Luzón-Toro et al. BMC Medical Genomics (2015) 8:83 DOI 10.1186/s12920-015-0160-7 RESEARCH ARTICLE Open Access Identification of epistatic interactions through genome-wide association studies in sporadic medullary and juvenile papillary thyroid carcinomas Berta Luzón-Toro1,2, Marta Bleda2,3,4, Elena Navarro2,5, Luz García-Alonso3,6, Macarena Ruiz-Ferrer1,2, Ignacio Medina3,7, Marta Martín-Sánchez1,2, Cristina Y. Gonzalez3,6, Raquel M. Fernández1,2, Ana Torroglosa1,2, Guillermo Antiñolo1,2, Joaquin Dopazo2,3,8*† and Salud Borrego1,2*† Abstract Background: The molecular mechanisms leading to sporadic medullary thyroid carcinoma (sMTC) and juvenile papillary thyroid carcinoma (PTC), two rare tumours of the thyroid gland, remain poorly understood. Genetic studies on thyroid carcinomas have been conducted, although just a few loci have been systematically associated. Given the difficulties to obtain single-loci associations, this work expands its scope to the study of epistatic interactions that could help to understand the genetic architecture of complex diseases and explain new heritable components of genetic risk. Methods: We carried out the first screening for epistasis by Multifactor-Dimensionality Reduction (MDR) in genome-wide association study (GWAS) on sMTC and juvenile PTC, to identify the potential simultaneous involvement of pairs of variants in the disease. Results: We have identified two significant epistatic gene interactions in sMTC (CHFR-AC016582.2 and C8orf37-RNU1-55P) and three in juvenile PTC (RP11-648k4.2-DIO1, RP11-648k4.2-DMGDH and RP11-648k4.2-LOXL1). Interestingly, each interacting gene pair included a non-coding RNA, providing thus support to the relevance that these elements are increasingly gaining to explain carcinoma development and progression. Conclusions: Overall, this study contributes to the understanding of the genetic basis of thyroid carcinoma susceptibility in two different case scenarios such as sMTC and juvenile PTC. Keywords: Sporadic medullary thyroid carcinoma, Juvenile papillary thyroid carcinoma, Epistasis, Multifactor- dimensionality reduction, Genome-wide association study Background thyroid cells from which thyroid carcinomas are de- Thyroid carcinoma is the most common endocrine rived. Medullary thyroid carcinoma (MTC) arises from malignancy, which account for more than 1 % of all calcitonin-producing parafollicular cells (thyroid C new malignant tumors [1]. There are several histo- cells) and constitutes around 2–5%ofallthyroidneo- logical types and subtypes according to the endocrine plasias [2]. Approximately 25 % of MTC cases present an autosomal dominant inherited disorder named * Correspondence: [email protected]; [email protected] Multiple Endocrine Neoplasia type 2 (MEN 2), which † Equal contributors includes three different clinical phenotypes: MEN 2A, 2Centre for Biomedical Network Research on Rare Diseases (CIBERER), Seville, Spain MEN 2B, and familial MTC (FMTC) [3]. In >95 % of 1Department of Genetics, Reproduction and Fetal Medicine, Institute of the cases, the three forms of MEN 2 are caused by Biomedicine of Seville (IBIS), University Hospital Virgen del Rocío/CSIC/ specific gain-of-function germline mutations of the University of Seville, Seville, Spain RET Full list of author information is available at the end of the article proto-oncogene [3, 4]. The remaining 75 % of © 2015 Luzón-Toro et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Luzón-Toro et al. BMC Medical Genomics (2015) 8:83 Page 2 of 9 MTC occurs as a non-inherited sporadic lesion involvement of pairs of variants in both diseases. The without associated endocrinopathy (sporadic MTC, determination of genome-wide epistasis encompasses sMTC). Unlike hereditary forms, very little is known statistical and computational challenges [17]. Here, we about the genetic basis of sMTC. So far, studies have have used the Multifactor-dimensionality reduction been focused on the analysis of specific single nucleo- (MDR) method to detect gene-gene interactions (GxG) tide polymorphisms (SNPs)/haplotypes and significant in GWAS, which has already successfully been used in associations to sMTC have been described for differ- the detection of multiple and joint genetic factors associ- ent susceptibility genes, suggesting that this phenotype ated with complex traits [18]. may be caused by a combination of multiple common We report here the first study that combines a screen- genetic variants [5–7].However,mostofthemhave ing for epistasis