HMGCR and Huntington disease: However, one study reported that fibroblasts from patients with HD have been found to show no alterations in cholesterol content or in the activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoAR), the key enzyme in the sterol synthesis pathway in fibroblasts cells [201], suggesting the brain cell-specific function in regulating cholesterol levels in HD.