A protein that is synthesized by a cancer-causing virus enables it to bypass immune systems and infect the immune cells. This mechanism uses an unusual gene and also ensures that the virus persists in the host.
The study examines the function of a protein called HBZ, which is made by the human T cell leukemia virus type 1 (HTLV-1), a retrovirus and a distant cousin to HIV, the cause of AIDS.
The findings indicate that HBZ enhanced the ability of HTLV-1 to establish a persistent infection in an animal host. The study by researchers with the Ohio State University Comprehensive Center and the College of Veterinary Medicine is published in the May issue of the journal Blood.
The gene that gives rise to HBZ is unusual because it lies on the wrong side of the virus's DNA molecule. Such genes are known as antisense genes, and they have been observed in only a few retroviruses, including HIV.
A DNA molecule is somewhat like a railroad track that is twisted into a double helix. The two rails correspond to the complementary strands of the DNA backbone, while the ties correspond to the chemical base pairs that hold the two strands together and encode genetic information.
Normally, that genetic information is encoded only along one DNA 'rail,' or strand. That strand is called the sense strand. The opposite strand is the antisense sense strand, and it generally carries no genetic information.
But HTLV-1 is a rare exception. Of its eight genes, (some of which have information for more than one protein), seven lie along the sense strand. The eighth, which encodes the HBZ gene, is on the antisense strand (where it lies across from portions of three genes on the sense strand).
'Encoding a gene on the antisense strand is one more way for a small, compact virus to pack more genetic information or genes into a very small space, and it is why viruses like HTLV and HIV are called complex retroviruses,' says principal investigator Patrick L. Green, professor of veterinary biosciences and of molecular virology, immunology and medical genetics, and a Comprehensive Cancer Center researcher.
'Our study is the first to show that this novel protein is important for survival of the virus, which suggests that a drug that targets it might disrupt viral replication and provide a new therapy for infected people.'
Some 15 to 25 million people are infected with HTLV-1 worldwide, and 1 to 4 percent of them will eventually develop adult T-cell leukemia or lymphoma, a cancer that responds poorly to treatment and that can cause death within six months. In others, it causes a crippling and painful autoimmune-like disorder, tropical spastic paraparesis.
For this study, Green and his collaborators first looked at how loss of the HBZ protein affected the virus's ability to infect and survive in its normal host immune cell, human T lymphocytes, or T cells, growing in culture.
They found little difference between the HBZ mutant HTLV and the normal virus. Both infected the cells and immortalized them equally well.
(Normally, T cells in culture die within a week or two. When the same cells are infected with HTLV-1, however, the virus causes changes that extend their life span indefinitely.)
Next, the scientists tested the ability of the mutant virus to infect and persist in a rabbit model, one of the few animals that duplicates human HTLV-1 infection. Those results indicated that the HBZ protein was required for prolonged infection in the body.
After eight weeks, rabbits that were infected with virus that lacked HBZ had one to 10 copies of the virus per 1,000 lymphocytes, whereas rabbits infected with normal virus had 50 to 100 HTLV-1 copies per 1,000 lymphocytes.
The virus may not survive well without HBZ because the immune system readily destroys cells infected by these viruses, Green says.
'We believe that HBZ acts as a brake on viral replication,' he says. 'Without HBZ, the virus replicates too fast, producing its proteins so quickly that the immune system readily detects infected cells and eliminates them.'
Green and his colleagues are now testing that hypothesis.