INVESTIGATING BIOCHEMICAL MARKERS OF ADULT AUTISM SPECTRUM DISORDER PHENOTYPES

Background: Recently, in the framework of a dimensional approach to autism-related psychopathology, increasing attention has been paid on investigating specific features of Autism spectrum disorder (ASD) during adulthood. Particular interest was paid on milder forms of ASD, without intellectual and language impairment, which may remain under-detected in early life. Moreover, several reports highlighted the presence of significant autistic-like traits in non-affected first-relatives of subjects with ASD, leading to the conceptualization of a “Broad autism phenotype” (BAP). Despite an increasing body of data from neuroimaging studies reported neurostructural and neurofunctional alterations in BAP, there is still a lack of evidence about potential biochemical correlates of ASD-like traits. The first attempts in researching biochemical markers of ASD focused on neurotransmitters and in particular on serotonin (5-HT), reporting possible different alterations between adulthood and childhood and/or between different kinds of biological samples. Brain-derived neurotrophic factor (BDNF), tryptophan (TRP), and metabolites of the TRP-derived kynurenine (KYN) shunt were also found altered in ASD children, and authors hypothesized their possible involvement in increased excitotoxicity or inflammatory activity. Other research also reported immune system activation among ASD children: results in this field seem promising, with studies stressing an association of cytokine levels with different grades of clinical severity and specific clusters of symptoms. Moreover, altered homocysteine (HCY) metabolism and altered trans-sulfuration and methylation processes are acquiring growing interest as possible metabolic signatures of ASD. Despite that, for most of these parameters scant research focused on adult samples and/or on BAP, without studies comparing adult ASD patients with their adult relatives. Aims: The aim of this work was to identify possible biochemical correlates for sub-threshold and over-threshold ASD symptomatology in adults. In particular, the study aimed to: evaluate presence and features of ASD symptoms in adult ASD patients without language or intellectual impairment and their first-degree relatives, as well as in controls, through suitable psychometric scales; evaluate levels of various biochemical parameters, among the three groups, and in particular: circulating levels of 5-HT, TRP, KYN, quinolinic acid (QA), chinurenic acid (KYNA), BDNF, IL-6, HCY; evaluate the possible correlations between clinical features, as measured by the psychometric scales, and biochemical parameters. Methods: A sample of adult ASD patients (ASD group) and first-degree relatives of the ASD probands (BAP group) were recruited among patients followed at the Psychiatric Section of Azienda Ospedaliera Universitaria Pisana, whereas unrelated controls (CTL group) were recruited on a voluntary basis. All subjects underwent a psychiatric and biochemical assessment. The psycometric instruments employed were: the Structured Clinical Interview for DSM-5, the Adult Autism Sub-threshold Spectrum (AdAS Spectrum), the Autism-Spectrum Quotient (AQ), the Ritvo Autism Asperger Diagnostic Scale,14-item version (RAADS-14), the Ruminative Response Scale (RRS) as well as the Work and Social Adjustment Scale (WSAS). A sample of peripheral venous blood was withdrawn from all the subjects and then processed for obtaining the different analytical specimen for biochemical assessment: the platelet poor plasma (PPP), platelet pellets and serum. All these parameters were measured by means of dedicated Enzyme-linked immunosorbent assay (ELISA) procedures. Results: ASD patients reported significantly higher total scores (greater severity of autistic traits/functional impairment) than the other groups on all psychometric scales. The BAP group reported intermediate scores, significantly higher than the CTL group. At the same time ASD patients reported significantly lower intra-platelet 5-HT, PPP 5-HT and TRP levels than BAP and CTL groups. Significant differences depending on pharmacological treatment within groups were reported for intra-platelet 5-HT levels only, and no difference in intra-platelet 5-HT was found when exluding subjects in treatment with antidepressants from the analysis. Moreover, significantly lower levels of KYNA were reported in both ASD and BAP group when compared with CTL subjects. IL-6 and HCY were instead significantly higher in the ASD group than in the CTL one, with BAP group showing intermediate levels, not significantly different from those reported in the other two groups. A multinomial logistic regression analysis identified higher levels of HCY and IL-6 as the statistically predictive variables of being in the ASD group, while increased IL-6 was statitstically predictive also of being in the BAP group. Specific patterns of association were found between autistic symptoms and biochemical variables. The biochemical parameters most associated with functional impairment were the increased levels of HCY and IL-6. Conclusions: our results confirm the need of further research on alterations of TRP metabolism in ASD and highlight the value of assessing the association of ASD with specific immune system alterations and impaired HCY-related metabolism, which may affect trans-sulfuration/methylation processes in this population. Moreover, our results confirm the presence of intermediate alterations in relatives of ASD patients also from a biochemical point of view, thus providing more support to the presence of a continuum between sub-threshold and full-threshold ASD phenotypes. Finally, our results stress the importance of evaluating metabolomics/proteomic signatures in the field of ASD patients’ care and management.