The challenges of gene and cell therapists can be divided into three broad categories based on disease, development of therapy, and funding.

Challenges based on the disease characteristics: Disease symptoms of most genetic diseases, such as Fabrys, hemophilia, cystic fibrosis, muscular dystrophy, Huntingtons, and lysosomal storage diseases are caused by distinct mutations in single genes. Other diseases with a hereditary predisposition, such as Parkinsons disease, Alzheimers disease, cancer, and dystonia may be caused by variations/mutations in several different genes combined with environmental causes. Note that there are many susceptible genes and additional mutations yet to be discovered. Gene replacement therapy for single gene defects is the most conceptually straightforward. However, even then the gene therapy agent may not equally reduce symptoms in patients with the same disease caused by different mutations, and even the samemutationcan be associated with different degrees of disease severity. Gene therapists often screen their patients to determine the type of mutation causing the disease before enrollment into a clinical trial.

The mutated gene may cause symptoms in more than one cell type. Cystic fibrosis, for example, affects lung cells and the digestive tract, so the gene therapy agent may need to replace the defective gene or compensate for its consequences in more than one tissue for maximum benefit. Alternatively, cell therapy can utilizestem cellswith the potential to mature into the multiple cell types to replace defective cells in different tissues.

In diseases like muscular dystrophy, for example, the high number of cells in muscles throughout the body that need to be corrected in order to substantially improve the symptoms makes delivery of genes and cells a challenging problem.

Some diseases, like cancer, are caused by mutations in multiple genes. Although different types of cancers have some common mutations, every tumor from a single type of cancer does not contain the same mutations. This phenomenon complicates the choice of a single gene therapy tactic and has led to the use of combination therapies and cell elimination strategies. For more information on gene and cell therapy strategies to treat cancer, please refer to the Cancer and Immunotherapy summary in the Disease Treatment section.

Disease models in animals do not completely mimic the human diseases and viralvectorsmay infect various species differently. The testing of vectors in animal models often resemble the responses obtained in humans, but the larger size of humans in comparison to rodents presents additional challenges in the efficiency of delivery and penetration of tissue.Gene therapy, cell therapy, and oligonucleotide-based therapy agents are often tested in larger animal models, including rabbit, dog, pig and nonhuman primate models. Testing human cell therapy in animal models is complicated by immune rejections. Furthermore, humans are a very heterogeneous population. Their immune responses to the vectors, altered cells, or cell therapy products may differ or be similar to results obtained in animal models.

Challenges in the development of gene and cell therapy agents: Scientific challenges include the development of gene therapy agents that express the gene in the relevant tissue at the appropriate level for the desired duration of time. There are a lot of issues in that once sentence, and while these issues are easy to state, each one requires extensive research to identify the best means of delivery, how to control sufficient levels or numbers of cells, and factors that influence duration of gene expression or cell survival. After the delivery modalities are determined, identification and engineering of a promoter and control elements (on/off switch and dimmer switch) that will produce the appropriate amount of protein in the target cell can be combined with the relevant gene. This gene cassette is engineered into a vector or introduced into thegenomeof a cell and the properties of the delivery vehicle are tested in different types of cells in tissue culture. Sometimes things go as planned and then studies can be moved onto examination in animal models. In most cases, the gene/cell therapy agent may need to be improved further by adding new control elements to obtain the desired responses in cells and animal models.

Furthermore, the response of the immune system needs to be considered based on the type of gene or cell therapy being undertaken. For example, in gene or cell therapy for cancer, one aim is to selectively boost the existing immune response to cancer cells. In contrast, to treat genetic diseases like hemophilia and cystic fibrosis the goal is for the therapeutic protein to be accepted as an addition to the patients immune system.

If the new gene is inserted into the patients cellularDNA, the intrinsic sequences surrounding the new gene can affect its expression and vice versa. Scientists are now examining short DNA segments that may insulate the new gene from surrounding control elements. Theoretically, these insulator sequences would also reduce the effect of vector control signals in the gene cassette on adjacent cellular genes. Studies are also focusing on means to target insertion of the new gene into safe areas of the genome, to avoid influence on surrounding genes and to reduce the risk of insertional mutagenesis.

Challenges of cell therapy include the harvesting of the appropriate cell populations and expansion or isolation of sufficient cells for one or multiple patients. Cell harvesting may require specific media to maintain the stem cells ability toself-renew and mature into the appropriate cells. Ideally extra cells are taken from the individual receiving therapy. Those additional cells can expand in culture and can be induced to becomepluripotent stem cells(iPS), thus allowing them to assume a wide variety of cell types and avoiding immune rejection by the patient. The long term benefit of stem cell administration requires that the cells be introduced into the correct target tissue and become established functioning cells within the tissue. Several approaches are being investigated to increase the number of stem cells that become established in the relevant tissue.

Another challenge is developing methods that allow manipulation of the stem cells outside the body while maintaining the ability of those cells to produce more cells that mature into the desired specialized cell type. They need to provide the correct number of specialized cells and maintain their normal control of growth and cell division, otherwise there is the risk that these new cells may grow into tumors.

Challenges in funding: In most fields, funding for basic or applied research for gene and cell therapy is available through the National Institutes of Health (NIH) and private foundations. These are usually sufficient to cover the preclinical studies that suggest a potential benefit from a particular gene and cell therapy. Moving into clinical trials remains a huge challenge as it requires additional funding for manufacturing of clinical grade reagents, formal toxicology studies in animals, preparation of extensive regulatory documents, and costs of clinical trials.Biotechnology companies and the NIH are trying to meet the demand for this large expenditure, but many promising therapies are slowed down by lack of funding for this critical next phase.

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