U.S. officials have approved the costliest medicine in history: a $2.1-million gene therapy that treats a rare childhood disease. Gene therapy is a medical treatment in which healthy genetic material is introduced into cells to replace abnormal genes. The new gene therapy targets a disease called spinal muscular atrophy (SMA), which severely weakens a child’s muscles. The treatment, called Zolgensma, was developed by Swiss drugmaker Novartis. Over time, SMA can make it impossible for the child to move, swallow or breathe. The disease affects about 1 in 10,000 births. In 90 percent of cases, the condition causes death or the need for permanent breathing assistance by age two. SMA is the leading cause of genetic infant death.