Genome-wide Association Study Identifies Four Genetic Loci Associated with Thyroid Volume and Goiter Risk
2011; Elsevier BV; Volume: 88; Issue: 5 Linguagem: Inglês
10.1016/j.ajhg.2011.04.015
ISSN1537-6605
AutoresAlexander Teumer, Rajesh Rawal, Georg Homuth, Florian Ernst, Margit Heier, Matthias Evert, Frank Dombrowski, Uwe Völker, Matthias Nauck, Dörte Radke, Till Ittermann, Reiner Biffar, Angela Döring, Christian Gieger, Norman Klopp, H.‐Erich Wichmann, Henri Wallaschofski, Christa Meisinger, Henry Völzke,
Tópico(s)Thyroid Disorders and Treatments
ResumoThyroid disorders such as goiters represent important diseases, especially in iodine-deficient areas. Sibling studies have demonstrated that genetic factors substantially contribute to the interindividual variation of thyroid volume. We performed a genome-wide association study of this phenotype by analyzing a discovery cohort consisting of 3620 participants of the Study of Health in Pomerania (SHIP). Four genetic loci were associated with thyroid volume on a genome-wide level of significance. Of these, two independent loci are located upstream of and within CAPZB, which encodes the β subunit of the barbed-end F-actin binding protein that modulates actin polymerization, a process crucial in the colloid engulfment during thyroglobulin mobilization in the thyroid. The third locus marks FGF7, which encodes fibroblast growth factor 7. Members of this protein family have been discussed as putative signal molecules involved in the regulation of thyroid development. The fourth locus represents a "gene desert" on chromosome 16q23, located directly downstream of the predicted coding sequence LOC440389, which, however, had already been removed from the NCBI database as a result of the standard genome annotation processing at the time that this study was initiated. Experimental proof of the formerly predicted mature mRNA, however, demonstrates that LOC440389 indeed represents a real gene. All four associations were replicated in an independent sample of 1290 participants of the KORA study. These results increase the knowledge about genetic factors and physiological mechanisms influencing thyroid volume. Thyroid disorders such as goiters represent important diseases, especially in iodine-deficient areas. Sibling studies have demonstrated that genetic factors substantially contribute to the interindividual variation of thyroid volume. We performed a genome-wide association study of this phenotype by analyzing a discovery cohort consisting of 3620 participants of the Study of Health in Pomerania (SHIP). Four genetic loci were associated with thyroid volume on a genome-wide level of significance. Of these, two independent loci are located upstream of and within CAPZB, which encodes the β subunit of the barbed-end F-actin binding protein that modulates actin polymerization, a process crucial in the colloid engulfment during thyroglobulin mobilization in the thyroid. The third locus marks FGF7, which encodes fibroblast growth factor 7. Members of this protein family have been discussed as putative signal molecules involved in the regulation of thyroid development. The fourth locus represents a "gene desert" on chromosome 16q23, located directly downstream of the predicted coding sequence LOC440389, which, however, had already been removed from the NCBI database as a result of the standard genome annotation processing at the time that this study was initiated. Experimental proof of the formerly predicted mature mRNA, however, demonstrates that LOC440389 indeed represents a real gene. All four associations were replicated in an independent sample of 1290 participants of the KORA study. These results increase the knowledge about genetic factors and physiological mechanisms influencing thyroid volume. From the clinical and the public-health point of view, thyroid disorders such as nontoxic and toxic goiter are relevant diseases in previously and currently iodine-deficient areas. Whereas goiters are highly prevalent in iodine-deficient regions, it is less commonly present in iodine-replete areas.1Laurberg P. Pedersen K.M. Hreidarsson A. Sigfusson N. Iversen E. Knudsen P.R. Iodine intake and the pattern of thyroid disorders: a comparative epidemiological study of thyroid abnormalities in the elderly in Iceland and in Jutland, Denmark.J. Clin. Endocrinol. Metab. 