by MDR in GWAS on these two rare failed to be replicated in other populations, maybe due thyroid carcinoma types. to the difficulty in collect enough patients to reach the necessary statistical power [6]. Methods Nonmedullary thyroid carcinoma (NMTC) comprises Patients and controls thyroid carcinomas of follicular cell origin and among Two different series of Spanish patients were recruited them papillary thyroid carcinoma (PTC) and follicular for this study: one with 66 sMTC patients, with absence thyroid carcinoma represent the two most common sub- of personal or family history suggestive of MEN 2 and types (85 and 10 %, respectively) [8]. Regarding PTC, the absence of traditional germline MEN 2-defining RET age is considered as one of the most important prognos- mutations and another with 38 jPTC (range from 5 to tic factors. Juvenile PTC (jPCT), a rare disease in chil- 24 years of age) with no history of head and/or neck dren and adolescents, presents with an aggressive initial irradiation (see Additional file 1: Table S1) and no famil- manifestation, though patients have an excellent overall iar aggregation. Additionally, 129 healthy controls com- prognosis, showing longer periods of survival and a prising unselected, unrelated, race, age, and sex-matched lower incidence of recurrence [9, 10]. However, PTC re- individuals without previous thyroid-related disease mains under-reported in children and adolescents and history were recruited and used as controls of both car- there is still disagreement about the standard treatment cinoma types. and optimal type of follow-up needed [9, 11]. The few All subjects underwent peripheral blood extraction for pathologic studies carried out in jPTC point to exposure genomic DNA isolation using MagNA Pure LC system to ionizing radiation as the only known environmental (Roche, Indianapolis, IN) according to the manufac- risk factor [12, 13]. Although it have reported several turer’s instructions. SNPs associated with the risk of PTC in the absence of A written informed consent was obtained from all the radiation, most of them were carried out with older participants for clinical and molecular genetic studies. patients (mean age around 45 years) and/or using som- The study was approved by the Ethics Committee for atic rather than germline DNA [6]. As in the case of clinical research in the University Hospital Virgen del sMTC, the molecular mechanisms that account for jPTC Rocío (Seville, Spain) and complies with the tenets of or explain the development of this tumour remain the declaration of Helsinki. largely unknown. Despite the success of GWAS [14] in the identifica- tion of hundreds of genetic variants associated to Genome-wide genotyping different diseases [15], its application to rare and DNA derived from peripheral blood was hybridized to multifactorial diseases still presents major drawbacks. Affymetrix Genome-Wide Human SNP 6.0 arrays. CEL Conventional GWAS approaches, based on single files were processed using Affymetrix Power Tools (APT markers, require of large cohorts, typically unavailable v1.15.0) and genotypes were obtained using the Birdseed in rare diseases. In fact, the prevalence of sMTC is approxi- (v2) calling algorithm. mately 7/100,000, according to Orphanet (see http:// www.orpha.net/orphacom/cahiers/docs/GB/Prevalence_of_ Data quality control rare_diseases_by_alphabetical_list.pdf) and there are no Quality control was carried out using PLINK software data available for PTC. Additionally, the complex genetic [19]. Individuals with more than 7 % of missing rate as nature of some diseases make single marker approaches well as those with excessive or reduced heterozygosity quite inefficient, while suggest taking approaches based on (+/-2 times the standard deviation of the mean) were systems biology [16]. removed. The data were also checked for duplicates We carried out a GWAS in two thyroid carcinoma and related individuals. Finally, divergent ancestry was types (sMTC and jPTC) followed by the detection of epi- checked using EIGENSTRAT [20] to detect possible static interactions to identify the potential simultaneous population stratification. Luzón-Toro et al. BMC Medical Genomics (2015) 8:83 Page 3 of 9 Markers with excessive missing rate (>5 %) were System (Applied Bio- systems, Foster City, California, discarded. SNPs with minor allele frequency < 1 % and/ USA) or not in Hardy-Weinberg equilibrium (in unaffected − samples; P <1×10 5) were also excluded. Functional interpretation