adult height variation in the general population. Here, we selected 150 tag SNPs from eight candidate genes in the growth hormone (GH)/insulin-like growth factor-1 (IGF1) axis (GHR, GHRH, GHRHR, IGF1, IGFALS, IGFBP3, JAK2, STAT5B), and genotyped them in ∼2,200 individuals ascertained for short or tall stature. Nominally significant tag SNPs were then tested in three additional replication cohorts, including a family-based panel to rule out spurious associations owing to population stratification. Across the four height cohorts (Nxa0=xa06,075 individuals), we did not observe any consistent associations between stature and common variants (≥5% minor allele frequency) in these eight genes, including a common deletion of the growth hormone receptor gene exon 3. Tests of epistatic interactions between these genes did not yield any results beyond those expected by chance. Although we have not tested all genes in the GH/IGF1 axis, our results indicate that common variation in these GH/IGF1 axis genes is not a major determinant of stature, and suggest that if common variation contributes to adult height variation in the general population, the variants are in other, possibly unanticipated genes.

Progress in Genome-Wide Association Studies of Human Height

Human height (stature) is a strongly genetic trait, with up to 90% of the variation in height within a population determined by a combination of multiple inherited factors. Recent advances in genetics and genomics now permit comprehensive genome-wide surveys of common genetic variations in those variants that are associated with stature. The first such studies have borne fruit, identifying over 40 genetic loci that can be reproducibly shown to have an influence on adult height. These unbiased searches throughout the genome identified several loci that also harbour rare mutations responsible for more severe alterations in height or skeletal growth. Although the predictive value of the common variants thus far discovered remains low, the identification of these loci has led to new insights into the biology of human growth, and may help identify genes that underlie previously uncharacterized syndromes of abnormal skeletal growth.

Promoter polymorphisms and allelic imbalance in ABCB1 expression

Objective The ABCB1 (MDR1) gene, encoding the transporter P-glycoprotein, is known to act on a broad range of prescription medicines. For this reason a large number of studies have assessed the functional consequences of variation in this gene. Particular attention has focused on the ABCB1_3435C>T polymorphism, an exonic variant resulting in a synonymous change. This variant has been associated with mRNA, protein and serum levels, and with responses to a number of medicines. The results of association studies have, however, been variable and it is not currently clear whether this polymorphism is functional or is in linkage disequilibrium with functionally distinct alleles. Results To identify functional variation in the ABCB1 gene we assessed allelic imbalance by pyrosequencing cDNA from 80 lymphoblastoid B cell lines from the Centre dEtude du Polymorphisme Humain (CEPH) collection, including 74 individuals heterozygous for 3435C>T. We found that the degree of ABCB1 allelic imbalance differed among B-cell lines. In an effort to fine-map variants that influence the proportion of the two allelic mRNA species we genotyped representative common variations near the 3435C>T polymorphism by using a tagging single nucleotide polymorphism (SNP) approach. In one approach, we assessed in segregating families the impact of cis-acting variants on mRNA levels by using allelic imbalance as the phenotype in a regression analysis that distinguishes the coupling arrangements (phase) among alleles. In a second approach, we assessed allelic imbalance levels in lymphoblastoid B-cell lines from unrelated HapMap individuals, and performed an association using tagSNPs in a 5-Mb region surrounding the gene. Two potential cis-acting variants, a promoter rs28656907/rs28373093 dinucleotide polymorphism (P=0.007) and the rs10245483 SNP (P=0.0003) located 2u2009Mb upstream from the gene, were predictors of ABCB1 expression. Conclusions The study outlines a general experimental approach for fine mapping gene variants that influence mRNA expression by using cultured cell lines.