Kranzler et al, in the April 2014 edition of The American Journal of Psychiatry, show that topiramate 200 mg daily led to very substantial reduction in alcohol use in heavy drinkers, compared to placebo. But this effect was dramatically present only for a subgroup of drinkers who have the CC genotype of the rs2832407 gene. This genotype is carried by about 42% of people having European ancestry.
Topiramate stands out as a very reasonable, safe, and relatively well-tolerated adjunct in the treatment of alcoholism. I don't think it is necessary to test for the genotype--it would be reasonable to offer an empirical trial, and to predict with the patient that there will be about a 40% chance of the medication having a dramatic effect. If it doesn't help, the risks would be minimal. Since topiramate is an anticonvulsant, it could theoretically treat or prevent withdrawal symptoms, even if it doesn't independently reduce the urge to drink.