Full-length ArticleHLA-G coding region polymorphism is skewed in autistic spectrum disorders
Introduction
An uncontrolled immune response is suspected to impair the central nervous system (CNS) development as observed in children with a diagnosis of Autism Spectrum Disorders (ASD) (Saresella et al., 2009, Westover et al., 2011, Onore et al., 2012). In line with this, a maternal immune attack on foetal tissues is suggested to be involved in the pathogenesis of ASD (Warren et al., 1996). This hypothesis is reinforced by the observation that autoantibodies (indicating an adaptive immune process) targeting neuronal components of the foetal brain, are observed in a subset of mothers of ASD patients (Braunschweig et al., 2013, Croen et al., 2005, Patterson, 2011, Singer et al., 2008).
Natural killer (NK) cells are effectors of innate immunity that are present in the peripheral blood and, during pregnancy, can also be identified within the uterine mucosa, at the foetal/maternal interface. NK cells are characterized by the expression of a receptor family (KIR) that interact with classical HLA-C and non-classical HLA-G molecules on foetal trophoblast. The net effect of these interactions determines whether NK will or will not be activated. A skewing of the KIR-HLA complexes, in which activating molecules prevail, was recently shown in ASD children (Torres et al., 2012) and their mothers (Guerini et al., 2014). HLA-G, in particular, is a non-classical HLA Class Ib molecule expressed by the placental and decidual trophoblast cells (Yelavarthi et al., 1991) as well as by foetal cells (Van Wijk et al., 2001), which is responsible for the induction of allogenic tolerance during pregnancy. The immunomodulatory functions of HLA-G are mediated by the interaction with KIR receptors on NK cells, including KIR2DL and ILT-2 (Favier et al., 2010); these interactions lead to suppression of T cell lymphocyte proliferation and stimulation of interferon (IFN)-c production by uterine NK cells (UNK), with the net effect of favouring foetal tolerance (Akhter et al., 2012, Matter and Sharif, 2013, Peter von Dadelszen, 2008). Besides being expressed on placental cell surface, HLA-G molecules can also be secreted. Soluble HLA-G (sHLA-G) induces the apoptosis of both activated NK and CD8 T-cells and a decreased amount of sHLA-G is seen in serum of women undergoing recurrent spontaneous abortions (Hviid et al., 2002). These observations suggest that the HLA-G/NK interaction at the maternal–foetal interface is critical in determining the pregnancy outcome.
Different HLA-G protein isoforms were recently identified; in fact, four membrane-bound HLA-G isoforms (G1, G2, G3, and G4) and three different soluble forms (G5, G6 and G7) -resulting by alternative splicing and very few amino acid polymorphisms (Castelli et al., 2014)- can be distinguished. To date, 44 HLA-G alleles encoding 14 distinct functional proteins have been described. Nevertheless, only four variation sites in the coding region of the HLA-G locus are related to the amino acid exchange frequently found in worldwide populations: (i) the +292 A/T SNP at exon 2 (codon 31), exchanging a threonine for serine in the G∗01:03 allele; (ii) the +755 C/A SNP at exon 3 (codon 110), exchanging leucine/isoleucine in the G∗01:04 allele group; (iii) the DeltaC deletion at exon 3 in the G∗01:05N allele; and (iv) the +1799 C/T SNP (codon 258), exchanging threonine/methionine in the G∗01:06 allele (Donadi et al., 2011). These isoforms are functionally different, as they are endowed with a differential ability to induce tolerance. Finally, a nucleotide insertion of 14 bp in the regulatory 3′UTR region of HLA-G gene (HLA-G∗14bp+) is shown to reduce tolerogenic activity (Christiansen et al., 2012, Hylenius et al., 2004), and suggested to be involved in ASD development (Guerini et al., 2015).
We evaluated HLA G∗01:01-∗01:06 allelic polymorphisms in a well-characterized cohort of Italian children affected by ASD and their relatives to investigate whether specific HLA-G alleles, which might play a role in ASD-associated immune activation, can be identified.
Section snippets
Patients and controls
A total of 111 ASD children (92 males/19 females; mean age: 8.7 ± 4.4 years) born in peninsular Italy and of Italian descent, 81 of their mothers, and 39 of their healthy siblings (13 males/26 females; mean age: 11 ± 5.9 years) were enrolled at the Don Gnocchi Foundation, at the C. Mondino National Neurological Institute and at the ANFFAS (National Association of the Families of People with Intellectual and/or Relational Disability).
In the 3 months preceding study enrollment, all ASD children
HLA-G distribution in ASD children and control group
HLA-G allelic distribution was analyzed both in the group of 111 ASD children and in their healthy siblings and was compared to that reported in two Caucasoid population of multiparous women and their partners from Brazil (N = 167) (Nardi et al., 2012) (Pop 1) and Denmark (N = 93) (Hviid et al. 2002) (Pop 2) (Fig. 1). A statistically different HLA-G distribution was observed when ASD children were compared to either Pop 1 (pc = 1 × 10−4 df = 12) or Pop 2 (pc = 1 × 10−3 df = 12). In particular, the HLA-G∗0105N
Discussion
HLA-G coding region polymorphisms were analyzed in ASD children, in their healthy siblings and in 81 of their mothers; results were also compared to those obtained in two different datasets of multiparous women with different ethnic backgrounds. Results showed the presence of a significant skewing of HLA-G polymorphisms in ASD children, being the HLA-G∗0105N allele significantly more and the HLA-G∗0101 allele significantly less frequent, respectively. Notably, the same skewing was detected in
Conflict of interest statement
No author declares any conflict of interest.
Acknowledgments
Supported by Ricerca Corrente 2015 and Ricerca Finalizzata 2009 [Italian Ministry of Health].
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