photo: Peter Jones, Ph.D., D.Sc. / by Van Urfalian
ANGELES — In healthy bodies, liver cells beget liver cells, while skin
cells beget skin cells. Previous research, however, has shown that —
sometimes — cells can be reprogrammed, from skin, for example, to
muscle or vice versa. This phenomenon has stumped scientists because
there are cellular mechanisms in place to prevent such changes from
New research from the Keck School of
shows how proteins called transcription factors can reprogram genes
that have been turned off, shedding light on what happens when a cell’s
fate changes course and why. The paper “Polycomb-repressed genes have
permissive enhancers that initiate reprogramming” appears in the Dec.
9, 2011, edition of the journal Cell
Stem cells are unique because they can divide and differentiate into
different types of cells in the body. Typically, as the stem cells
become specialized — into organs, blood and bone — genes are suppressed
so the cells are no longer able to switch from one type of cell to
another. While scientists have been able to force specialized cells to
revert back to stem cells, they have not understood the mechanism by
which it happened.
Promoters and enhancers are regulatory regions on a gene. They regulate
transcription, which is the first step to gene expression. When a gene
is not expressed, the promoter is occupied by nucleosomes, cellular
structures that contain DNA. Promoters on genes that are expressed are
not occupied by nucleosomes, and are receptive to transcription.
“We think that an embryonic stem cell is able to differentiate because
most of the gene enhancers are open — that’s what we expect to see,”
Jones, Ph.D., D.Sc.
distinguished professor of urology and biochemistry & molecular
biology at the Keck School and principal investigator of the study.
“Think of the promoter as the front door and the enhancer as the back.
If we look at differentiated cells, we were surprised to find that, in
many cases, when the front door is shut, the back door was still open.
We don’t know why that is, but it explains how you can reprogram cells
— it’s because the back door is open.”
Jones and colleagues targeted MYOD1, a protein that plays a key role in
“MYOD1 is a master transcription factor — it is what makes a muscle
cell a muscle cell. You would not expect that gene to be expressed in a
skin cell. The gene is not expressed, but its enhancer remained
receptive,” Jones said.
The team inserted MYOD1 into fibroblast cells and found that it bound
to the enhancer and was transferred to the promoter. This forced out
the nucleosomes and established a permissive state for expression. The
structure of the MYOD1 enhancer in the fibroblast was indistinguishable
from the enhancer in a muscle cell.
Study co-author Xianghong
Jasmine Zhou, Ph.D.
, associate professor of biological sciences and
computer science in the USC Dana
and David Dornsife College of Letters, Arts and Sciences
analyzed the structure of five differentiated cell types and found that
many genes suppressed by the protein Polycomb, and therefore not
expressed, have a permissive enhancer.
The study of enhancers is relatively new, but they play a potentially
major role in biology.
“The difference between species — for example, between us and monkeys —
is probably due to different enhancers and not to different genes.
Differences in disease susceptibility among people are probably due to
changes in enhancers,” Jones said.
Other co-authors include Phillippa C. Taberlay, Theresa K. Kelly,
Chun-Chi Liu, Jueng Soo You, Daniel D. de Carvalho, Tina B. Miranda,
and Gangning Liang, all from USC. Liu also is affiliated with the National
Chung Hsing University
in Taiwan. Funding for their research came
from the National Institutes of Health
and National Science Foundation