SMA is an autosomal recessive genetic disorder that is caused by mutations in the survival motor neuron (SMN) genes, which lead to motor neuron degeneration and progressive paralysis. The authors generated iPS cells by infecting fibroblasts from the patient with SMA and his unaffected mother with viral vectors encoding the transcription factors OCT4 (also known as POU5F1), SOX2, NANOG and LIN28, which have previously been shown to reprogramme human somatic cells to a pluripotent state. Both cells from the patient (iPS-SMA cells) and cells from the unaffected control (iPS-WT cells) were able to generate teratomas harbouring tissue from the three primary embryonic germ layers (ectoderm, mesoderm and endoderm), but the iPS-SMA cells showed markedly lower levels of SMN1 than the iPS-WT cells.
Given that the lack of SMN proteins specifically affects the viability of motor neurons, the authors differentiated these iPS cells into motor neurons using various growth factors, including retinoic acid and sonic hedgehog. Differentiation was confirmed by checking for the expression of motor neuron transcription factors (HOXB4, OLIG2, ISL1 and HB9 (also known as MNX1)) and markers of mature motor neurons (SMI-32 and choline acetyltransferase). Although similar numbers of motor neurons were initially generated from the WT and SMA iPS cultures, after 6 weeks of differentiation there was a decrease in the size and number of the motor neurons differentiated from the iPS-SMA cultures. Furthermore, after 8 weeks in culture the iPS-SMA-derived cells did not exhibit punctate synapsin staining, suggesting that presynaptic maturation was impaired.
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