BioWorld International Correspondent
LONDON - The efficiency and accuracy of cervical screening programs could be greatly boosted by a new cervical smear test developed by scientists in the U.K. Preliminary studies suggest the new test can detect some abnormal smears that are missed by conventional screening methods. The new method may also hasten the day when smears can be read by machines.
The test, which uses antibodies to proteins found only in dividing cells, is being developed by Santa Clara, Calif.-based diaDexus. Large-scale trials, using both the new test and the conventional one, are expected to start in the U.S. within the next 18 months. If these prove successful, the new test should be available worldwide in three years.
Gordon McVie, director general of the Cancer Research Campaign, the U.K. cancer charity which funded the work, said: "Spotting a handful of abnormal cells out of up to 300,000 healthy ones in a cervical smear can be extremely difficult. This discovery is amazing because it effectively makes any suspect cells stand out from the crowd. If large-scale trials prove successful, I believe it could be an ideal solution to the mistaken cervical smear diagnoses which we have, sadly, all heard so much about."
The cervical screening program in the U.K. has been dogged by numerous failures. In one example, at Kent & Canterbury Hospital, eight women died and 30 had hysterectomies after screening errors which occurred between 1990 and 1995. Ninety-one thousand smear tests were reviewed following the discovery of the mistakes.
Earlier this month, the All Party Public Accounts Committee in the U.K. blamed the National Health Service for failing to tackle the problem vigorously. The committee's report said errors in the cervical screening program were still putting thousands of lives at risk, with "worrying failures at every stage of the screening process."
Technique Uses Pap Stain On Same Smear
It is against this background that Frank Dobson, secretary of state for health, appeared at a Cancer Research Campaign press conference last week to welcome the new test. "The system was failing and the system is being changed. But this new development, if it proves effective and reliable, could make the job a lot easier by easing the task of the cytology screeners and giving a massive boost to the accuracy of the test," he said.
Details of the new test were published in the Proceedings of the National Academy of Sciences last week, in a paper titled "Improved cervical smear assessment using antibodies against proteins that regulate DNA replication." A key feature of the new technique is the use of the conventional Pap stain on the same smear, so that both methods contribute information.
Ronald Laskey, of the Wellcome Trust/Cancer Research Campaign Institute at the University of Cambridge, one of the senior authors of the paper, told BioWorld International that researchers "are obviously very excited about the promise of the new test, but we recognize that a substantial amount of validation has to be done before it can enter the clinic."
Laskey and his colleagues were studying the proteins that regulate DNA replication, particularly those which ensure that only one copy of DNA is made in each cell division cycle. During the synthesis of DNA, a complex of proteins builds up on the original DNA chain, called the pre-replication complex. Some of the proteins belonging to this complex are called minichromosome maintenance proteins (Mcms), and these are loaded onto unreplicated DNA by a protein called Cdc6 (the Cdc standing for cell division cycle).
"We found that both the Mcm proteins and Cdc6 are displaced from chromatin and broken down when cells leave the division cycle and become quiescent," Laskey said. "Therefore, we had markers that had some potential for recognizing cells which were in the division cycle rather than resting." Gareth Williams, a Cancer Research Campaign clinical fellow in Laskey's laboratory, who had previously worked in pathology, and Nick Coleman, also a pathologist at the University of Cambridge, realized that these markers might be useful in solving some of the problems in cervical smear analysis.
The researchers therefore looked to see whether they could detect these proteins in frozen sections of cervical biopsies with the appropriate antibodies. They found that the antibodies stained the proliferating basal cell layer in the normal cervix, but not the non-dividing cells at the surface - which are those sampled for the smear test. "In contrast," Laskey said, "in those biopsies with dividing dysplastic or neoplastic cells - which are precancerous and cancerous, respectively - up to 95 percent of the cells stained."
The next step was to find out whether smears could be stained with the antibodies. To increase the chances of finding "positive" smears, the researchers studied those taken at colposcopy clinics. They found that, out of 28 cases assessed as positive by the Pap stain, all 28 were classified as positive using the new test. However, out of 30 cases classified as normal by the Pap stain, only 26 were also identified as negative with the new test. Of the remaining four, two showed harmless inflammatory changes which were readily diagnosed using the Pap stain.
"The other two cases," Williams and his colleagues write in PNAS, "were confirmed as containing abnormal (low-grade squamous intraepithelial lesions) cells on re-examination of the Pap stain by the local pathologist; i.e., they were false-negatives of the type we are attempting to eliminate. On subsequent excision biopsy, each of these two latter cases actually were found to contain areas of high-grade squamous intraepithelial lesions on histological examination, yet they had been reported as negative by conventional Pap stain."
Automated Test Could Save U.K. Millions
The Cancer Research Campaign has called the new test the "campaign test." McVie told last week's press conference: "In the short term, we hope it could be used in addition to normal screening procedures to improve human assessment of cervical smears. In the future, the Campaign test could be easily automated so that only suspect cervical smears would be scrutinized by the human eye." This could save the U.K. cervical screening program - which currently carries out more than 4.5 million smears annually -millions of pounds in reduced manpower costs, he predicted.
Next, Laskey and his colleagues plan to examine whether the antibodies to Mcm5 and Cdc6 can help improve the early diagnosis of other types of cancer. n