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Clues from the resilient
http://www.ScienceMag.org/content/344/6187/970.full Potential 2nd site mutations that neutralize #Mendelian-disease mutations

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“For 127 catastrophic Mendelian diseases (those caused by a single gene such as cystic fibrosis and ataxia-telangiectasia), there are presently 164 genes harboring 685 known recurrent variants that are highly penetrant and causal for deleterious traits, most typically manifesting in individuals before the age of 18 (). …For common diseases, the observed small effect sizes of individual gene variants limit diagnostic potential, and given that most variants identified have an unclear function, how to target the corresponding gene for therapeutic intervention is typically unclear. For rarer Mendelian disorders, although genetics directly implicate a specific gene in a disease, a majority of such cases relate to loss-of-function mutations. Designing small molecules to fix the corresponding broken protein has proven difficult….
The prominent role of second-site mutations and environmental factors that enable resistance to (or buffer against) disease traits has been well established in a multitude of model organisms from yeast to mice (–). Screening for second-site mutations in “resilient” individuals that prevent disease-causing alleles from manifesting their effects could identify targets to which drugs would be designed to disrupt their function, as opposed to targeting the disease-causing gene directly. Genetic studies examining seemingly healthy people have revealed, for example, rare mutations in chemokine (C-C motif) receptor type 5 (the co-receptor for human immunodeficiency virus) that block HIV infection (), and secondary mutations in fetal globin genes that modify the severity of sickle cell disease by buffering primary mutations in β-globin genes ()
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