Last year was the annus mirabilis of the first breast-cancer gene, BRCA1. A jury of 55 researchers found itsmutation guilty in the familial form, or 5 percent of thecases diagnosed each year. That left the far largerpopulation of sporadic breast cancer victims without asuspect gene, let alone an alleged genetic perpetrator.
But not any more.
Today's Science ties BRCA1 at last to the sporadic cases,in a convoluted cellular hide-and-seek conspiracy. Thisnew indictment carries the cryptic title: "Aberrantsubcellular localization of BRCA1 in breast cancer."
Its senior author is molecular biologist Wen-Hwa Lee,director of the Biotechnology Institute at the University ofTexas Health Sciences Center in San Antonio. In 1987,Lee was one of the three pioneer investigators whorevealed the existence of tumor suppressor genes, viatheir analysis of the retinoblastoma gene.
Lee told BioWorld Today that when Science reportedcloning of the BRCA1 gene last year (see BioWorldToday, Sept. 15, 16, 19, 1994, p. 1) but with only 50percent in familial cases, and none at all in sporadic, "thatbothered me a lot." It's against the so-called tumor-suppressor theory, of which the inactivation gives rise tocancer.
Whereupon, Lee and his co-authors decided to investigatethis anomaly, by looking not at BRCA1's DNA sequence,but at the protein it encodes.
"The method," he explained, "was very easy." His groupraised polyclonal fluorescence-tagged antibodies toimmunostain the protein in tissue-culture tumor cells."Then we went to the microscope and took a look. Wecould see it immediately."
What they saw was "quite a surprise," namely, that "inmost of the sporadic cancer cell lines, the protein is quiteintact," not deformed by the mutant gene. What's more,"In a normal cell, the protein is found in the nucleus, butin essentially every tumor cell we looked at, it's in thecytoplasm."
That finding led the Texans to the first inkling of whatnormal BRCA1 is up to: The protein it expresses acts as atranscription factor in the nucleus, where it belongs,turning other genes on and off as needed. This fact cameto light, Lee said, "because from the primary DNAsequence, we found that they contain a zinc-fingerdomain. This motif is the signature of a transcriptionfactor."
When tumorous, the protein gets stuck in the cytoplasm,"where it can no longer function normally," Leeobserved. But "this malfunction," he pointed out, "is notdue to any mutation of this protein. The fact that it cannotbe transported back to the nucleus may be caused bysome other gene or genes gone wrong."
He surmises that this bad company could include "theBRCA2 and 3 genes too." (See BioWorld Today, Oct. 24,1994, p. 1.)
Lee allows that he is "quite excited" about thismislocation discovery's potential use in the medicalmanagement of breast cancer. "The BRCA1 gene'sprotein location in the tumor cell," he explained, "will bea very excellent prognostic marker. If this protein islocated in the nucleus, we know the tumor is very earlystage.
"But," he continued, "in case we see that this protein islocated in the cytoplasm, it suggests that the other gene orgenes are mutated. Therefore, this tumor will be gettingworse and worse."
Molecular pathologist Craig Allred, a co-author oftoday's Science report, picks up the story:
"A patient whose tumor has metastasized to the under-arm lymph nodes, pretty much automatically getsadjuvant chemotherapy after surgery," he pointed out."Someone with negative nodes, whom we think we'vecured with surgery, still has a statistical chance ofsuffering a recurrence later."
Preventive chemotherapy is itself horrendously toxic, sowhether or not to administer it involves complexguesswork. "The idea in node-negative disease," Allredadded, "is really to find reasons to withhold suchtherapeutic toxicities from the patient."
That's where the new mislocation insight comes in.
San Antonio has one of the world's largest breast-cancertumor banks. Allred, Lee and their research co-authors"are planning to look in our tumor bank at 1,000 patientswith primary breast cancer that has not metastasized, andcompare them with 1,000 that have.
"One advantage of looking for BRCA1 localization in thecell," Allred explained, "is that for the first time it givesus clues as to what the function of that gene may be, byits relationship to other clinical factors, such as hormonereceptors. And protein mislocalization could be a reasoneither to withhold, or to prescribe, adjuvant therapy for apatient free of cancer."
Among the 55 co-authors in last year's discovery of theBRCA1 gene, a principal investigator was geneticistMark Skolnik, a founder and research vice president ofMyriad Inc., of Salt Lake City. He now welcomes Lee'sfinding as "the first strong indication that the gene plays arole in non-hereditary as well as familial breast cancer."
Moreover, Skolnik added, "Lee's second finding, whenhe looks at tissue samples of breast cancer, sees that theprotein is sometimes nuclear, sometimes cytoplasmic,sometimes both. This indicates that the role of theBRCA1 as a marker of tumor progression may indeed bevery important. It's a way of trying to classify a lesionthat's been biopsied."
Meanwhile, Myriad is putting the finishing touches on asequence test to diagnose predisposition to cancer ofmembers of a familial breast cancer kindred. "We willcollect a blood sample and analyze the DNA for BRCA1mutations," Skolnik said. "A woman who is about to betreated for breast cancer, knowing that she has thegermline BRCA1 mutation, would undergo treatmentdifferent from that offered a well woman with the gene,to prevent breast or ovarian cancer."
Myriad plans to provide the test on a commercial basis,Skolnik added, "beginning in the second half of 1996. It'sa fairly complicated blood test. We don't yet know theprice, but it will be somewhat expensive." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.