Enhanced activity of human serotonin transporter variants associated with autism

Abstract

Rare, functional, non-synonymous variants in the human serotonin (5-hydroxytryptamine, 5-HT) transporter (hSERT) gene (SLC6A4) have been identified in both autism and obsessive-compulsive disorder (OCD). Within autism, rare hSERT coding variants associate with rigid-compulsive traits, suggesting both phenotypic overlap with OCD and a shared relationship with disrupted 5-HT signalling. Here, we document functional perturbations of three of these variants: Ile425Leu; Phe465Leu; and Leu550Val. In transiently transfected HeLa cells, the three variants confer a gain of 5-HT transport phenotype. Specifically, enhanced SERT activity was also observed in lymphoblastoid lines derived from mutation carriers. In contrast to previously characterized Gly56Ala, where increased transport activity derives from catalytic activation, the three novel variants exhibit elevated surface density as revealed through both surface antagonist-binding and biotinylation studies. Unlike Gly56Ala, mutants Ile425Leu, Phe465Leu and Leu550Val retain a capacity for acute PKG and p38 MAPK regulation. However, both Gly56Ala and Ile425Leu demonstrate markedly reduced sensitivity to PP2A antagonists, suggesting that deficits in trafficking and catalytic modulation may derive from a common basis in perturbed phosphatase regulation. When expressed stably from the same genomic locus in CHO cells, both Gly56Ala and Ile425Leu display catalytic activation, accompanied by a striking loss of SERT protein.

Serotonin transporter variant drives preventable gastrointestinal abnormalities in development and function

“…Platelets do not synthesize 5-HT (6) but utilize the 5-HT transporter (SERT, encoded by Slc6a4) (7) to take it up (8) as they circulate through the gut. Multiple hyperactive coding variants of SERT have been found in subjects with ASD (9)(10)(11). Knockin mice expressing the most common of these, SERT Ala56 (12), display hyperserotonemia, increased 5-HT clearance, and hypersensitivity of central 5-HT 1A and 5-HT 2A receptors, as well as repetitive behaviors and deficits of socialization that are reminiscent of ASD (13).…”

Control of serotonin transporter phosphorylation by conformational state

“…Within these families, individuals with the mutation (Ile425Val, or I425V) were more strongly affected with several psychiatric disorders, including obsessive-compulsive disorder and autism spectrum disorders. Subsequently, additional families with this mutation and others in SERT have been identified (2)(3)(4)(5)(6)(7)(8).…”

Autism associated with low 5-hydroxyindolacetic acid in CSF and the heterozygous SLC6A4 gene Gly56Ala plus 5-HTTLPR L/L promoter variants

“…Among them, polymorphisms within the 5-HTTLPR promoter sequence, mutations in the coding sequence, or intronic mutations of this serotonin transporter were reported to be linked to autism by some studies, but other studies have been inconclusive [8][9][10]. The SLC6A4 5-HTTLPR promoter sequence, located about 1 kb upstream of the transcription initiation site, contains two variable repeat length polymorphisms known as the long (L) with 16 repeat elements, or the 44 bp shorter (S) variant with 14 repeat elements [11,12], which determines a different expression of the serotonin transporter in pre-synaptic axonic membranes.…”

“…[14]). In addition to these non-coding variants in the promoter region, a number of mutations in the coding region modulate SERT activity [10]. Among them, the non-synonymous variant Gly56Ala in exon 3 of the SLC6A4 gene was reported to be autism-associated and showed constitutively elevated SERT activity [15].…”