Increased platelet serotonin level (PSL) has been consistently found in a portion of autistic patients. Suggested mechanisms
for hyperserotonemia in autism have been increased synthesis of serotonin (5HT) by tryptophan hydroxylase
(TPH), increased uptake into platelets through 5HT transporter (5HTt), diminished release from platelets through 5HT2A
receptor (5HT2Ar) and decreased metabolism by monoamine oxydase (MAOA). The allelic influence of genes, encoding
the mentioned 5HT elements, on PSL was investigated in 63 autistic subjects. Our study shows that 5HTt-LPR and
–1438AG 5HT2Ar genotypes did not significantly affect PSL. However, significantly higher PSLs were observed in subjects
with »cc« genotype of a218c TPH and subjects with »4« genotype of uVNTR MAOA. In addition, when TPH-cc and
MAOA-4 were combined as »high 5HT« genotypes, a correlative increase in PSL was observed with the increase in the
number of »high 5HT« genotypes. These results suggest a possible synergistic effect of genes regulating 5HT synthesis/
degradation in dysregulation of the peripheral 5HT homeostasis of autistic patients
