Genetic construct

A genetic construct comprising;

• a promoter operably linked to a first coding sequence, which encodes tyrosine hydroxylase (TH) and a second coding sequence, which encodes GTP cyclohydrolase 1 (GCH1);
• wherein the first coding sequence comprises a sequence of SEQ ID NO:
• 1 or SEQ ID No:
• 2, or a fragment or variant thereof having greater than 85% sequence identity to SEQ ID No:
• 1 or SEQ ID No:
• 2;
• or a nucleotide sequence encoding an amino acid sequence as set out in SEQ ID NO:
• 21 or SEQ ID No:
• 22, or a fragment or variant thereof having greater than 85% sequence identity to SEQ ID No:
• 21 or SEQ ID No:
• 22;
• wherein the second coding sequence comprises a sequence of SEQ ID NO:
• 4, or a fragment or variant thereof having greater than 85% sequence identity to SEQ ID No:
• 4;
• or a nucleotide sequence encoding an amino acid sequence as set out in SEQ ID NO:
• 23, or a fragment or variant thereof having greater than 85% sequence identity to SEQ ID No:
• 23;
• wherein the second coding sequence is 3′ to the first coding sequence and the first and second coding sequences are part of a single operon;
• wherein the genetic construct further comprises between the first and second coding sequences a sequence encoding a 2A peptide spacer so as to encode a fusion protein in which the TH protein is fused to the GCH1 protein via the 2A peptide spacer that is cleavable to thereby yield a functional TH protein and a functional GCH1 protein;
• and
• wherein the genetic construct does not encode aromatic amino acid decarboxylase (AADC).