1998; 83: 765-769Crossref PubMed Scopus (275) Google Scholar, 2Teng W. Shan Z. Teng X. Guan H. Li Y. Teng D. Jin Y. Yu X. Fan C. Chong W. et al.Effect of iodine intake on thyroid diseases in China.N. Engl. J. Med. 2006; 354: 2783-2793Crossref PubMed Scopus (528) Google Scholar The effect of iodine deficiency on goiter risk is pronounced by cigarette smoking, whereas this association is not present in regions with optimal iodine supply.3Völzke H. Schwahn C. Kohlmann T. Kramer A. Robinson D.M. John U. Meng W. Risk factors for goiter in a previously iodine-deficient region.Exp. Clin. Endocrinol. Diabetes. 2005; 113: 507-515Crossref PubMed Scopus (36) Google Scholar, 4Ittermann T. Schmidt C.O. Kramer A. Below H. John U. Thamm M. Wallaschofski H. Völzke H. Smoking as a risk factor for thyroid volume progression and incident goiter in a region with improved iodine supply.Eur. J. Endocrinol. 2008; 159: 761-766Crossref PubMed Scopus (20) Google Scholar, 5Knudsen N. Laurberg P. Perrild H. Bülow I. Ovesen L. Jørgensen T. Risk factors for goiter and thyroid nodules.Thyroid. 2002; 12: 879-888Crossref PubMed Scopus (167) Google Scholar Additional environmental factors include gender, age, and body mass index.3Völzke H. Schwahn C. Kohlmann T. Kramer A. Robinson D.M. John U. Meng W. Risk factors for goiter in a previously iodine-deficient region.Exp. Clin. Endocrinol. Diabetes. 2005; 113: 507-515Crossref PubMed Scopus (36) Google Scholar, 5Knudsen N. Laurberg P. Perrild H. Bülow I. Ovesen L. Jørgensen T. Risk factors for goiter and thyroid nodules.Thyroid. 2002; 12: 879-888Crossref PubMed Scopus (167) Google Scholar There is no doubt that genetic factors also play a substantial role in the etiology of simple goiter.6Brix T.H. Kyvik K.O. Hegedüs L. Major role of genes in the etiology of simple goiter in females: a population-based twin study.J. Clin. Endocrinol. Metab. 1999; 84: 3071-3075PubMed Google Scholar, 7Hansen P.S. Brix T.H. Bennedbaek F.N. Bonnema S.J. Kyvik K.O. Hegedüs L. Genetic and environmental causes of individual differences in thyroid size: a study of healthy Danish twins.J. Clin. Endocrinol. Metab. 2004; 89: 2071-2077Crossref PubMed Scopus (75) Google Scholar Sibling studies from Denmark, a region with previously mild to moderate iodine deficiency, demonstrated a higher intraclass correlation for thyroid volume in monozygotic twins as compared to dizygotic twins, suggesting that genetic factors account for approximately 61%–78% of the interindividual variation of the thyroid volume.7Hansen P.S. Brix T.H. Bennedbaek F.N. Bonnema S.J. Kyvik K.O. Hegedüs L. Genetic and environmental causes of individual differences in thyroid size: a study of healthy Danish twins.J. Clin. Endocrinol. Metab. 2004; 89: 2071-2077Crossref PubMed Scopus (75) Google Scholar Whereas genetic loci associated with clinically overt euthyroid multinodular goiter were already mapped in linkage analyses, genome-wide association studies (GWAS) investigating genetic factors with regard to thyroid enlargement have not been conducted so far. Thus, we have performed a GWAS on thyroid volume in Germany, a previously iodine-deficient area with moderate iodine deficiency in the northeast and moderate to severe iodine deficiency in the south.8Meng W. Schindler A. Horack S. Lux E. Muche A. Renal iodine excretion by students in East Germany. A prospective study 1989 to 1996.Med. Klin. (Munich). 1998; 93: 347-351Crossref PubMed Google Scholar, 9Meng W.S.A. Iodine Supply in Germany.in: Delange F. Robertson A. McLoughney E. Gerasimov G. Elimination of Iodine Deficiency Disorders (IDD) in Central and Eastern Europe, the Commonwealth of Independent States and the Baltic States. World Health Organization, Munich, Germany1998: 21-27Google Scholar A voluntary iodine fortification program was introduced in Germany during the 1980s. In December 1993, improved legislations concerning the iodization of table salt became effective, which contributed to an increase in the use of iodized salt for food production, resulting in a stable iodine supply during the past 15 years. In the discovery-stage GWAS, 3620 individuals, aged 20–79 years, from the baseline examinations of the Study of Health in Pomerania (SHIP-010Volzke H. Alte D. Schmidt C.O. Radke D. Lorbeer R. Friedrich N. Aumann N. Lau K. Piontek M. Born G. et al.Cohort Profile: The Study of Health in Pomerania.Int. J. Epidemiol. 