Clonetics Conditionally Immortalized Human Cells. Relevant Cells for High Throughput Screening

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1 Clonetics Conditionally Immortalized Human Cells Relevant Cells for High Throughput Screening

2 Clonetics Conditionally Immortalized Cell Strains Conditionally immortalized cell strains are primary cells that behave normally in cell culture, can be epanded beyond 40 population doublings, yet maintain normal behaviors throughout the culture time. This powerful breakthrough is achieved by combining the telomere rejuvenating effects of telomerase with a specially modified temperature sensitive large-t (ts LT) antigen in the same cell. The ts LT mutant drives cell proliferation while controlling the classical behaviors of SV40 that impact karyotypic stability over multiple population doublings. Consequently, the conditionally immortalized cell strain acts like a primary cell, provides homogeneous cell populations at the high cell counts of cell lines, and allows researchers to eliminate poor drug candidates early to focus on more promising candidates. 1

3 The Problem Today 20% 12% It takes years to develop a drug candidate into a commercially available product; consequently, the most promising compounds need to be identified early. Twenty-two percent of new drugs fail due to efficacy. The drug discovery teams need to be confident, early on, that they are investing development time in the best candidates with the highest probability of surviving the clinical trials gauntlet. The pharmaceutical organizations that don t do this successfully, fail. 23% 22% 23% Clinical Efficacy Portfolio Reasons Clinical Safety Toicity ADME Figure 1. Five reasons why drug candidates fail in clinical trials. Twenty-two percent of new drugs fail due to efficacy. Poor efficacy could be identified early by using normal human cells. Limitations of Today s Tools Live animal models yield a rich supply of data, but are not cost effective for high throughput screening. One must also engage in species etrapolation to reach a best guess of how the drug will work on humans. Cell lines provide the high cell count and homogeneous populations for high throughput applications, but don t ehibit normal behavior, as do primary cells. It is well accepted that primary cells behave more normally than cell lines. Yet, key limitations eist that preclude one from using primary cells during initial phases of drug screening. The limitations include cost, homogeneity of cell population and total cell count achievable, donor to donor variability, and loss of key cellular behaviors when the cell is outside its home organ. 2

4 33 C prb E2 F E2F prb Ts T Antigen 37 C prb E2 F E2F Cell Division Blocked prb Proliferation S Phase Conditional Immortalization The Breakthrough! Temperature sensitive large-t (ts LT) Our construct of a specially modified ts LT drives cells to proliferate for many population doublings, can be switched off to allow the cells to differentiate under the proper media conditions, and confers stability to the host genomic background. The ts LT mutant codes for a gene product that is functional at 33 C, but non-functional at 37 C. Though the construct overcomes the cells natural senescence behavior at low passage and provides a strong proliferative signal, the cells are not able to ehibit a full range of differentiated functions. By switching the culture conditions to 37 C, one stops cell proliferation and the cells are responsive to media growth factors that drive differentiation. If the line is carried at 37 C for sufficient time, the strain permanently looses its ability to proliferate. A second mutation in the construct controls SV40 s ability to auto-ecise. Consequently, this mutant conveys a higher level of karyotype stability to the host genomic background. Ts T Antigen Proliferation S Phase Figure 2. Temperature sensitive T antigen. Using temperature sensitive T antigen, we can switch off the proliferation by a simple change in temperature. Chromosome Telomere Repeats Each time cell divides, the telomeres become shorter provides chromosomal stability over multiple cell division events. During mitosis, cells make copies of their genetic material. To make sure that information is successfully passed from one generation to the net, each chromosome has a special protective cap of a (TTAGGG) repeat sequence called a telomere located at the end of its arms. Telomeres can reach a length of 15,000 base pairs and function by preventing chromosomes from losing base pair sequences at their ends. They also stop chromosomes from fusing to each other. However, each time a cell divides, some of the telomere is lost (usually base pairs per division). When the telomere becomes too short, the chromosome reaches a critical length and can no longer replicate. At this point, cells senesce and may eventually be removed by apoptosis. Telomere activity is controlled by two mechanisms: erosion and addition. Erosion, as mentioned, occurs each time a cell divides. Addition is determined by the activity of telomerase. When telomeres shorten, chromosomes become unstable and cell division is blocked can etend telomeres and allow continuation of cell division, also called telomere terminal transferase, is an enzyme made of protein and RNA subunits that elongates chromosomes by adding TTAGGG sequences to the end of eisting chromosomes. is found in fetal tissues, adult germ cells, and tumor cells. activity is regulated during development and has a very low, almost undetectable, activity in somatic (body) cells. Since these somatic cells do not regularly use telomerase, they age. If telomerase is activated in a cell, the cell will continue to grow and divide; consequently, active telomerase is critical to avoid senescence and generate billions of daughter cells for high throughput screening. Figure 3. activity in normal cells. Addition of telomerase to a somatic cell can lengthen telomeres, allowing cell division. 3

5 The Power of the Combination Sequential transfection at low passage using ts LT, followed by telomerase, is critical to yielding the most prolific strains, highly responsive to differentiation. 37 C Normal Phenotype Cells isolated from human tissues HMF3 Transfect with Large T-antigen p6 ts p4 ts p12 p12 p4 ts p6 Cells are created epressing temperature sensitive Large T-antigen and telomerase p18 p18 = 45 PD ts p17 p18 = 49 PD p20 = 54 PD p24 = 64 PD p25 = 66 PD Split ratio 1:4 Split ratio 1:10 / 1:20 33 C Large T-antigen Active Immortalized Phenotype (gene epression altered) Cell population epanded due to immortalization 37 C Normal phenotype Immortalization Switched ON Immortalization Switched OFF HMF3A p64 = 195 PD HMF3B p56 = 147 PD HMF3C p55 = 170 PD HMF3D p68 = 252 PD Cells revert to a normal phenotype Fig. 4. Retroviral transduction with ts LT and htert. Sequence of retroviral gene transduction and resulting population-doubling (PD) potential of human adult mammary fibroblasts from a single 19-year-old donor (HMF3). Figure 5. How conditional immortalization works. Normal Cells for HTS and Drug Discovery References Fauth, C., O Hare, M. J., Lederer, G., Jat, P. S., and Speicher, M. R. (2004) Order of genetic events is critical determinant of aberrations in chromosome count and structure. Genes, Chromosomes & Cancer 40: O Hare, M. J., Bond, J., Clarke, C., Takeuchi, Y., Atherton A. J., Berry, C. (2001) Conditional immortalization of freshly isolated human mammary fibroblasts and endothelial cells. PNAS 98: By combining the proliferation driving power of ts LT and telomerase to manage cellular senescence, one achieves conditional immortalization. Conditional immortalization allows, at a permissive temperature, production of large, uniform cell populations. At this permissive temperature, telomerase is actively reversing telomere shortening, as daughter cells are spawned. Concurrently, at the permissive temperature, the special ts LT is actively driving cell proliferation while maintaining karyotype stability over 40+ population doublings. By increasing the culture temperature, the cells stop dividing. One then converts to a differentiation medium, so that the cells can epress normal differentiated function and phenotype once again. Now drug discovery researchers have access to an unlimited supply of differentiated cells of the same genotype that can be used as models in drug discovery programs for functional cell-based assays and long-term gene epression studies. 4

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