Investigating the Relation Between Serotonin (5-HT) and Insistence on Sameness in Autism Spectrum Disorders Using Genetic and Biological Markers

Background:

The serotonergic system may play an important role in the manifestation of autism spectrum disorders (ASDs).  Genetic studies using linkage and association have identified the serotonin transporter gene (SLC6A4) as a candidate gene for ASDs.  Recent cross disciplinary research has shown that haplotypes in this region alter 5-HT levels.  Biological measures of serotonin (5-HT) are correlated in relatives of individuals with ASDs.  Given the utility of selective serotonin reuptake inhibitors in reducing problem behaviors attributed to restricted and repetitive behavior in ASDs, several questions exist about the relation between 5-HT and RRB.  We focus on aggression and the RRB Insistence on Sameness (IS), a potentially debilitating phenotype, first described by Kanner and now used to reduce heterogeneity in genetic studies.

Objectives:

The primary purpose of this study is to investigate the relation between 5-HT and IS using genetic and biological markers including genotypes within SLC6A4 and whole blood platelet 5-HT.  A secondary objective is to analyze the relations among the 5-HT measures and individual characteristics including age and sex in a sample with ASDs. 

Methods: Individuals (N = 52, 3-27-years-old) participating in an Autism Center of Excellence study visited a university center for comprehensive diagnostic testing.  Separate blood samples were drawn and analyzed for genotype and 5-HT in independent laboratories blind to sample identity.  Genotyping included the functional variants (i.e. Lg, La, Sg) of the 5-HTTLPR polymorphism and the following SNPs which may modify 5-HTTLPR expression: rs25532, rs16965628, rs2020936, rs2020937.  Whole blood platelet 5-HT was measured by HPLC with fluorescent detection.  Parents completed several measures about their children including the Autism Diagnostic Interview-Revised (ADI-R) and the Repetitive Behavior Scale-Revised (RBS-R).  Lifetime and current IS were characterized using items from the ADI-R factor (Cuccaro et al., 2003).  Current behavior was also measured on the RBS-R Ritualistic/Sameness and Compulsive Behavior subscales (Lam & Bodfish, 2007).  Aggression was measured on both instruments.  Correlations between 5-HT, IS, and aggression were produced.  5-HT, IS, and aggression scores were compared across genotype groups including haplotypes.  Individuals with high IS and aggression were compared to others on 5-HT measures.  Lastly, extensive phenotype information will be explored for individuals who were outliers on measures of 5-HT.

Results:

Preliminary analysis included 23 Caucasian, non-Hispanic subjects free of medications which influence serotonin.  One outlier with high 5-HT was excluded.  Age was negatively correlated with 5-HT (p<.01), but none of the behavior measures.  IS on the ADI-R and RBS-R were correlated (p<.01).  5-HT positively correlated with total score on the RBS-R (p=.08), but not IS.  Lifetime IS scores on the ADI-R were highest for the S/S and La/La groups at 5-HTTLPR (p=.07).  The La/La group scored significantly higher on RBS-R Compulsive subscale (p=.05).  The High IS with aggression group did not significantly differ on 5-HT level or 5-HTTLPR genotype. 

Conclusions: Additional analyses in a larger group will test more thoroughly whether genetic and biological markers of 5-HT relate to IS and aggression in a sample with ASDs. The preliminary findings raise interesting questions about the relation between 5-HT and IS.