Evidence for an association with the serotonin transporter promoter region polymorphism and autism

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Abstract
We have examined three functional polymorphisms, serotonin transporter promoter region polymorphism (5‐HTTLPR), dopamine D4 exon III repeat region (DRD4), and catechol‐O‐methyltransferase (COMT), in a small family‐based design toward identifying candidate genes that confer risk for autism. A significant excess of the long/long 5‐HTTLPR genotype was observed (likelihood ratio = 7.18; P = 0.027; 2 df; n = 33 families) as well as preferential transmission of the long allele of the 5‐HTTLPR (TDT chi‐square = 5.44; P<0.025; 1 df). No association was observed between the COMT and DRD4 polymorphisms and autism in this sample. Some previous studies have observed linkage between autism and the 5‐HTTLPR polymorphism and the current results are similar to those first reported by Klauck et al. [1997: Hum Genet 100:224–229; 1997: Hum Mol Genet 6:2233–2238]. Additionally, elevated serotonin levels have been consistently found in 30%–50% of autistic patients and may represent a marker for familial autism. Hyperserotonemia in autism appears to be due to enhanced 5‐HT uptake, as free 5‐HT levels are normal and the current report of an excess of the long/long 5‐HTTLPR genotype in autism could provide a partial molecular explanation for high platelet serotonin content in autism.