2010; 40: 294-307Crossref PubMed Scopus (686) Google Scholar) in West Pomerania (northeast Germany) were analyzed for associations of SNPs with the phenotypes "thyroid volume" and "goiter." The lead SNPs of the four identified loci that exhibited genome-wide significant associations for "thyroid volume" and the corresponding SNPs of these loci that showed the strongest associations to "goiter" were replicated in 1290 individuals, aged 30–79, years from the Kooperative Gesundheitsforschung in der Region Augsburg (KORA F4, southern Germany11Wichmann H.E. Gieger C. Illig T. MONICA/KORA Study GroupKORA-gen—resource for population genetics, controls and a broad spectrum of disease phenotypes.Gesundheitswesen. 2005; 67: S26-S30Crossref PubMed Scopus (334) Google Scholar). Finally, we performed a combined GWAS analysis using data from both studies. All participants were of European ancestry. Approval was obtained by local ethic committees, and informed consent was given by all participants. Goiter was defined as a thyroid volume of > 18 ml in women and of > 25 ml in men.12Gutekunst R. Becker W. Hehrmann R. Olbricht T. Pfannenstiel P. Ultrasonic diagnosis of the thyroid gland.Dtsch. Med. Wochenschr. 1988; 113: 1109-1112Crossref PubMed Scopus (108) Google Scholar Subjects with known thyroid disease or those with previous or current antithyroid treatment were excluded from the analyses, because potentially relevant treatment effects on thyroid volume cannot be quantified.13Brix T.H. Kyvik K.O. Hegedüs L. Validity of self-reported hyperthyroidism and hypothyroidism: comparison of self-reported questionnaire data with medical record review.Thyroid. 2001; 11: 769-773Crossref PubMed Scopus (32) Google Scholar, 14Bergmann M.M. Jacobs E.J. Hoffmann K. Boeing H. Agreement of self-reported medical history: comparison of an in-person interview with a self-administered questionnaire.Eur. J. Epidemiol. 2004; 19: 411-416Crossref PubMed Scopus (127) Google Scholar The detailed characteristics of the study populations, exclusion criteria, and quality control procedures are described in Table 1. Genotyping information and GWAS details are specified in Table S1 (available online).Table 1Cohort CharacteristicsStudy of Health in Pomerania (SHIP)Cooperative Health Research in the Region of Augsburg, Survey 4 (KORA F4)Study designpopulation-basedpopulation-basedSample size36201290Age in years (range)49 (20–81)60 (32–79)Females (%)1716 (47.4)541 (41.9)Current smokers (%)1143 (31.57)210 (16.29)BSA in m2 (SD)1.89 (0.21)1.90 (0.21)Thyroid volume in ml (SD)21.28 (11.43)21.55 (10.9)Presence of goiter (%)1325 (36.6)467 (36.2)Thyroid measurementUltrasound VST-Gateway 5 MHz linear array transducer (Diasonics)SONOLINE G50 5 MHz linear array transducer (Siemens Medical)Thyroid volume calculationlength × width × depth × 0.479 [ml] for each lobe34Brunn J. Block U. Ruf G. Bos I. Kunze W.P. Scriba P.C. Volumetric analysis of thyroid lobes by real-time ultrasound (author's transl).Dtsch. Med. Wochenschr. 1981; 106: 1338-1340Crossref PubMed Scopus (544) Google ScholarThyroid measurement QCIntra- and interobserver reliabilities within and between both studies were assessed before the start of each study and afterwards annually during the studies; analyses were performed according to Bland and Altman.35Bland J.M. Altman D.J. Regression analysis.Lancet. 1986; 1: 908-909Abstract PubMed Scopus (103) Google Scholar All measurements of the thyroid volume for within and between study comparisons showed Spearman correlation coefficients of > 0.85 and mean differences (+ 2 SD) of the mean bias of < 5% (<25%).Sample exclusions by phenotypeIndividuals taking thyroid medication or reporting thyroid disorders, women pregnant at the time of thyroid measurement Open table in a new tab All SNPs with a minor allele frequency < 0.01 were excluded. Since all X-linked SNPs were excluded from imputation,15Soranzo N. Spector T.D. Mangino M. Kühnel B. Rendon A. Teumer A. Willenborg C. Wright B. Chen L. Li M. et al.A genome-wide meta-analysis identifies 22 loci associated with eight hematological parameters in the HaemGen consortium.Nat. Genet. 