A third gene locus for tuberous sclerosis is closely linked to the phenylalanine hydroxylase gene locus.

Following the observation of a patient suffering from tuberous sclerosis (TSC) with a de novo reciprocal translocation t(3;12) (p26.3;q23.3), we have undertaken a linkage study in 15 TSC families using polymorphic DNA markers neighbouring the chromosome breakpoints. Significant lod scores have been obtained for markers D12S7 (Zmax = 2.34, theta = 0.14) and PAH (phenylalanine hydroxylase (Zmax = 4.34, theta = 0.0). In multipoint linkage analysis, the peak lod score was 4.56 at the PAH gene locus. These data suggest the existence of a third gene locus for TSC (TSC3) on chromosome 12q22-24.1. The regions that have been found to be linked to TSC in different families map to the positions of three enzymes, phenylalanine hydroxylase (12q22-24), tyrosinase (11q14-22), and dopamine-beta-hydroxylase (9q34), all of which are involved in the conversion of phenylalanine to catecholamine neurotransmitters or melanin. Disorders of these biochemical pathways might be involved in the pathogenesis of TSC.