Assuming that issues with the production of pluripotent cells from patients' tissues, and the generation of differentiated cells from them, can be resolved, what other concerns remain? Unfortunately, there are many fundamental questions that have still not been addressed.
One important issue relates to the degree of maturity of the cells that are produced. For instance, ALS and SMA are both motor neuron diseases, but they affect different motor neuron populations. In ALS, neurons innervating distal muscles are most sensitive while, in SMA, those innervating proximal muscles are most at risk. Furthermore, in both diseases certain rare populations of motor neurons are completely unaffected so, in theory, when trying to model those diseases, using very specific types of motor neurons would be most appropriate. In fact, motor neurons produced by the most common differentiation protocols have a rather generic rostral cervical identity, although there is good reason to think that they can be induced to differentiate further by additional morphogens [44
]. Presumably, a transdifferentiation approach in which the correct motor neuron pool-specific transcription factors are expressed in the motor neurons could also be successful.
This is not the only consideration, though. Typically, cells derived from pluripotent cells resemble their embryonic or immature counterparts (for example, [45
]). Can they be induced to mature sufficiently to model adult disease? This has been hotly debated, especially in the context of late onset disease [46
]. Many neurodegenerative disorders, such as AD and PD, take decades to affect humans and even many months to affect transgenic mice, so is it reasonable to think that neurons derived from stem cells could be induced to adopt a disease phenotype? It is possible that even in the late onset diseases, some of the pathological changes, such as protein aggregation, occur long before clinical symptoms. Another possibility relates to the fact that many of these diseases are primarily sporadic and may be initiated by the presence of particular environmental factors. Exposing cells to high concentrations of, or prolonged incubation with, these factors might greatly accelerate the appearance of pathology in the cell culture environment. For example, the addition of cellular stressors, such as pro-oxidants or other compounds that compromise mitochondrial function, might bring on disease-related alterations [45
Another important issue concerns the nature of the diseases that realistically can be modeled by applying a reprogramming or even a transdifferentiation method to patient-derived cells. Naturally, monogenic diseases seem most amenable to this technique, and monogenic diseases that affect predominantly one cell type are likely to be better still. For these conditions, the expectation is that the reprogramming process will maintain the mutations involved, as will the differentiation protocol. However, what about diseases that are mostly sporadic and might involve epigenetic modifications of the genome? In those cases, reprogramming would tend to erase most of the epigenetic marks. Perhaps the transdifferentiation method will help in this regard, but this is not yet clear.
Certainly, the major degenerative diseases of the nervous system are primarily late onset and, while mostly sporadic in nature, are known to involve a small percentage of cases with well known disease-causing mutations. One way forward that may be both doable and instructive is to establish an in vitro
phenotype using the genetic variants of the disease first, and then test the sporadic cases to determine if there are culture conditions that will produce the same disease pathology. Alternatively, it might be possible to identify pathology-producing cell culture manipulations that are informative about identifying the causative factors for the disease: for example, addition of certain insecticides may accelerate the onset of disease features in a PD model [45
Starting just a few years ago, there have been many attempts to apply an overall stem cell strategy to the understanding of specific diseases. The typical starting point has been the production of patient-specific iPSCs. One of the first comprehensive reports was that of Park and colleagues [47
], who derived them from patients with adenosine deaminase deficiency-related severe combined immunodeficiency, Shwachman-Bodian-Diamond syndrome, Gaucher disease type III, Duchenne and Becker muscular dystrophy, PD, Huntington's disease, juvenile-onset diabetes, and Down syndrome/trisomy 21. More recent efforts have included production of iPSCs from patients with SMA [48
], ALS [49
], and Hutchinson-Gilford Progeria Syndrome (premature aging, associated with vascular defects) [50
]. A few illustrative cases will be presented.