2009; 41: 1182-1190Crossref PubMed Scopus (404) Google Scholar 27.399 directly genotyped SNPs of chromosome X and 46 directly genotyped SNPs from mtDNA (SNP call rates ≥ 80%, pHWE > 0.001, MAF > 1%) were tested in an additional analysis in the discovery stage. None of these SNPs showed genome-wide significant associations with "thyroid volume" or "goiter." Associations were tested with the use of a linear additive model on natural log-transformed thyroid volume (ml) for the "thyroid volume" phenotype and a logistic regression analysis for the "goiter" phenotype, respectively. Adjustment for age, gender, current smoking state (yes or no), and body surface area (BSA) was performed for all analyses. All p values of the discovery GWAS and the results of the meta-analysis were corrected for genomic control. Only SNPs, for which association data from both studies were available, were included in the meta-analysis. Associations were considered to have genome-wide significance below a p value of 5 × 10−8.16Pe'er I. Yelensky R. Altshuler D. Daly M.J. Estimation of the multiple testing burden for genomewide association studies of nearly all common variants.Genet. Epidemiol. 2008; 32: 381-385Crossref PubMed Scopus (519) Google Scholar Genomic control was applied both for the individual cohorts and for the combined results. The estimated genomic control was low for "thyroid volume" and "goiter" for both the individual-cohort analysis (Table S1) and the combined analyses (λGC = 1.058 and λGC = 1.021, respectively), suggesting little residual confounding due to population stratification (Figure S1). All SNPs found to be associated with one of the two phenotypes of interest in the discovery GWAS, the replication stage, or the combined analysis were in Hardy-Weinberg equilibrium (p > 0.001) in both studies. To identify independently associated loci, SNPs were clumped with the use of the PLINK17Purcell S. Neale B. Todd-Brown K. Thomas L. Ferreira M.A. Bender D. Maller J. Sklar P. de Bakker P.I. Daly M.J. Sham P.C. PLINK: a tool set for whole-genome association and population-based linkage analyses.Am. J. Hum. Genet. 2007; 81: 559-575Abstract Full Text Full Text PDF PubMed Scopus (16805) Google Scholar clumping algorithm of (r2 > 0.1, 1 Mb distance) based on genotype data of 4105 SHIP participants. To validate the independence of the four loci for "thyroid volume" in the discovery stage, the lead SNPs of these loci were analyzed together in a multivariate linear regression model, in which the associations remained significant and mostly unchanged, indicating statistical independence of the four SNPs from each other (Table 2).Table 2Results from the Analysis of Independence and the Explained Variance of the Lead SNPs of the Four Loci Associated with the "Thyroid Volume" Phenotype in the Discovery-Stage GWASDiscovery-Stage GWASSHIPKORALocusLead SNPp ValueEffectSEVariance Explainedp ValueEffectSEVariance Explained1rs121389504.700 × 10−130.0930.0133.33%3.988 × 10−60.1030.0223.18%2rs13549202.530 × 10−90.0600.0106.619 × 10−30.0480.0183rs177674917.390 × 10−110.0630.0104.437 × 10−40.0600.0174rs120910473.250 × 10−8−0.0540.0102.775 × 10−4−0.0590.016The values were calculated by analyzing all SNPs adjusted for sex, age, smoking status, and body surface area in a linear regression model. Compared to the results in Table 3, the p values have changed only marginally, indicating statistical independence of the four SNPs from each other. Open table in a new tab The values were calculated by analyzing all SNPs adjusted for sex, age, smoking status, and body surface area in a linear regression model. Compared to the results in Table 3, the p values have changed only marginally, indicating statistical independence of the four SNPs from each other. The discovery analysis identified four loci associated with "thyroid volume" at a genome-wide significance level. Two of these loci were also significantly associated with "goiter," whereas the other two missed genome-wide significance in the discovery stage (see Table 3). The strongest associations were found for the CAPZB region on chromosome 1p36. Within this region, two independent loci were significantly associated with both "thyroid volume" and "goiter": at the locus upstream of CAPZB, rs12138950 represented the lead SNP for both phenotypes. Within CAPZB, rs12091047 represented the lead SNP for "thyroid volume," whereas rs12033437 was the lead SNP for "goiter." For the third locus at 15q21, the lead SNPs for both phenotypes (rs1354920 for "thyroid volume" and rs1023683 for "goiter") were located within C15orf33 in close vicinity of FGF7.Table 3SNPs within the Four Loci that Show the Strongest Association with the "Thyroid Volume" and "Goiter" Phenotypes in the Discovery-Stage GWASLocusChr.PositionLead SNP SHIPpGC SHIPp KORApGC METAAllele 1Allele 2Freq1ImpQualR2Thyroid Volume1119711702rs121389509.340 × 10−142.333 × 10−062.876 × 10−18CA0.1540.989same SNP21547612815rs13549201.130 × 10−087.656 × 10−034.923 × 10−10TC0.3020.9700.6031678302988rs177674911.240 × 10−108.571 × 10−048.071 × 10−13GA0.3221.0001.004119644512rs120910472.530 × 10−098.400 × 10−051.777 × 10−12TC0.3430.9930.97Goiter1119711702rs121389506.666 × 10−117.111 × 10−053.641 × 10−14CA0.1540.9890.6021547494739rs10236831.651 × 10−089.199 × 10−073.864 × 10−13TA0.7580.9910.8331678306777rs38135798.138 × 10−088.457 × 10−043.868 × 10−10GA0.4820.997same SNP4119622481rs120334371.499 × 10−072.844 × 10−054.055 × 10−11TC0.3341.0000.97Allele 1, effect allele; Allele 2, other allele; Freq1, frequency of allele 1; ImpQual, imputation quality measurement (observed by expected variance ratio); pGC, p value of the association after genomic control has been applied; R2, linkage disequilibrium value for the respective lead SNP with the lead SNP of the meta-analysis GWAS of the same locus. Open table in a new tab Allele 1, effect allele; Allele 2, other allele; Freq1, frequency of allele 1; ImpQual, imputation quality measurement (observed by expected variance ratio); pGC, p value of the association after genomic control has been applied; R2, linkage disequilibrium value for the respective lead SNP with the lead SNP of the meta-analysis GWAS of the same locus. The fourth locus at 16q23 was located within a 110 kb distance of the next annotated gene (MAF) and was not in linkage disequilibrium with it, and it was therefore initially designated as a "gene desert" (see Figure 2). The lead SNPs of this locus were rs17767491 for "thyroid volume" and rs3813579 for "goiter." All four loci were positively replicated in the second stage in KORA-F4 (Table 3). Finally, a GWAS using the SNP data from both studies was performed, including a study population of n = 4910. This combined analysis did not reveal additional, yet-unidentified genome-wide significant associations with the two phenotypes. However, all four loci detected in the primary stages were confirmed and exhibited even stronger associations than those in the discovery analysis (Table 4, Figure 1). In several cases, the combined analysis yielded lead SNPs that were different from those of the discovery GWAS but were consistently found to be in distinct linkage disequilibrium with the former. The lead SNPs of the loci at 16q23 ("gene desert") and at 1p36 within CAPZB now exhibited clear genome-wide significant associations for "goiter" (Table 4, Figure 1). We tested for interactions between the lead SNPs of the four "thyroid volume" loci in SHIP, but we did not observe any significant results (p > 0.05). For each individual, the number of alleles of all four loci increasing the thyroid volume was counted, and the mean increment of the log thyroid volume per allele was estimated. The results are shown in Table 5. Sensitivity analyses included anti-TPO antibody status and serum concentrations of TSH, free T3, and free T4 as potential confounders without substantially affecting the key results (Table S2).Table 4SNPs within the Four Loci with Strongest Association with the "Thyroid Volume" and "Goiter" Phenotypes in the Combined Meta-AnalysisLocusChr.PositionLead SNPpGCEffectSEAllele 1Allele 2Freq1Thyroid Volume1119711702rs121389502.876 × 10−18−0.1020.012AC0.84721547522589rs43387401.441 × 10−12−0.0670.009TC0.73831678302049rs177674199.418 × 10−150.0680.009TC0.3214119638105rs120454403.237 × 10−140.0670.009TG0.664Goiter1119715941rs109174681.114 × 10−140.660.59–0.73TC0.78321547522589rs43387402.843 × 10−130.690.63–0.76TC0.73931678306777rs38135793.868 × 10−101.321.21–1.44AG0.5184119638105rs120454401.649 × 10−111.381.26–1.51TG0.664Chr, chromosome; Allele 1, effect allele; Allele 2, other allele; Freq1, frequency of allele 1; pGC, p value of the association after genomic