Identification of Genetic Susceptibility Factors for Fibromyalgia / Identificació de factors de susceptibilitat genètica per a fibromiàlgia

Abstract

Fibromyalgia (FM) is a highly disabling syndrome defined by a low pain threshold and a permanent state of pain. Widespread pain is accompanied by a constellation of symptoms such as fatigue, sleep disturbances and cognitive impairment, among others. The mechanisms explaining this chronic pain remain unclear. Nowadays, the most established/ plausible hypothesis underlying FM ethiopathogenesis is the existence of a dysfunction in pain processing, as supported by alterations in neuroimaging and neurotransmitters levels. The etiology of FM involves the interaction of environmental and genetic susceptibility factors. The genetic contribution to FM has been proven by the presence of a higher concordance of monozygotic than dizygotic twins as well as family aggregation. However, the individual genetic and environmental factors involved have not been identified. The aim of this thesis was to elucidate genetic susceptibility factors for fibromyalgia. We assessed this objective through three main approaches: the identification of FM clinically homogeneous subgroups with a two step cluster analyses, a genome-wide association study in order to evaluate the possible contribution of single nucleotide polymorphisms with Illumina 1 million duo array, and array comparative genomic hybridization experiments to identify regions varying in copy number that could be involved in FM susceptibility,using Agilent 2X400K platform. 48 variables were evaluated in 1,446 Spanish FM cases fulfilling 1990 ACR FM criteria. A partitioning analysis was performed to find groups of variables similar to each other. Variables clustered into three independent dimensions: “symptomatology”, “comorbidities” and “clinical scales”. Only the two first dimensions were considered for the construction of FM subgroups, classifying FM samples into three subgroups: low symptomatology and comorbidities (Cluster 1), high symptomatology and comorbidities (Cluster 2), and high symptomatology but low comorbidities (Cluster 3). These subgroups showed differences in measures of disease severity and were further implemented in genetic analysis. Genome-wide association study was performed in 300 FM cases and 203 controls. No SNP reached GWAS association threshold, but 21 of the most associated SNPs were chosen for replication in over 900 cases and 900 pain free-controls. Four of the strongest associated SNPs selected for replication showed a nominal association in the joint analysis. In particular, rs11127292 (MYT1L) was found to be associated to FM with low comorbidities. Array comparative genomic hybridization detected 5 differentially hybridized regions. They were followed up and one of these regions was validated though a multiplex PCR experiment. An intronic deletion in NRXN3 showed to be associated to female cases of FM and in particular those with low levels of comorbidities. Replication analysis showed a stronger association when considering only female cases and controls and low comorbidities. This enhance the importance of gender in FM etiopathogenesis and could be pointing to the existence of a different genetic background for FM in males and females highlights the importance of identifying FM homogeneous subgroups for the detection of FM genetic susceptibility factors. If the proposed FM candidate genes are further validated in replication studies, this would constitute a change in the FM ethiologycal concept, as several of these candidates are known neuropsychiatric disease associated genes (autism, addiction, mental disability). This would highlight a novel neurocognitive involvement in this disorder, currently considered musculoskeletal and affective.

La fibromialgia (FM) es una enfermedad de etiología desconocida que se caracteriza por dolor crónico generalizado, junto a una amplia constelación de síntomas acompañantes. La base etiopatogénica que explica este estado permanente de dolor es aún desconocida. Hasta la fecha la teoría más plausible es la existencia de una disfunción en la transmisión del dolor. Los estudios familiares han mostrado una considerable agregación familiar en FM,sugiriendo la importancia de los factores genéticos en el desarrollo de estos cuadros. Con la presente tesis se ha pretendido estudiar e identificar variantes del genoma (polimorfismos de base única (SNPs)- y variantes en el número de copia –CNVs) asociadas a FM, con el objetivo de profundizar en la etiología de la enfermedad. Para ello, se han llevado a cabo tres grandes aproximaciones: la identificación de subgrupos clínicos homogéneos de FM mediante un análisis de clusters, un estudio de genoma completo (GWAS) para el análisis directo de SNPs y experimentos de hibridación genómica comparada mediante arrays (aCGH) con el fin de identificar regiones variables en el número de copia asociadas a FM. El análisis de clusters ha permitido la identificación de tres subgrupos de FM en función de los niveles de sintomatología y de comorbilidad personal y familiar. Los resultados del GWAS indican una posible contribución del sistema nervioso central en el desarrollo de FM, ya que las enfermedades neurológicas aparecen como sobrerrepresentadas en el estudio de pathways realizado en los SNPs que presentaban mayor asociación, y un SNP en el gen MYT1L ha presentado asociación estadísticamente significativa con FM con niveles bajos de comorbilidad, poniendo de manifiesto la importancia de identificar subgrupos clínicamente homogéneos para la detección de factores de susceptibilidad genética para FM. Un CNV en el gen neurexina3 ha mostrado, asimismo, asociación en mujeres con FM, y en particular, en aquellas con bajos niveles de comorbilidad, siendo un nuevo argumento a favor de la implicación del SNC. La confirmación de las variantes detectadas en nuevas cohortes de fibromialgia supondría un giro conceptual de la enfermedad hacia una visión más neurocognitiva que osteomuscular.