The Beery Twins Story and Sepiapterin Reductase

Transcription

1 The Beery Twins Story and Sepiapterin Reductase Sepiapterin reductase (SPR) is an enzyme that makes tetrahydrobiopterin an important cofactor used by other enzymes to make the neurotransmitters dopamine and serotonin. The Beery Twins Story is an example of how whole genome sequencing can be used to practice personalized medicine. Noah and Alexis Beery are fraternal twins. Soon after their birth, they were diagnosed as having DRD (Dopamine Responsive Dystonia) a clinically heterogeneous movement disorder. As they grew older, it became more apparent that this diagnosis was not correct. Although Noah and Alexis were unlucky to have been born with this health condition, they were very lucky to have parents who were relentless in their search for a definitive diagnosis and treatment for their kids. And so, when Joe Beery was hired by Life Technologies (a biotech company involved in NextGen DNA sequencing) as VP of Information Technology, he arranged to have the twins genomes sequenced. To meet the Beerys, visit Family Changed Forever By Sequencing Technology Meet The Beerys.aspx.

2 (from Bainbridge et.al, Sci Transl Med 3, 87re3, pp 1 6 (2011))

3 Sequencing of DNA extracted from the peripheral blood of the twins identified ~2.5 million variants that is, 2.5 million places where each twin s DNA sequence was different from the standard human reference sequence. Of these 2.5 million variants, 1.6 million were shared by each twin. Various bioinformatic filters were then applied in an effort to eliminate variants that were not thought to be responsible for the disease. In the end, only three variants remained as candidate genes. One of these was SPR (sepiapterin reductase) with a known association with dystonia. Examination of the twins SPR genes revealed that they were compound heterozygotes meaning that each twin had two different mutations in their SPR genes each present on a different allele. Subsequent DNA analysis of the SRP gene in other family members established the pedigree shown here: (from Bainbridge et.al, Sci Transl Med 3, 87re3, pp 1 6 (2011))

4 The SPR variants identified by whole genome sequencing was confirmed by Sanger sequencing resulting in the sequencing traces shown below. The p.arg150gly mutation is a A G mutation on chromosome 2 at nucleotide 72,969,094 leading to the replacement of Arginine with Glycine at amino acid 150 of SRP. The p.lys251x mutation is a A T mutation on chromosome 2 at nucleotide 72, resulting in the conversion of a Lysine codon (AAG) to a STOP codon (UAG). This is known as a nonsense mutation. (from Bainbridge et.al, Sci Transl Med 3, 87re3, pp 1 6 (2011)) As shown in the sequencing traces, each twin inherited a SRP gene containing the Arg150Gly mutation for their mother, and a SRP gene containing the Lys251X mutation form their father.

5 Although the molecular structure of the human sepiapterin reductase protein has not been determined, the structure of the mouse enzyme has been solved (see 1SEP.pdb). By mapping the changes caused by these two mutations onto the structure of the homologous mouse protein, we can understand how each mutation could result in an inactive enzyme: The arginine150 which is found at the end of a strand of beta sheet normally functions to stabilize the overall structure of the protein by forming an electrostatic interaction with aspartic acid 144 which is located at the beginning of an alpha helix. This salt bridge stabilizes the turn between the beta strand and the next alpha helix. It seems likely that the elimination of this salt bridge would reduce the stability of the protein, and perhaps even prevent its folding into a functional protein. Mutating the Lysine codon at amino acid 251 into a translation STOP codon results in the premature termination of the protein and the loss of the final ten amino acids at the C terminus of the protein. Remarkably, the third amino acid form the end of the protein is an aspartic acid that makes a critical interaction with tetrahydrobiopterin the product of the sepiapterin reductase enzyme. Therefore, the loss of this critical interaction with the product of the enzyme would undoubtedly affect the activity of the enzyme. In the case of the Beery twins, whole genome sequencing led to a more complete understanding of the molecular basis of their disease, and informed a change in their medical treatment. Since the tetrahydrobiopterin co factor is needed to make the neurotransmitters dopamine, noradrenalin and serotonin this diagnosis resulted in the addition of 5 HTP (a serotonin precursor, downstream from the step requiring tetrahydrobiopterin) to the twins medication. Together with the L dopa that was already being prescribed, this new treatment has resulted in a dramatic improvement in the twins health.