QnAs with Fred Gage.

With the advent of rapid genome sequencing, the view of the human genome as a vast desert dotted with far-flung oases of functional genes has contended with a growing appreciation of the role of regulatory sequences in shaping gene expression. The fraction of the human genome’s noncoding component that can be considered functional remains to be established, but strewn across the seeming wasteland and interspersed with the protein-coding genes are hotbeds of activity, including mobile elements called retrotransposons. Molecular cousins of “jumping genes,” which plant biologist Barbara McClintock discovered in maize in the 1950s, retrotransposons can move through genomes, disrupting genes and displacing regulatory DNA. One group of retrotransposons, called long interspersed nuclear element-1 (LINE-1)—several of which are active in human somatic and sex cells— has become an intense focus of evolutionary research in recent years. Salk Institute neuroscientist Fred Gage has documented higher levels of LINE-1 mobility in certain cell types in nonhuman primates, such as chimpanzees and bonobos, compared with humans. His team has identified proteins that restrict the movement of LINE-1 in human cells, serving as putative guardians of genome integrity. Early hints suggest that the proteins in turn might be regulated through epigenetic mechanisms. Gage recently described his findings at the Arthur M. Sackler colloquium Epigenetic changes in the developing brain: Effects on behavior at the National Academy of Sciences in Washington, DC. PNAS spoke to Gage about the role of retrotransposons in genomic variation. PNAS: In what ways can retrotransposons act as an evolutionary force? Gage: When Barbara McClintock discovered transposons, she proposed that these mobile elements might regulate gene expression, a notion that did not get much attention at the time, even though the idea that DNA can move around in the genome was revolutionary. Since then, we have learned that mobile genetic elements can be an evolutionary force, reshaping the genomic landscape near sites where they land on the genome, potentially creating new regulatory elements and altering gene expression. Retrotransposons, which are active in somatic and germ-line cells, have been hypothesized to contribute to the genetic diversity that arises in evolution. We have shown, for example, that these elements are active in neural progenitor cells and can contribute to somatic evolution. Also, it has been shown that these mobile elements can contribute to disease. PNAS: You set out to create a repository of induced pluripotent stem (iPS) cells from nonhuman primates to generate precursors of neuronal cells. What was the ultimate goal of this effort? Gage: I am the codirector of a San Diegobased group of international researchers of diverse expertise, called the Center for Academic Research and Training in Anthropogeny. As a molecular biologist, I had long wanted to develop a model to address human origins and evolution. The goal was to compare brain development in humans and our closest primate living relatives: chimps and bonobos. When somatic cell nuclear transfer came along, we took advantage of the technology to create stem cells that could be differentiated into neuronal progenitors. Around 2007, iPS cell technology burst onto the scene, and that allowed us to create a bank of stem cells from fibroblasts derived from a range of nonhuman primates. PNAS: Your stem cell bank is something of a primate “cellular zoo,” as you put it. Can you describe the zoo? Gage: The cellular zoo, which is comprised of frozen fibroblasts, iPS cells, and neural progenitor cells, represents several nonhuman primates, namely chimps, bonobos, gorillas, orangutans, macaques, and marmosets. The cells are housed at the Salk Institute, and we collaborate with researchers interested in using them for their own experiments; the goal is to make this a national resource. The idea behind the zoo was to use standardized protocols to differentiate the iPS cells from different primates into various types of neural cells—neurons, oligodendrocytes—and perform genomic, epigenomic, and transcriptional analyses as the cells develop. This way, we can create a lexicon of developmental differences between the groups. More recently, we have been using direct reprogramming of fibroblasts into neural progenitors, circumventing iPS technology, partly because direct reprogramming might better preserve epigenetic marks in the cells. PNAS: You found more copies of LINE-1 in the genomes of chimpanzee iPS cells than in iPS cells derived from human fibroblasts. You also found that two proteins, produced at higher levels in humans than in chimpanzees and bonobos, might be tied to the restricted mobility of LINE-1 in humans. Can you describe the proteins? Gage: One of the proteins, APOBEC3B, is a DNA cytosine deaminase enzyme. Others have found that the enzyme, when overexpressed in cells, dramatically reduces experimental retrotransposition. We are collecting fibroblasts from people around the world with varying levels of the enzyme, including people who don’t produce the enzyme (the loss of the gene or a drop in its expression level does not manifest as a disease in humans). Fred Gage. Image courtesy of Joe Belcovson (Salk Institute for Biological Studies, La Jolla, CA).