ZURICH (Reuters) - Roche Holding (NYSE: ROG) and Spark Therapeutics (NASDAQ: ONCE) announced on Tuesday another extension of the Swiss drugmaker's $4.3 billion takeover offer for the U.S. gene therapy specialist as regulatory reviews continue.

The offer for Spark shares now runs to Oct. 1, Roche said.

Around 24.1% of Spark's outstanding shares had been tendered as of the end of August.

"The offer was extended to provide additional time for the U.S. Federal Trade Commission (the 'FTC') and the UK Competition and Markets Authority (the 'CMA') to complete their previously disclosed reviews of Roche's pending acquisition of Spark. The parties remain committed to the transaction and are working cooperatively and expeditiously with the FTC and the CMA," a statement said.

(Reporting by Michael Shields, editing by John Revill)