Genetics 101 for cardiologists: rare genetic variants and monogenic cardiovascular disease.

Monogenic diseases have a distinctive familial inheritance that follows Mendel's laws, showing patterns like dominant, recessive, or X-linked. There are > 7000 monogenic diseases curated in databases, and together they account for up to 10% of all illnesses encountered in the emergency room or clinic. Despite the rarity of individual monogenic conditions, mapping their causative genes and mutations is important for several reasons. First, knowing the causative gene and mutation could provide actionable information for genetic counselling. Sometimes, knowing the gene and mutation allows for early diagnosis in affected families, which is important if there is an evidence-based intervention. Second, the implication of a mutant gene as being causative for a clinical phenotype provides strong evidence of the importance of the gene product in a cellular or biochemical pathway. Discovery of new molecular pathways in families with rare diseases can serve as the first step toward developing rational therapies to help not only affected families, but also patients with less extreme, nongenetic forms of the same condition. For instance, the study of rare patients with familial hypercholesterolemia helped in developing statin drugs, initially as a treatment for familial hypercholesterolemia but now a widely used therapy to reduce low-density lipoprotein cholesterol and cardiovascular disease risk.