The Swiss company said it was extending its offer to May 2 from the original deadline of Wednesday, having received support from holders of only 29.4 percent of Spark shares. It needs a majority for its offer to go through.
A spokesman said Roche remained confident the deal would be completed by the end of June.
"All terms and conditions of the offer shall remain unchanged during the extended period," Roche said in a statement on the $114.50 per share offer it announced in February.
Roche also said a review of the acquisition by the U.S. Federal Trade Commission (FTC), the government agency which seeks to prevent anti-competitive practices, was taking more time than anticipated and its plans had had to be refiled.
It did not give any further detail on the refiling but said it had been agreed with the FTC and with Spark.
"The FTC review and clearance is required for the deal to be completed and needs more time," a Roche spokesman said.
"The deal is not in doubt at all and we expect it will be completed according to our guidance in the first half of 2019. There needs to be more than 50 percent of the shares to be tendered, but we believe our offer to be full and fair and it has been recommended by the board of Spark," the spokesman added.
Spark shares closed at $114.01 on Tuesday. Roche shares were down 0.3 percent in early trade.
Gene therapies use specially engineered viruses to deliver genetic material into defective cells, in hopes of improving or potentially even curing an inherited condition.
Roche offered more than twice the Philadelphia-based company's closing price on Feb. 22 for a portfolio that includes a blindness treatment that has U.S. and European approval and other projects for haemophilia and neurodegenerative disorders such as Huntington's disease.
The offer came as rivals including Novartis also move into gene therapy, where treatments for rare, inherited diseases command some of the highest prices in medicine.
Spark's blindness therapy Luxturna for instance is priced at $850,000 per patient. The treatment had sales of $27 million in 2018.
However Spark faces at least three lawsuits in the United States by shareholders challenging the sale, on grounds that it undervalues Spark's stock and is unfair to shareholders.
Roche on Wednesday declined to comment on the lawsuits.
Roche Chief Executive Severin Schwan is interested in gene therapies to help compensate for patent losses on his $21 billion per year trio of cancer medicines Rituxan, Herceptin and Avastin, which are facing competition from cheaper copies.
Spark, which also has product candidates that have shown promising early results in patients with haemophilia, reduced its net loss to $78.7 million in 2018, while revenue increased to $64.7 million.
(Editing by David Holmes)
By John Revill and John Miller