No abstract

Tretinoin is a safe and highly effective agent for inducing complete remission in patients with acute promyelocytic leukemia. Clinical response to this agent is associated with leukemic-cell differentiation and is linked to the expression of an aberrant RAR-alpha nuclear receptor. Molecular detection of the aberrant receptor may serve as a useful marker for residual leukemia in patients with this disease.

To better understand genetic alterations in oral premalignant lesions, we examined 84 oral leukoplakia samples from 37 patients who had been enrolled in a chemoprevention trial. The samples were analyzed for two microsatellite markers located at chromosomes 9p21 and 3p14. Loss of heterozygosity (LOH) at either or both loci was identified in 19 of the 37 (51%) patients. Of these 19 patients, seven (37%) have developed head and neck squamous cell carcinoma (HNSCC) while only one of 18 (6%) of patients without LOH developed HNSCC. Our data suggest that clonal genetic alterations are common in oral premalignant lesions; that multiple genetic alterations have already occurred in oral premalignant lesions, allowing at least a focal clonal expansion; and that losses of the 9p21 and 3p14 regions may be related to early processes of tumorigenesis in HNSCC. These genetic alterations in premalignant tissues may serve as markers for cancer risk assessment.

Genetic alterations at chromosomal sites containing putative tumor-suppressor genes (i.e., 3p14 and the FHIT gene, 9p21 and the p16 gene [also known as CDKN2], and 17p13 and the p53 gene [also known as TP53]) occur frequently in the histologically normal or minimally altered bronchial epithelium of chronic smokers.

KIT gain of function mutations play an important role in the pathogenesis of gastrointestinal stromal tumors (GISTs). Imatinib is a selective tyrosine kinase inhibitor of ABL, platelet-derived growth factor receptor (PDGFR), and KIT and represents a new paradigm of targeted therapy against GISTs. Here we report for the first time that, after imatinib treatment, an additional specific and novel KIT mutation occurs in GISTs as they develop resistance to the drug. We studied 12 GIST patients with initial near-complete response to imatinib. Seven harbored mutations in KIT exon 11, and 5 harbored mutations in exon 9. Within 31 months, six imatinib-resistant rapidly progressive peritoneal implants (metastatic foci) developed in five patients. Quiescent residual GISTs persisted in seven patients. All six rapidly progressive imatinib-resistant implants from five patients show an identical novel KIT missense mutation, 1982T3 C, that resulted in Val654Ala in KIT tyrosine kinase domain 1. This novel mutation has never been reported before, is not present in pre-imatinib or post-imatinib residual quiescent GISTs, and is strongly correlated with imatinib resistance. Allelic-specific sequencing data show that this new mutation occurs in the allele that harbors original activation mutation of KIT.

We have utilized antibody probes to examine the expression of DNA topoisomerases I and II and chromosome scaffold protein Sc-2 in normal and transformed cells. Neither topoisomerase I nor Sc-2 shows significant fluctuations in content or stability across the cell cycle. In contrast, topoisomerase II undergoes significant cell cycle-dependent alterations in both amount and stability. As cells progress from mitosis into G1, much of the topoisomerase II is degraded. During the first 2 hr of G1, the half life of topoisomerase II is decreased from that measured in asynchronous cell populations by a factor of 7. This suggests that the chromosome condensation/decondensation cycle is coupled to a parallel cycle of synthesis and degradation of topoisomerase II. In control experiments, we also found that the half-life of topoisomerase II is shorter in normal cells than in transformed cells by a factor of 4. Since the number of copies of topoisomerase II per cell is also lower in normal cells, this suggests that control of topoisomerase II stability is altered upon transformation. The stability of topoisomerase I and Sc-2 does not differ significantly between normal and transformed cells. (6)(7)(8).Topoisomerase I is apparently not a structural protein, as it is extractable from chromatin at relatively low ionic strength (9) and is distributed throughout the large =100-kilobase (kb) chromatin loops of the mitotic chromosome (5). In addition, topoisomerase I is not essential for growth in yeast (6,10,11).Assays of topoisomerase II activity both in cell-free extracts and in vivo suggested that enzyme levels are greater in proliferating cells than in their quiescent counterparts (12)(13)(14)(15)(16)(17). Quantitative immunological methods demonstrated that topoisomerase II is rapidly lost upon cessation of mitotic activity in either immature erythroblasts or myoblasts (18), thus confirming that the enzyme is a specific and sensitive marker for cell proliferation. Topoisomerase I, on the other hand, is present in many different quiescent cell types (12, 18).The disappearance of topoisomerase II upon the cessation of mitotic activity suggested to us that the transition from mitosis to interphase might be accompanied by facilitated degradation of this protein. Therefore, we have used antibody probes to examine the expression and stability of the enzyme across the cell cycle, with particular emphasis on the period surrounding mitosis. As controls, we performed parallel studies on topoisomerase I and on Sc-2, a 135-kDa polypeptide that is the second most abundant component of the mitotic chromosome-scaffold fraction (19). The results indicate that the transition from mitosis into the subsequent G1 phase is accompanied by a dramatic decrease in the stability of topoisomerase II (but not topoisomerase I or Sc-2). These observations suggest that degradation of topoisomerase II may be intimately associated with the process of chromosome decondensation. tTo whom reprint requests should be addressed. MATERIALS AND METHODS 1086T...

The ubiquitously expressed 14-

Thrombopoietin, administered as a single dose, is a potent stimulus for prolonged platelet production in humans. It merits further evaluation for the prevention and treatment of thrombocytopenia.