Elsevier

Journal of Affective Disorders

Volume 165, 20 August 2014, Pages 135-141
Journal of Affective Disorders

Research report
Association between toll-like receptor 2 gene diversity and early-onset bipolar disorder

https://doi.org/10.1016/j.jad.2014.04.059Get rights and content

Abstract

Background

Toll-like receptor 2 (TLR2) molecules play a pivotal role in innate immune responses by their ability to recognize and sense a wide repertoire of infectious and endogenous cellular structural elements. Here we evaluated whether genetic variants in TLR2 influence the age of the disease onset in bipolar disorder (BD).

Methods

DNAs from 571 BD patients 229 early-onset (EO-BD) and 342 late-onset (LO-BD) and 199 healthy controls (HC) were analyzed for the following TLR2 polymorphisms: the 5′-UTR −196 to −174 insertion/deletion (ins/del), the intron 1 rs4696480 A/T, and the exon 3 rs3804099 C/T and rs3804100 C/T. PHASE software was used for haplotype reconstruction. Genetic associations were examined using a chi-square test.

Results

We found that the TLR2 rs3804099 TT was significantly more prevalent in EO-BD than in LO-BD patients (corrected p (pc)=0.024). After excluding family history of psychiatric disorders, we also found that the TLR2 rs4696480 TT genotype was significantly more prevalent in EO-BD as compared to LO-BD and controls (pc=0.002 and 0.002). Homozygous state for the insTTT haplotype, carrying the above mentioned risk genotypes, was significantly more frequent in EO-BD than in LO-BD patients (pc=0.007) and in EO-BD without family history of psychiatric disorders as compared to (i) those with positive history (pc=0.03), (ii) with LO-BD without family history (pc=0.001) and (iii) with HC (pc=0.009).

Limitations

Confirmation by replication in independent BD cohorts is warranted.

Conclusions

Our data suggest the potential role of TLR2 genetic variants in the pathogen-mediated susceptibility to BD.

Introduction

Bipolar disorder (BD) is one of the leading causes of morbidity in age-active adults (Gore et al., 2011). According to the World Health Organization, BD ranks within the top twenty medical causes of disability and sixth among mental disorders (Vos et al., 2012). Although BD is a clinically defined nosological entity, its etiopathological mechanisms are poorly understood and valid biomarkers are yet to be uncovered which are essential prerequisites for developing appropriate prevention/control programs and innovative therapeutic approaches.

Given the genetic and phenotypic complexities of BD with potential involvement of environmental triggers, wherever possible, sub-phenotype analyses based on clinically homogeneous groups (candidate symptom approach) are expected to facilitate the identification of genetic factors that contribute to the disease subtype (Leboyer et al., 1998). Several independent cohort studies have identified and validated the existence of such sub-phenotype groups in BD based on the age-at-onset of the disease (Bellivier et al., 2003, Bellivier et al., 2001, Lin et al., 2006, Manchia et al., 2008, Tozzi et al., 2011). Indeed, early-onset BD (EO-BD) has been shown to be clinically and genetically much more homogeneous than the late-onset BD (LO-BD) (Leboyer et al., 2005). In addition, the EO-BD distinguishes from LO-BD by its relatively severe course with more manic episodes and psychotic symptoms, longer time to remission, rapid cycling, poorer response to treatment and raised incidence of comorbidities at younger age with forty percent of adolescents presenting elevated serum CRP levels, a prominent pro-inflammatory marker (Bellivier et al., 2001, Bromet et al., 2005, Goldstein et al., 2011, Goldstein et al., 2009, Leboyer et al., 2012, Leverich et al., 2007, Perlis et al., 2004, Schürhoff et al., 2000).

Besides the repeatedly observed elevation of anti- and pro-inflammatory cytokines in the serum (Modabbernia et al., 2013), an inflammatory activation of the T cell/monocyte-macrophage system (Breunis et al., 2003, Drexhage et al., 2011) with a pro-inflammatory mRNA signature in monocytes (Padmos et al., 2008) in BD patients, along with raised levels of interleukin-1 beta (IL-1β), IL-1 receptor (IL-1R), myeloid differentiation primary response 88 (MYD88) and nuclear factor-kappa B (NF-kB) subunits (p50 and p65) in the BD post-mortem frontal cortex (Rao et al., 2010) collectively implicate a dysfunctional immune/inflammatory process in this apparently multisystemic disorder.

Since a significant proportion of the molecules involved in the modulation of the immune/inflammatory processes are encoded by polymorphic gene loci, a potential “immunogenetic” contribution to BD was postulated and explored (Clerici et al., 2009, Debnath et al., 2013). Exploring the innate immunogenetic background of BD patients, we recently demonstrated that the genetic variants of a pattern recognition receptor (PRR), namely Toll-like receptor 4 (TLR4), were associated with EO-BD suggesting a less efficient anti-infectious response in this subgroup (Oliveira et al., 2014). This finding could be in agreement with the admitted implication of infectious events in BD as mothers of BD patients have been recently found to have had more frequently influenza infection during pregnancy (Parboosing et al., 2013) and an increased proportion of BD patients found to have anti-Toxoplasma gondii antibodies as compared to controls (Dickerson et al., 2013, Hamdani et al., 2013, Pearce et al., 2012, Tedla et al., 2011).

