It was Victoria Gray’s first time in London and, despite a sleepless plane ride across the Atlantic Ocean, she wasn’t about to skip sightseeing. While crossing Trafalgar Square, Gray paused briefly to reflect on her experience. “I would never have been able to walk this long before,” she told a NPR reporter. “I feel like I got a second chance.”Four years earlier, in 2019, Gray had become the first patient with sickle cell anemia — a genetic disorder that causes red blood cells to become sticky a...