control has been applied. Open table in a new tab Table 5Mean of Log Thyroid Volumes of Individuals Carrying the Specified Number of "Thyroid Volume"-Increasing Alleles of the Four Lead SNPs in the Combined AnalysisSHIPKORANo. of Risk AllelesMean95% CIFreq.Freq. %Mean95% CIFreq.Freq. %02.77[2.68,2.85]762.102.75[2.60,2.89]292.2612.84[2.8,2.87]45912.692.81[2.75,2.87]16012.4522.90[2.87,2.92]96226.592.91[2.87,2.95]34727.0032.97[2.94,2.99]107729.772.95[2.91,2.99]41232.0643.01[2.99,3.04]72620.073.06[3.01,3.11]24919.3853.10[3.05,3.15]2536.993.25[3.17,3.34]745.7663.16[3.06,3.26]611.693.26[3.04,3.46]131.0173.32[2.94,3.7]40.113.46[2.70,4.23]10.08Total3618100.001285100.00The number of risk alleles ranges from 0 to 7. There were no individuals homozygous for the thyroid increasing allele at all four loci. Freq denotes frequency; the number of individuals carrying the corresponding number of risk alleles. Freq %, percentage of individuals carrying the corresponding number of risk alleles. Open table in a new tab Chr, chromosome; Allele 1, effect allele; Allele 2, other allele; Freq1, frequency of allele 1; pGC, p value of the association after genomic control has been applied. The number of risk alleles ranges from 0 to 7. There were no individuals homozygous for the thyroid increasing allele at all four loci. Freq denotes frequency; the number of individuals carrying the corresponding number of risk alleles. Freq %, percentage of individuals carrying the corresponding number of risk alleles. The two CAPZB loci associated with both phenotypes are clearly independent from each other, as their lead SNPs are not in significant linkage disequilibrium (r2 = 0.004). Furthermore, the effects of their minor alleles on "thyroid volume" and "goiter" are inverse. CAPZB encodes the two β subunit isoforms of the capping protein (CP), also known as the barbed-end actin binding protein. CP represents a heterodimeric protein composed of α and β subunits. The α1 and α2 subunit isoforms of CP are encoded by CAPZA1 and CAPZA2, respectively. Both β subunit isoforms encoded by CAPZB are specified by differential pre-mRNA splicing.18Schafer D.A. Korshunova Y.O. Schroer T.A. Cooper J.A. Differential localization and sequence analysis of capping protein beta-subunit isoforms of vertebrates.J. Cell Biol. 1994; 127: 453-465Crossref PubMed Scopus (122) Google Scholar In spite of pronounced amino acid sequence differences between the carboxy termini of β1 and β2, both subunit isoforms exhibit comparable actin-binding activities.19Schafer D.A. Jennings P.B. Cooper J.A. Dynamics of capping protein and actin assembly in vitro: uncapping barbed ends by polyphosphoinositides.J. Cell Biol. 1996; 135: 169-179Crossref PubMed Scopus (323) Google Scholar Therefore, the β subunit carboxy termini, besides binding actin, may interact with different target proteins that might regulate their activity.20Yamashita A. Maeda K. Maéda Y. Crystal structure of CapZ: structural basis for actin filament barbed end capping.EMBO J. 2003; 22: 1529-1538Crossref PubMed Scopus (119) Google Scholar This is substantiated by the fact that the two β isoforms exhibit tissue-specific expression: whereas β2 represents the prevailing isoform of nonmuscle tissues, β1 predominates in muscle tissues.18Schafer D.A. Korshunova Y.O. Schroer T.A. Cooper J.A. Differential localization and sequence analysis of capping protein beta-subunit isoforms of vertebrates.J. Cell Biol. 1994; 127: 453-465Crossref PubMed Scopus (122) Google Scholar In the thyroid, TSH-induced engulfment of the colloid by extension of microvilli and filopodia protruding in the thyroid follicular lumen from the surface of thyrocytes represents a key step for thyroglobulin mobilization. The resulting endocytotic vesicles fuse with lysosomes, and proteolysis of thyroglobulin releases mono-, di-, tri-, and tetraiodthyronine (T1, T2, T3, and T4, respectively). Polymerization of actin is crucial in the formation and extension of microvilli and filopodia. CP represents a major antagonist of filopodia formation.21Mejillano M.R. Kojima S. Applewhite D.A. Gertler F.B. Svitkina T.M. Borisy G.G. Lamellipodial versus filopodial mode of the actin nanomachinery: pivotal role of the filament barbed end.Cell. 2004; 118: 363-373Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar, 