Study Shows That Cells Have A Natural Defense Against HIV
Scientists here have discovered a previously unknown
mechanism that cells use to fight off the human
immunodeficiency virus (HIV), the cause of AIDS The findings
indicate that two proteins that normally help repair
cellular DNA can also destroy the DNA made by HIV after it
enters a human cell. This HIV DNA is essential for the virus
to survive and reproduce. The study was led by researchers
at The Ohio State University Comprehensive Cancer Center –
Arthur G. James Cancer Hospital and Richard J. Solove
Research Institute (OSUCCC – James) and published in the
Proceedings of the National Academy of Sciences.The findings
could lead to a possible new strategy for treating HIV
infection and AIDS, one that might complement current
therapies and would probably be less susceptible to viral
drug resistance – an increasingly urgent dilemma for
patients and doctors.Currently, doctors treat people with
AIDS using combinations of drugs that target the virus
itself. These drugs do not eliminate HIV from the body, but
they do block its ability to reproduce and spread, and they
restore most people with AIDS to good health.In time,
however, HIV can develop mutations that render those drugs
ineffective.“Our findings identify a new potential drug
target, one that involves a natural host defense,” says
principal investigator Richard Fishel, professor of
molecular virology, immunology and molecular genetics and a
researcher with the OSUCCC – James. “HIV treatments that
target cellular components should be far less likely to
develop resistance.”Fishel's laboratory colleague and first
author Kristine Yoder discovered the role of the cellular
repair proteins while trying to answer a different
question.Before HIV infects a cell, it carries its genetic
material in the form of RNA, or ribonucleic acid. Once
inside a cell, the virus makes a copy of its genes in the
form of DNA. This DNA copy – known as cDNA – then travels to
the cell nucleus. There, it becomes inserted, or integrated,
into the cell's DNA. There it is known as a provirus, and it
will generate new HIV in an infected patient and eventually
cause AIDS.The process of integration, which is absolutely
required for a productive infection, begins with the help of
an enzyme, integrase, which is supplied by HIV. But the job
is finished by DNA repair enzymes provided by the host
cell.Yoder originally wanted to identify which repair
enzymes were involved.During these experiments, Yoder
learned that cells with high levels of two proteins called
XPB and XPD had lower levels of HIV provirus in their
chromosomes. Both proteins help the cell repair damaged
DNA.Yoder, Fishel and their collaborators then introduced
mutations into the genes for the two proteins, which
crippled the proteins' ability to repair DNA. When cells
with these mutations were then infected with HIV, they
showed higher levels of provirus in their chromosomes.“When
we weakened a DNA repair pathway, we got more integration of
the provirus,” Yoder says. “This was a total surprise.”Next,
the researchers wanted to learn whether the normal cells
used in the study had lower proviral levels because they
were making less HIV cDNA or because the HIV cDNA was being
destroyed before it integrated.To answer that question, the
researchers used antiretroviral drugs known as
non-nucleoside reverse transcriptase inhibitors (NNRTIs).
These drugs prevent HIV from making the cDNA copy of its RNA
genetic material. The researchers exposed newly infected
cells to the drugs and then measured changes in the amount
of cDNA over time.These experiments showed that the cDNA was
destroyed faster in cells with normal XPB and XPD compared
to cells with mutant XPB or XPD. Cells with normal XPB
protein lost half their proviral DNA after 4.6 hours, while
cells with low levels of the protein lost half after about
7.7 hours. Similarly, cells with normal XPD protein lost
half the proviral DNA after 3.5 hours, while cells with
mutated protein lost half after five hours.These experiments
also showed that the two proteins destroyed the HIV cDNA
before it is integrated into the chromosome.“Overall, our
results indicate that these two DNA repair proteins
participate in the destruction of HIV cDNA in cells,” Fishel
says. “This process reduces the pool of HIV cDNA that can
integrate into host chromosomes, thereby protecting cells
from infection.”The researchers are now working to learn how
the proteins destroy the HIV cDNA. These studies could lead
to drugs that might help the proteins destroy more HIV cDNA
and in shorter time.Funding from the National Cancer
Institute and the National Institute of General Medical
Sciences supported this research.
