In the new era of gene replacement and gene-editing therapeutics, the development focus is higher efficiency and higher fidelity gene transfer vectors and gene engineering technologies. Yet, the most important factor for success may prove to be tissue stem cells, and in particular their number.
Solving one difficult problem at a time is how biomedicine sometimes appears to move forward, especially when problems are serially dependent. However, this impression can also be made when solving one challenge reveals new ones. Previously non-limiting factors can emerge as new rate-limiting problems at the very moment when an earlier more significant problem is solved. Gene replacement and gene-editing therapeutics may be entering such a period of trial now.
In the early days of gene therapy, the singular challenge was getting therapeutic DNA expressed in cells before it was destroyed or loss. Advances in both approach and technology reduced this problem dramatically. The advance in approach was removing cells and genetically engineering them in vitro before returning them to the body. The advance in technology includes myriad developments in molecular genetic engineering that inform the expression and regulation of gene-encoding DNA.
More recent progress in molecular genetics engineering has brought the power of high efficiency lentivirus vectors designed to improve gene replacement success in rare tissue stem cells and high fidelity gene-editing that reduces the risk of cancer, which plagued earlier gene therapy technologies that used extrinsic promoter elements to express transferred genes. Though these new advances are not without new risks (e.g., off-target mutations), they have moved into clinical trial development without any apparent overt technical challenges.
However, an important, and perhaps presently unrecognized, technical problem that may lie in the path of the future success of gene replacement and gene-editing clinical trials and therapies is unknown stem cell number. Hematopoietic stem cells are the curative gene therapy cell target for these early trials. Recent trials have proceeded without knowing the number of hematopoietic stem cells used for genetic engineering, the number successfully engineered, or the number of engineered cells transplanted into patients. The same deficiencies will be present in the first gene-editing trials that may occur later this year.
A priori, it should be self-evident that knowing the number of the target stem cells for gene replacement or gene-editing would be a important factor for success. In practice, the noted progress in molecular genetics may have uncovered stem cell number as the new rate-limiting problem for successful gene therapy. No matter how efficient the gene transfer or how faithful the gene-editing, failure to transplant enough genetically engineered stem cells will result in treatment failure in the long term.
Committed progenitor cells, the non-stem progeny of hematopoietic stem cells, out-number their parents significantly. Depending on the tissue cell preparation, committed progenitors can be present in 100-fold to as much as 10000-fold excess. Therefore, whether or not sufficient hematopoietic stem cells have been effectively modified is currently really only a guess that is not affirmed until a patient obtains a long-term cure. It is more likely that abundant, genetically modified committed progenitor cells will be responsible for initial positive responses, which are not stable and fail in the long term. Of course, there can be other causes of treatment failure, too, that are related to expression of the curative gene itself (e.g., suppression by methylation). Deciphering between these possible causes of failures, and preventing the former cause altogether, is not possible without being able to count stem cells, specifically.
At the end of 2015, Asymmetrex listed the following entry as the sixth way in which stem cell biomedicine could improve in 2016, if a method for counting tissue stem cells were available.
“6. Gene therapy trials, which depend on correcting defective genes or introducing curative genes in long-lasting tissue stem cells, would no longer be attempted without being certain that sufficient tissue stem cells were present for effective therapy.”
2016 is now under way, and a technology for counting human hematopoietic stem cells is now available. Gene therapy and gene-editing companies are invited to visit here to learn more about counting hematopoietic stem cells and other targeted human adult tissue stem cells to accelerate progress in their gene replacement therapy and gene-editing therapy clinical trials.
In an upcoming free RegMedNet webinar, scheduled for March 22, Director James L. Sherley will review Asymmetrex’s new “AlphaSTEM” technology for counting adult tissue stem cells and its applications for gene replacement and gene-editing therapeutics development.