All agree that any drug getting schizophrenics back to normal would be a blockbuster. The more we study its genetics and biochemistry the harder the task becomes. Here’s one target — neuregulin1, one variant of which is strongly associated with schizophrenia (in Iceland).
Now that we know that neuregulin1 is a potential target, why should discovering a drug to treat schizophrenia be so hard? The gene stretches over 1.2 megaBases and the protein contains some 640 amino acids. Cells make some 30 different isoforms by alternative splicing of the gene. Since the gene is so large one would expect to find a lot of single nucleotide polymorphisms (SNPs) in the gene. Here’s some SNP background.
Our genome has 3.2 gigaBases of DNA. With sequencing being what it is, each position has a standard nucleotide at each position (one of A, T, G, or C). If 5% of the population have any one of the other 3 at this position you have a SNP. By 2004 some 7 MILLION SNPs had been found and mapped to the human genome.
Well it’s 10 years later, and a mere 23,094 SNPs have been found in the neuregulin gene, of which 40 have been associated with schizophrenia. Unfortunately most of them aren’t in regions of the gene which code for amino acids (which is to be expected as 640 * 3 = 1920 nucleotides are all you need for coding out of the 1,200,000 nucleotides making up the gene). These SNPs probably alter the amount of the protein expressed but as of now very little is known (even whether they increase or decrease neuregulin1 protein levels).
An excellent review of Neuregulin1 and schizophrenia is available [ Neuron vol. 83 pp. 27 - 49 '14 ] You’ll need a fairly substantial background in neuroanatomy, neuroembryology, molecular biology, neurophysiology to understand all of it. Included are some fascinating (but probably incomprehensible to the medicinal chemist) material on the different neurophyiologic abnormalities associated with different SNPs in the gene.
Here are a few of the high points (or depressing points for drug discovery) of the review. Neuregulin1 is a member of a 6 gene family, all fairly similar and most expressed in the brain. All of them have multiple splicing isoforms, so drug selectivity between them will be tricky. Also SNPs associated with increased risk of schizophrenia have been found in family members numbers 2, 3 and 6 as well, so neuregulin1 not be the actual target you want to hit.
It gets worse. The neuregulins bind to a family of receptors (the ERBBs) having 4 members. Tending to confirm the utility of the neuregulins as a drug target is the fact that SNPs in the ERBBs are also associated with schizophrenia. So which isoform of which neuregulin binding to which iso form of which ERBB is the real target? Knowledge isn’t always power.
A large part of the paper is concerned with the function of the neuregulins in embryonic development of the brain, leading the the rather depressing thought that the schizophrenic never had a change, having an abnormal brain to begin with. A drug to reverse such problems seems only a hope.
The neuregulin/EBBB system is only one of many genes which have been linked to schizophrenia. So it looks like a post of a 4 years ago on Schizophrenia is largely correct — http://luysii.wordpress.com/2010/04/25/tolstoy-was-right-about-hereditary-diseases-imagine-that/
Happy hunting. It’s a horrible disease and well worth the effort. We’re just beginning to find out how complex it really is. Hopefully we’ll luck out, as we did with the phenothiazines, the first useful antipsychotics.