16 April 1997 AUA97\PCA13Q.MS-- 1,000 words

Disrupted spots on 3 chromosomes
hint at prostate cancer genes

New Orleans--Prostate cancer is caused by several hits that damage different genes, each one destroying another control on the cell--growth, adhesion, vascularization. Researchers are narrowing those hits down more and more specifically to locations on chromosomes 6, 8, 10, 13, 16 and 18.

At the AUA's 92nd meeting, urologists reported progress on three chromosomes. Abnormalities on chromosome 13 suggest that there is a prostate cancer gene near 3 other cancer genes. Chromosome 6 holds a gene that seems to predict recurrence. And chromosome 9 has a gene which is highly specific to prostate cancer, and may be useful for diagnoses and targeted therapy.

Breast cancer gene's neighbors

The breast cancer gene, BRCA2, lies on chromosome 13. Men who inherit one copy of that gene are three times as likely to have prostate cancer. Is BRCA2 responsible for prostate cancer? No, said Chunde Li, MD, PhD, a researcher at the Karolinska Hospital, Stockholm, Sweden.

But there is a deletion of an unknown gene in the same region that does seem to cause prostate cancer, said Dr. Li.

The BRCA2 gene is on the long (or q) arm of chromosome 13, in a region known as 13q12-14. In that same region, not too far from BRCA2, is also RB1, the retinoblastoma gene, and DBM, the disrupted-in-B-cell-malignancy gene.

Dr. Li wanted to find out whether any of these 3 genes were deleted or disrupted in prostate cancer as well. Because their sequences were already known, he could use a polymerase chain reaction (PCR) probe to see whether the genes were missing in the chromosomes of tumor pathology samples. He could also probe for 8 other standard markers between and around these genes in the same region of chromosome 13 to locate deletions more precisely. Combining another standard technique, he compared the maternal chromosome to the paternal chromosome to see whether pieces were missing from one, a condition called loss of heterozygosity (LOH).

He examined 35 prostate cancers, 25 from primary localized tumors, 6 from local lymph nodes and 4 from brain metastases.

Seventeen out of the 35 (49%) displayed LOH on that region. Two regions were frequently deleted.

The presence of the BRCA2 gene was not correlated with prostate cancer at all, Dr. Li found. The RB1 and DBM genes did have a high frequency of deletion in these samples of prostate cancer cells. But there was no correlation between tumor grade, stage, or two-year short-term survival. This suggests that the prostate cancer was not caused by the deletion of the RB1 or DBM gene either, but by the disruption of some other genes in the same neighborhood.

The 8 other markers did indicate disruption in the same region. So there must be a deletion of gene in that same region that causes prostate cancer, Dr. Li concluded, "but we don't know what it is yet."

Marker for recurrence

Over on chromosome 6, researchers at the Uniformed Services University of Health Sciences found another site on the long arm where the loss or disruption of a gene seems to be associated with prostate cancer.

The marker had useful predictive value: When those sections of the chromosome were missing on cancer cells, patients were twice as likely to have a recurrence, according to staff scientist Vasantha Srikantan, PhD.

"It's been known by comparative genomic hybridization that losses do occur on chromosome 6 in prostate cancer," said Dr. Srikantan. But the location of those losses was only known approximately. She tried to locate it more precisely.

"So we did a comprehensive analysis on prostate cancer tissues obtained from Walter Reed Army Hospital," said Dr. Srikantan. They used samples from 37 radical prostatectomy patients, microdissecting the tumor from the normal tissue.

"We analyzed chromosome 6q using the 10 markers spanning 6q," said Dr. Srikantan. They detected losses, or LOH, in 10 of the 37 patients. "So we know where precisely on chromosome 6q the losses occur," she said.

Unlike Dr. Li's markers on chromosome 13, Dr. Srikantan's markers were correlated with pathologic stage, and with recurrence.

After 15 months, 36% of the patients with 6q LOH had a recurrence, whereas only 15% without LOH had a recurrence, although the trend didn't reach statistical significance.

"We're trying to develop a molecular pedigree on the behavior of prostate cancer, explained Judd W. Moul, MD, director of the Center for Prostate Disease Research. "That will help the doctor to determine which patients are going to fail after radical prostatectomy, and which patients are going to fail at which therapy. You might have a certain biomarker that predicts failure for surgery, whereas a different one might predict failure for radiation."

Most specific marker yet

And on chromosome 9, Dutch researchers found another gene, DD3, which is the most highly specific to prostate cancer discovered so far.

"We found the gene expressed in 47 out of 50 human prostatic tumors in pathological samples, whereas we couldn't find any expression in that messenger RNA in the normal prostate tissue from the same patient," said Marion J.G. Bussemakers, PhD, senior research associate at University Hospital, Nijmegen, the Netherlands.

Using more sensitive probes, she found a small amount of expression in normal prostate cells, but there was no detectable expression in normal human artery, brain, breast, bladder, colon, or other tissues. "So it's specific for tumor and specific for prostate," said Dr. Bussemakers.

Typical of these investigators, Dr. Bussemakers assumed that prostate cancer cells and normal cells would be synthesizing different proteins, so she compared the entire protein output of normal cells and cancer cells in pathology samples from the same patient. She did this by looking at the mRNA from each cell type, a technique called differential display analysis. Using standard methods, she subtracted the normal mRNA from the prostate cancer mRNA, and was left with the sequence she named DD3. DD3 was on the long arm of chromosome 9, in the 21-22 region.