H1 (WA01) ES cells were obtained from WiCell Research Resources (Wicell, WI); the original untargeted iPS cell lines were derived from dermal fibroblasts or keratinocytes of two patients with dystrophic epidermolysis bullosa. Targeting of the COL7A1 locus and successful excision of both the reprogramming cassette and the positive selection cassette were achieved in both lines33. The clones that were karyotypically normal were used in the study and were named as iPSC#1 line and iPSC#2 line. iPSC#3 was derived from stored peripheral T cells obtained from the Rutgers University Cell and DNA Repository (RUCDR). ES and iPS cells were maintained as feeder-free cells in mTeSR1 medium (Stem Cell Technologies; ref. 34). Mouse glial cells were isolated from the forebrain of newborn wild-type CD1 mice35. Briefly, newborn mouse forebrain homogenates were digested with papain and EDTA for 15 min, cells were dissociated by harsh trituration to avoid growing of neurons and plated onto T75 flasks in MEM (Invitrogen) supplemented with 10% fetal bovine serum (FBS). Upon reaching confluence, glial cells were trypsinized and replated at lower density for a total of three