22Cooper J.A. Sept D. New insights into mechanism and regulation of actin capping protein.Int. Rev. Cell Mol. Biol. 2008; 267: 183-206Crossref PubMed Scopus (165) Google Scholar The elongation status of the barbed end of actin filaments can be regarded as the net result of the interplay between capping and anticapping activities, with CP as the major barbed-end terminator.21Mejillano M.R. Kojima S. Applewhite D.A. Gertler F.B. Svitkina T.M. Borisy G.G. Lamellipodial versus filopodial mode of the actin nanomachinery: pivotal role of the filament barbed end.Cell. 2004; 118: 363-373Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar On the other hand, the barbed-end actin-binding activity of CP is modulated by additional regulatory proteins that are, in turn, able to bind to and sequester the former. Known examples of this are the CARMIL and V-1/myotrophin proteins that can bind to CP and inhibit its binding to the barbed end of the actin filament, i.e., its capping function.22Cooper J.A. Sept D. New insights into mechanism and regulation of actin capping protein.Int. Rev. Cell Mol. Biol. 2008; 267: 183-206Crossref PubMed Scopus (165) Google Scholar Interestingly, the CP-binding activity of V-1/myotrophin is regulated by cAMP, the most important second messenger involved in the TSH signal transduction.23Kitazawa M. Yamakuni T. Song S.Y. Kato C. Tsuchiya R. Ishida M. Suzuki N. Adachi E. Iwashita S. Ueno S. et al.Intracellular cAMP controls a physical association of V-1 with CapZ in cultured mammalian endocrine cells.Biochem. Biophys. Res. Commun. 2005; 331: 181-186Crossref PubMed Scopus (11) Google Scholar The lead SNP showing the strongest association with both phenotypes in our combined analysis, rs12138950, is located in the upstream region of CAPZB (Figure 2). Therefore, it can be speculated that the causative sequence variant underlying the association of this locus with "thyroid volume" and "goiter" might influence the activity of the CAPZB promoter, thereby modulating the expression level of the gene. As the minor allele of this locus is associated with increased thyroid volume and risk of goiter, the postulated causative polymorphism most probably represents a promoter-up variant, causing an increased β2 subunit amount in the thyrocyte. Given that both subunits are unstable in the absence of the other subunit but stabilized in its presence,24Amatruda J.F. Gattermeir D.J. Karpova T.S. Cooper J.A. Effects of null mutations and overexpression of capping protein on morphogenesis, actin distribution and polarized secretion in yeast.J. Cell Biol. 1992; 119: 1151-1162Crossref PubMed Scopus (95) Google Scholar the total amount of the active CP heterodimer might also be increased. According to this model, alleviated reception of the incoming TSH/cAMP signal as a result of attenuated uncapping activity would result in reduced thyroglobulin engulfment by filopodia, decreased T3/T4 release, and, in turn, compensatory thyroid hyperplasia and increased thyroid volume. In the second CAPZB locus, the lead SNP for both phenotypes in the combined analysis, rs12045440, is located within the first of the nine introns of CAPZB (Figure 2). Because this locus is marked by a genic lead SNP, one may hypothesize that at least one, yet-unidentified, sequence polymorphism in linkage disequilibrium with the associated SNPs represents a nonsynonymous SNP causing an amino acid exchange in the encoded β2 subunit. Because the minor allele of this locus is associated with decreased thyroid volumes, the β2 variant specified by this allele might exhibit a more sensitive response to the TSH/cAMP signal, resulting in enhanced uncapping compared to the major allele. This could be caused by an improved interaction of CP with at least one protein that negatively regulates the capping activity, such as cAMP-activated V-1/myotrophin. Accordingly, improved reception of the incoming TSH/cAMP signal would result in accelerated thyroglobulin engulfment by filopodia, increased T3/T4 release, and, in turn, compensatory thyroid hypoplasia and decreased thyroid volume. Recently, Panicker et al. described an association between the SNP rs10917469 and TSH serum concentration.25Panicker V. Wilson S.G. Walsh J.P. Richards J.B. Brown S.J. Be
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