Dec 20 (Reuters) - Bluebird bio Inc (BLUE.O) said on Monday its gene therapy trial for sickle cell disease has been placed on partial hold for children while the company investigates the potential side-effect of persistent anemia in a patient. Bluebird shares were down 18% in premarket trade following the news. The U.S. Food and Drug Administration has imposed the partial suspension of lovotibeglogene autotemcel gene therapy trial in patients under the age of 18, the company said. However, enrollment of adult sickle cell disease patients in three studies, as well as follow up for treated patients of all ages in all studies, is continuing as planned, it said. The gene therapy developer said it is evaluating what impact, if any, the partial suspension may have on its projected timing for the submission of its application for the treatment, which is currently the first quarter of 2023.