UPDATE 1-Spark to charge $850,000 per patient for blindness gene therapy

(Adds company comment, updates share price)

Jan 3 (Reuters) - Spark Therapeutics Inc said on Wednesday it will charge $850,000 per patient for its breakthrough gene therapy to treat a rare form of blindness, a lower than expected price that the company said reflects patient and insurer concerns about access and cost.

Investors had expected the company to charge closer to $1 million for the treatment, Luxturna, which is designed to be given just once. The product, approved in December by the Food and Drug Administration, is the first U.S.-approved gene therapy for an inherited disease.

Spark Therapeutics said it has created a number of payment models to help reduce costs for insurers by allowing installment payments over a number of years and to ensure financial concerns do not hinder access for patients.

"The price reflects the stakeholder considerations we have learned about these past months," the company said in a statement, "and our need to build a sustainable company that addresses the unmet needs of patients with genetic diseases."

Spark's shares slipped 0.4 percent to $53.12 in early trading.

Luxturna treats inherited retinal disease caused by defects in a gene known as RPE65, which tells cells to produce an enzyme critical for normal vision. The condition affects between 1,000 and 2,000 people in the United States. (Reporting by Toni Clarke in Washington and Akankshita Mukhopadhyay in Bengaluru; Editing by Arun Koyyur and Bill Trott)