On December 2, 2009, NIH director, Francis S. Collins, M.D., Ph.D., announced the approval of the first 13 human embryonic stem cell (hESC) lines for NIH-funded research under the NIH Guidelines for Human Stem Cell Research adopted last July. And 12 days later, he sanctioned another 27 hESC lines. In accordance with the NIH guidelines, these stem cell lines were derived from embryos that were donated under ethically sound, informed-consent processes.
While investigators breathed a sigh of relief, scientists from Scripps Research Institute and separately from the University of Michigan reported a significant lack of ethnic diversity among available hESC lines. This will pose various problems as this technology becomes more widely used in research and clinical settings.
Cytochrome P-450 2D6 (CYP2D6) provides a significant example of ethnic variations at the genetic level. This enzyme is involved in the metabolism of close to 50% of all currently marketed drugs. Genotypic and phenotypic variations of the enzyme, however, often lead to major differences in the concentration of various drugs in the blood and the brain. As a result considerable differences in therapeutic responses and adverse drug effects are observed.
Analysis Points to Under-Represented Groups
The University of Michigan and Scripps groups used SNP analysis to study hESC lines to determine their ethnic origins, linking them to genetic signatures that could affect medical outcomes. Jeanne Loring, Ph.D., senior author of the Scripps paper published in Nature Methods and director of the Center for Regenerative Medicine at The Scripps Research Institute, and colleagues used data from the International HapMap Project.
The HapMap catalogs SNPs from 269 individuals for several million well-defined polymorphisms. It includes data from populations of four regions: Ibadan, Nigeria; Northern and Western Europe; 44 individuals from Tokyo; and 45 individuals from Beijing.
The Scripps team's analysis focused on 47 hESC lines and five widely used induced pluripotent stem cell (iPSC) lines. They demonstrated that these cells originated largely from Caucasian and East Asian populations, with little representation from populations originating in Africa.
The University of Michigan team used published data from 2,001 individuals of known ancestry whose genotypes were included in the HapMap and Human Genome Diversity projects. Their data included more than 480,000 SNPs in the human genome and also represented 47 hESC lines that had been developed in the U.S., Europe, Israel, and Singapore. More than half were cultured at Harvard University.
Their findings, reported in the New England Journal of Medicine, suggest that most hESC lines are of Northern and Western European ancestry. The Michigan group concluded that “efforts to derive and disseminate new stem cell lines should now emphasize under-represented populations to allow researchers to assess the extent to which the ancestry of stem cell lines influences disease models, cellular therapies, and drug screening. Availability of more diverse lines will reduce the risk that the potential benefits of stem cell research will be limited to patients with certain ancestries.”
For example, in 2008, scientists at the African Institute of Biomedical Science and Technology, Harare, Zimbabwe, and the University of Nairobi in Kenya suggested that due to a gene variation, seen more often among the sub-Saharan population, Bristol-Myers Squibb’s HIV drug Sustiva is not metabolized or cleared from the body as quickly. This led to above normal concenrtrations and lack of compliance due to increased toxicity. The research team established a biobank and pharmacogenetics database of African populations, which revealed a high frequency of individuals were homozygous for the CYP2B6*6 allele (18–25%). In Caucasians, on the other hand, CYP2B6*6 frequencies of 5–10% are more typical.
This is consistent with clinical observations that Africans have significantly higher plasma concentrations of Sustiva, a non-nucleoside reverse transcriptase inhibitor used as part of a highly active antiretroviral therapy compared to Caucasians when given at the standard 600 mg per day doses. The scientists thus indicate the need to lower the dose of this drug in people of sub-Saharan origin homozygous for the CYP2B6*6 allele. They believe that this could increase patient compliance and reduce costs.
Drug Development Leveraging hESC Lines
“Lack of genetic diversity among the currently available hESC lines will absolutely limit the value of hESC cells to the pharmaceutical industry,” Dr. Loring told GEN. Yet pharma companies have made and continue to make investments in hESCs for drug discovery, screening, development, and drug-toxicity assessments.
Last June GE Healthcare and Geron formed an alliance to develop and commercialize cellular assay products derived from hESCs that could be used in early in vitro screening of drug candidates to predict toxicity and pharmacological characteristics. The companies said that, for example, derivation of functional cell types from hESCs, in particular hepatocytes of the liver and cardiomyocytes of the heart, could provide a reliable supply of cells to perform metabolism, biodistribution, and toxicity testing of drug candidates.
The program will use hESC lines listed on the NIH Human Pluripotent Stem Cell Registry. The agreement grants GE Healthcare an exclusive license under Geron's extensive intellectual property portfolio covering the growth and differentiation of hESCs. It also gained a sublicense under Geron's rights to the hESC patents held by the Wisconsin Alumni Research Foundation.
Cellartis, a provider of stem cell derived products and technologies, and AstraZeneca initiated an alliance in 2006 to develop assays for target and lead validation, compound screening, drug metabolism studies, and safety assessment. The companies hope the hESC-derived cells will help researchers predict the human body's response to drugs more effectively than immortalized human cell lines, animal-derived cell lines, and tissues that are currently used by most drug makers, said Aileen Allsop, vp of R&D science policy for AstraZeneca.
Allsop pointed out that the firms began the collaboration as a very speculative, small-scale agreement, but AstraZeneca decided to extend the arrangement because it was beginning to give us some promising data. Jan Lundberg, who until November was AstraZeneca’s head of drug discovery, noted “Such a breakthrough could improve research and development productivity by reducing attrition rates and ultimately lead to safer medicines for patients.”
The iPSC Option
Scripps’ Dr. Loring thinks that availability of ethnically representative cell banks will require production of iPSCs, starting with normal fibroblast cell lines or similar. Her group at Scripps has begun to respond to the lack of diversity in hESCs by using skin cells from an individual of West African Yoruba heritage to create a new stem cell line. They claim that it is the first to carry the genetic profile of this ethnic group, expressing distinct genetic markers for disease risk and drug metabolism.
Dr. Loring’s group is working to differentiate the stem cells into cell types useful for drug discovery, screening, and toxicology. She cites the failure of Sustiva to completely address a population in critical need of an HIV drug as a stark example of requiring cells representative of distinct populations. She believes the derivation of iPSCs using cells from distinct populations will create more relevant models of drug development. Dr. Loring describes this work as the foundation of a new database of human pluripotent stem cell genetic information that will be available for other researchers to access for studies on specific genes, stem cell transplantation, and other topics.
iPSCs, however, come with their own set of drawbacks in terms of assessing and maintaining pluripotency. Indeed, hESCs as well require much research and investigation to become a viable source of therapies and a robust technology for drug development. Obtaining greater diversity is a good starting point to ensure that its potential is fully realized in all aspects of healthcare.