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Comparison of Human and Chimpanzee Stem Cells Reveals Genetic Differences

By Fazale Rana - December 12, 2013
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Researchers from the Salk Institute and UC San Diego identified a key genetic difference between humans and chimpanzees when they compared gene expression patterns in induced pluripotent stem cells from both primates. They observed higher levels of expression in the human genome for two genes that repress the movement of the retrotransposon, LINE-1—a type of DNA that moves around the genome. Even though humans and chimpanzees display significant genetic similarities, they also exhibit significant genetic differences in accord with the Genesis 2 creation account.

I really like Facebook. Perhaps what I enjoy most is connecting with old high school classmates. It’s fascinating to find out what people have been doing since graduation. Some of my friends, like me, have moved all over the country. And others live just a few miles away from where they grew up, never straying too far from home.

High school friends aren’t the only things that move around (or stay put). Recently, researchers from the Salk Institute and the University of California, San Diego (UCSD) discovered that mobile pieces of DNA (called retrotransposons—specifically LINE-1sequence elements) move around differently within the genomes of humans and chimpanzees. And the difference in the mobility of LINE-1 elements may help explain the biological and behavioral differences between these primates.1

Similarities vs. Differences in Humans and Chimps

It’s not necessary to be on Facebook all the time to know that humans and chimpanzees display a high degree of genetic similarity, on the order of 95 to 98 percent (the exact value depends on the specific types of comparisons that are made). Many people regard the high degree of genetic similarity between humans and chimpanzees as evidence that we share a close evolutionary connection. However, it is possible to make sense of these similarities from a creation model perspective. In fact, I would maintain that a careful reading of the Genesis 2 creation account anticipates this genetic similarity.

In spite of all the hoopla surrounding human-chimpanzee genetic similarities, manybiologists don’t think that a simple comparison of DNA sequences is all that meaningful. The emerging consensus views gene regulation (or gene expression) as the basis for the biological differences and cognitive gap between humans and chimpanzees.

In other words, there are meaningful genetic differences between humans and chimpanzees. These differences have little to do with the set of genes found in the genomes (which for all intents and purposes is the same for humans and chimpanzees) or the close correspondence of the DNA sequences. Instead, the most biologically meaningful comparisons focus on how the genes are used—in other words, the patterns of gene expression.

Researchers have also discovered other genetic differences as well. For example, humans and chimpanzees display differences in alternate gene splicing. As important as these insights may be, researchers interpret these studies with caution because the comparisons have relied primarily on preserved tissues. These samples do not reflect the behavior of live tissues and cells, nor do they provide access to differences in gene expression that likely take place during the course of embryonic development.

Studying Induced Pluripotent Stem Cells

To remedy this problem, scientists from the Salk Institute and UCSD prepared induced pluripotent stem cells (iPSCs) from chimpanzee and bonobo fibroblasts (a type of skin cell) and compared their gene expression activity with human iPSCs. (iPSCs behave just like embryonic stem cells in that they can be coaxed to form a wide range of different adult cell types. These cells allow researchers a window into the gene expression differences from the earliest stages of embryological development and provides them with a gene expression profile for each of the different cell types in the human body.)

The investigators discovered no significant differences for around 11,500 protein-coding genes. But they also found 1,375 genes in human iPSCs with increased levels of expression and 1,050 different genes in chimpanzee and bonobo iPSCs with increased activity. In human iPSCs, two of the genes with the greatest expression increase play a role in suppressing the movement of LINE-1 retrotransposons. In support of this observation, the research team also discovered more copies of LINE-1 sequence elements in the genomes of chimpanzees and bonobos than in the human genome.

LINE-1 DNA plays a role in X-chromosome inactivation and influences monoallelic gene expression. These pieces of DNA can also lead to the inactivation of genes and even disrupt genome integrity as they move around. It is quite reasonable to think that these mobile pieces of DNA can account for some of the biological differences between humans and the great apes.

This latest work adds to the growing recognition that gene expression is the key to understanding the biological and behavioral uniqueness of humans. Even though, we display a high degree of genetic similarity with the great apes (and other animals), we also manifest significant genetic differences (i.e., gene expression) in a way thataccords with the Genesis 2 creation account.

Endnotes
  1. Maria C. N. Marchetto et al., “Differential L1 Regulation in Pluripotent Stem Cells of Humans and Apes,” Nature (published electronically October 23, 2013), doi:10.1038/nature12686.

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