Alnylam Pharmaceuticals Inc on Wednesday received an early U.S. approval for its gene silencing drug to treat patients with a rare genetic disorder that can cause severe pain. The drug, Givlaari, uses a mechanism known as RNA interference to target and "silence" specific genetic material, blocking the production of the protein that causes the disease called acute hepatic porphyria. The buildup of the protein can cause acute seizures that occur suddenly and can produce permanent neurological damage and death, according to the U.S. Food and Drug Administration. Alnylam did not immediately respond to a Reuters request for pricing details. Shares of the company were up 7.5% before trading was halted ahead of the announcement. "Givlaari will be an entirely new therapy for a poorly treated rare disease patient population," said Oppenheimer analyst Leland Gershell, adding he expected peak sales of $560 million.