Plasma cytokine profiles in Fragile X subjects: Is there a role for cytokines in the pathogenesis?

Abstract

Background

Fragile X syndrome (FXS) is a single-gene disorder with a broad spectrum of involvement and a strong association with autism. Altered immune responses have been described in autism and there is potential that in children with FXS and autism, an abnormal immune response may play a role.

Objectives

To delineate specific patterns of cytokine/chemokine profiles in individuals with FXS with and without autism and to compare them with typical developing controls.

Methods

Age matched male subjects were recruited through the M.I.N.D. Institute and included: 19 typically developing controls, 64 subjects with FXS without autism and 40 subjects with FXS and autism. Autism diagnosis was confirmed with ADOS, ADI-R and DSM IV criteria. Plasma was isolated and cytokine and chemokine production was assessed by Luminex multiplex analysis.

Results

Preliminary observations indicate significant differences in plasma protein levels of a number of cytokines, including IL-1α, and the chemokines; RANTES and IP-10, between the FXS group and the typical developing controls (p < 0.01). In addition, significant differences were observed between the FXS group with autism and the FXS without autism for IL-6, eotaxin, MCP-1 (p < 0.04).

Conclusions

In this study, the first of its kind, we report a significantly altered cytokine profile in FXS. The characterization of an immunological profile in FXS with and without autism may help to elucidate if an abnormal immune response may play a role and help to identify mechanisms important in the etiology of autism both with and without FXS.

Introduction

Fragile X syndrome (FXS) is a single-gene disorder with a broad spectrum of involvement including cognitive and behavioral impairments of varying degrees associated with distinct physical features. One behavioral phenotype of FXS is also characterized by autistic symptoms, including social and communication deficits, and stereotypic behavior. From the most recent studies, approximately 25–33% of children with FXS have autism (Kaufmann et al., 2004, Rogers et al., 2001) whereas pervasive developmental disorder-not otherwise specified (PDD-NOS) occurs in an additional 30%, and approximately 2–6% of children with autism have FXS (Estecio et al., 2002, Hagerman and Hagerman, 2002, Harris et al., 2008, Reddy, 2005, Wassink et al., 2001).

Fragile X syndrome is nearly always caused by an expansion (>200) of a CGG trinucleotide repeat in the 5′ untranslated region (5′UTR), followed by methylation and silencing of the Fragile X mental retardation 1 (FMR1) gene, with subsequent deficiency or absence of FMR1 protein (FMRP). It is the absence of FMRP, important for normal brain development that causes FXS (Irwin et al., 2000, Weiler and Greenough, 1999). Since lack of FMRP leads to immature dendritic spines, it is thought that FMRP is involved in synaptogenesis, especially in the cerebral cortex, cerebellum and hippocampus, and, more specifically, in synaptic plasticity (Hagerman, 2006).

Although the etiology of idiopathic autism without FXS is unknown, other causes of autism are associated with known gene defects including neuroligin, neurorexin or SHANK protein (Hagerman et al., 2008). In addition, multiple genetic association studies have implicated genes that are relevant to the function of the immune system including human leukocyte antigens (HLA) complement C4, MET tyrosine kinase pathway, serine and threonine kinase C gene PRKCB1, macrophage inhibitory factor (MIF), Reelin and PTEN (reviewed in Enstrom et al. (2009)). Moreover, altered immune responses have been described in autism (Ashwood et al., 2006) and it is possible that an abnormal immune response may play a role in children with FXS and autism. For example immune abnormalities such as increased frequency of infections, particularly otitis media and sinusitis infections, especially in early childhood, have been described in at least a subgroup of boys with FXS (Hagerman and Hagerman, 2002). An increased susceptibility to infections in FXS may underlie a dysfunctional immune response which might also play a role in increased autism susceptibility in these patients. Persistent gastrointestinal (GI) symptoms, such as loose stools have also been described in FXS and are consistent with similar reports of GI symptoms and increased mucosal immune activation in a subset of children with autism (Ashwood et al., 2003, Ashwood et al., 2004, Hagerman et al., 1987, Hagerman and Hagerman, 2002). Several patients studied with FXS, demonstrated a transient hypogammaglobulinemia particularly IgG subclass 1 and 3 (Hagerman and Hagerman, 2002). Interestingly, in autism, reduced total IgG levels have been observed (Heuer et al., 2008) and are directly correlated with worsening in aberrant behaviors. Immune dysfunction may also be present in some carriers of the premutation of FXS, as women who are carriers have a higher rate of hypothyroidism and fibromyalgia related to autoimmune dysfunction (Coffey et al., 2008), however, immune dysregulation has not been studied in individuals with FXS.

Cytokines are key mediators of cell–cell communication in the immune system. Within the nervous system, cytokines play important roles in conveying signals between cells, with neurons and glia not only able to produce different cytokines but can also respond to different cytokines through a diverse array of cytokine receptors expressed on the cell surface. Cytokines have assorted effects on neuronal tissue such as the modulation of systemic and central nervous system (CNS) responses to infections or injury and can modulate brain function affecting cognitive and emotional processing. The cytokine milieu has been shown to directly affect neural tissue function and development, especially the pro-inflammatory cytokines such as IL-1, IL-6, IL-12, IFNγ and TNFα, which have pleiotropic effects in the CNS including in neurodevelopment. Abnormal immune profiles in FXS may play a role in the development of cognitive deficits and may also lead to behaviors characteristics of autism. In this study, we assessed specific cytokine and chemokine profiles in individuals with FXS who have autism spectrum disorders and those that do not have autism spectrum disorders and compared these cytokine profiles to the profiles seen in typical developing controls.