Belonging also to the PRR family, the Toll-Like receptor 2 (TLR2) molecules are of interest in the context of BD given their equally important role in the innate immune system as TLR4. Indeed the TLR2 molecules, expressed on the cell surface of immune cells, intestinal epithelial cells and in the central nervous system recognize a broad range of pathogen-associated (bacterial, viral, fungal or parasite-derived) molecular patterns (PAMPs) viz peptidoglycans, lipoarabinomannan, hemagglutinin, phospholipomannan and glycosylphosphatidylinositol or host-derived damage-associated molecular patterns (DAMPs) and can induce an inflammatory response including neuro-inflammation (Akira et al., 2006, Cario, 2008, Debierre-Grockiego et al., 2007, Hoffmann et al., 2007, Kumar et al., 2009). For the latter, examples include the microglial inflammation in Parkinson and Alzheimer disorders attributed, at least partly, to the interaction of TLR2 molecules with some endogenous ligands (α-synuclein and amyloid β peptide respectively) and the TLR2-dependent herpes simplex virus type 1-induced neuro-inflammation (Aravalli et al., 2005, Kim et al., 2013, Liu et al., 2012).

The TLR2 molecules are encoded by a polymorphic gene located on chromosome 4 (4q31), and functionally relevant common variants could affect the inter-individual ability to attenuate or aggravate the inflammatory/immune processes. Indeed the TLR2 genetic variants have been associated with susceptibility to Gram-positive bacterial infections (Schröder and Schumann, 2005) and to several immune-related disorders including type-1 diabetes, allergic asthma (Bjørnvold et al., 2009), cancer (Wang et al., 2013) and Alzheimer׳s disease (Yu et al., 2011). To our knowledge, in the context of psychiatric disorders, only one study had addressed the genetic association between TLR2 polymorphisms (rs3804099 and rs3804100) and schizophrenia in a Korean cohort but no significant association with the disease was found (Kang et al., 2013).

Given these findings, we and others have postulated that a genetically-defined defective immune/inflammatory response to infectious insults early in life (even in utero) will set a background of susceptibility to BD during development and will require, later in life, external (infectious) triggers for disease expression and/or relapse. Such subgroup of patients may differ from those with a family history of psychiatric disorders who carry one or multiple gene variants directly involved in neurodevelopmental processes. Thus comparison of patients with or without a family history of psychiatric disorders could constitute an approach to uncover other genetic susceptibilities to environmental stressors including infection. This approach, by restricting the influence of known genetic risk loci, has already proven its efficacy in the field of immunogenetic association studies of human leukocyte antigen (HLA) alleles in a variety of conditions such as ankylosing spondylitis or hematopoietic stem cell transplantation (Amroun et al., 2005, Boukouaci et al., 2011, Boukouaci et al., 2009, Tamouza et al., 2006).

In this report we have used such strategy to understand the potential influence of TLR2 polymorphism in BD by analyzing the relatively homogeneous EO-BD cohort with and without a family history of psychiatric disorders.

Section snippets

Samples

Five hundred and seventy one patients meeting DSM-IV criteria (American Psychiatric Association, 1994) for BD (type I or II), consecutively admitted to three French university-affiliated psychiatric departments (Paris-Créteil, Bordeaux and Nancy), were interviewed by trained psychiatrists with the French version of the Diagnostic Interview for Genetic Studies (DIGS version 3.0) (Nurnberger et al., 1994). The mean age of whole cohort was 42 years (range: 14 years–80 years). The age-at-onset was

Results

Genotype and haplotype distributions in EO-BD, LO-BD patients and HC are summarized in Table 1, Table 2, Table 3. For the four studied polymorphisms (TLR2 5′-UTR −196 to −174 ins/del, intronic TLR2 rs4696480 A/T and exonic TLR2 rs3804099 C/T and rs3804100 C/T) the observed genotype distribution satisfied the expected Hardy–Weinberg proportions for control samples with overall frequencies comparable to those previously published in public database (http://www.ncbi.nlm.nih.gov).

We found that the

Discussion

Recognition that immuno-inflammation may represent a major mechanism in BD is now well accepted, thus studying the genetic control of the innate arm of the immune response in BD is needed. At this end, we analyzed the genetic diversity of the TLR2 locus because of the key-role played by the encoded TLR2 molecules in innate immunity and the previously reported associations of TLR2 variants with a wide range of immune disorders (Bjørnvold et al., 2009, Schröder and Schumann, 2005, Wang et al.,

Role of funding source

This work was supported by INSERM (UMRS 1160 and U955), Assistance Publique des Hôpitaux de Paris, Agence Nationale pour la Recherche (ANR MNP2008-VIP-Project) and fondation FondaMental.

Conflict of interest

The authors declare no competing financial interests.

Acknowledgments

We thank patients with bipolar disorders and controls who agreed to participate in this study. We thank E. Abadie, the Cochin Hospital cell repository (Prof. J. Chelly), the Clinical Center of Investigations (Dr. O. Montagne), the Biological Resources Center (Prof. B. Ghaleh) of Mondor Hospital, and the blood bank Center of EFS Créteil (Drs. J.L. Beaumont and B. Mignen) for technical assistance. We thank Nita Nguyen for data management.

We thank B. Cochet, R. Cohen, O. Elgrabli, A. Raust and L.

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