Unusual distribution of chromosome 12 in a testicular germ-cell tumor cell line (833K) and its cisplatin-resistant derivative (64CP9)

Molecular Mechanisms of Cisplatin Chemoresistance and Its Circumventing in Testicular Germ Cell Tumors

PURPOSE OF REVIEW

Testicular germ cell tumors (TGCTs) represent the most common solid tumors affecting young men. Majority of TGCTs respond well to cisplatin-based chemotherapy. However, patients with refractory disease have limited treatment modalities associated with poor prognosis. Here, we discuss the main molecular mechanisms associated with acquired cisplatin resistance in TGCTs and how their understanding might help in the development of new approaches to tackle this clinically relevant problem. We also discuss recent data on the strategies of circumventing the cisplatin resistance from different tumor types potentially efficient also in TGCTs.

RECENT FINDINGS

Recent data regarding deregulation of various signaling pathways as well as genetic and epigenetic mechanisms in cisplatin-resistant TGCTs have contributed to understanding of the mechanisms related to the resistance to cisplatin-based chemotherapy in these tumors. Understanding of these mechanisms enabled explaining why majority but not all TGCTs patients are curable with cisplatin-based chemotherapy. Moreover, it could lead to the development of more effective treatment of refractory TGCTs and potentially other solid tumors resistant to platinum-based chemotherapy. This review provides additional insights into mechanisms associated with cisplatin resistance in TGCTs, which is a complex phenomenon, and there is a need for novel modalities to overcome it.

The relative activity of cisplatin, oxaliplatin and satraplatin in testicular germ cell tumour sensitive and resistant cell lines

BACKGROUND

Germ cell tumours (GCT) can become resistant to cisplatin, which is associated with a relatively poor prognosis. Oxaliplatin and satraplatin have been developed to overcome cisplatin resistance in other cancers, but their effect in cisplatin resistant (cisR) GCTs is unclear. In this work we address this issue by comparing their efficacy in three paired sensitive and cisR GCT cell lines.

METHODS

Three established cisplatin sensitive (cisS) and resistant cell line pairs were used (GCT27, GCT27r: SUSA, SUSAr: 833k, 833kr). Viability was assessed using a luciferase based ATP assay and EC(50) and EC(80) concentrations were calculated. Western blot analysis and flow cytometry was used for further assessment.

RESULTS

Sensitivity to the three platinum compounds was broadly similar in the three cisS lines GCT cell lines (EC(50) = 0.27-0.51 microM for cisplatin, 0.52-0.79 microM for oxaliplatin, 0.31-1.26 microM for satraplatin). EC(50) values for cisplatin in the three cisR sub lines were 1.8- to 3.8-fold higher than in the sensitive parental lines. Cross resistance to satraplatin and oxaliplatin occurred in all three cisR cell lines (resistance factor 1.9-4.4), with the exception of oxaliplatin in the 833Kr (resistance factor 0.9). Differences in the effect of specific drugs on cell cycle distribution, p53, p21 and MDM2 were observed.

CONCLUSIONS

These data suggest that satraplatin and oxaliplatin could theoretically be used in chemo-naive GCTs and support the further clinical evaluation of these agents in this setting. The mechanism of cross resistance to these drugs appears multifactorial.

Similar chromosomal changes in cisplatin and oxaliplatin-resistant sublines of the H69 SCLC cell line are not associated with platinum resistance

Small cell lung cancer (SCLC) initially responds well to DNA damaging drugs such as cisplatin, however this is transitory as resistance normally develops. To investigate whether changes in chromosomal copy number caused by platinum drug treatment contributes to platinum resistance, we have analyzed H69 SCLC cells and two low-level platinum-resistant sublines, H69CIS200 and H69OX400, derived by cisplatin and oxaliplatin treatment, respectively. Affymetrix 10K SNP array showed that cisplatin and oxaliplatin have independently caused similar changes including loss of segments 6q21-qter and 13pter-13q.14.11 and duplication of chromosome 21. Interestingly, despite using equally cytotoxic doses of drug in the development of the cell lines, oxaliplatin caused three times more chromosomal changes than cisplatin. The resistant cell lines lose their resistant phenotype after 3 months of drug-free culture. The revertant cell lines, denoted H69CIS200-S and H69OX400-S, were also analyzed by Affymetrix array to determine if chromosomal changes associated with resistance remain after the resistant phenotype is lost. In the H69OX400-S many of the changes observed in the resistant cells were absent suggesting that they contributed to the resistant phenotype including: loss of 1q23.3-qter, 10q11.23, and 19q13.12-q13.2 and duplication of segments 6p21.2-p12.3, 16q12.1-16q13, 16q21-q23.1, and 19q12. However, out of the similar changes induced by cisplatin and oxaliplatin, both the loss of 6q21-qter and gain of 21 were still present in the H69CIS200-S and H69OX400-S cells. This suggests that cisplatin and oxaliplatin induced similar changes due to inherent vulnerabilities in the H69 cells rather than changes associated with platinum resistance.

Comparative genomic hybridization in pineal germ cell tumors

Fifteen primary pineal germ cell tumors (8 germinomas, 4 mixed teratomas-germinomas, 2 immature teratomas, and 1 yolk sac tumor) and 2 recurrences of the yolk sac tumor were studied by comparative genomic hybridization (CGH). An average of 1.8 chromosomal changes per germinoma (0.5 gains vs 1.3 losses), 5.5 per mixed teratoma-germinoma (3.0 gains vs 2.5 losses), 3.5 per immature teratoma (2.0 gains vs 1.5 losses), and 2.0 in the yolk sac tumor (2 gains vs 0 losses) were found; the first recurrence showed 7 (4 gains vs 3 losses), the second 13 imbalances (8 gains vs 5 losses). The most frequent imbalances were gains on 12p (40%), 8q (27%), and 1q (20%) as well as losses on 13q (47%), 18q (33%), 9q and 11q (20% each). Among germinomas, the most common chromosomal changes were -13q and -18q (38% each), in mixed teratomas-germinomas +8q (100%), +12p (75%), -13q (75%) and -9q (50%). Seven high-level gains were identified: 5 in mixed teratomas-germinomas (+8q: 3 cases, + 12p: 2 cases), 1 each in a germinoma (+2p) and an immature teratoma (+12p). Minimal common regions of over- and underrepresentation were found on +8q11.22-21.1, +12p11.1-12.1, -9q32-qter, -11q23.2-qter, -13q32-qter and -18q22-qter. Our findings suggest, that imbalances in cerebral germ cell tumors affect the same chromosomes as among their extracerebral counterparts, albeit in a considerably lower frequency among cerebral germinomas where +12p does not seem to play a major role.

Characterization of multiple 12p rearrangements in testicular germ cell tumor cell line 833K and its subclone 64CP by chromosome microdissection

Rearrangements of chromosome arm 12p are known to be common in germ cell tumors (GCT). Previous studies, using fluorescence in situ hybridization (FISH) with a whole chromosome 12 painting probe, showed unusual distributions of chromosome 12-derived chromatin in GCT cell line 833K and its cisplatin-resistant subclone, 64CP, located next to AgNOR (silver staining nucleolus organizer regions), some of which were ectopic. In this study, the ectopic stalk regions were shown by FISH to be composed of 18s and 28s rDNA, but were flanked by beta-satellite DNA, which may form a barrier around the rDNA. In order to determine the specific origins of the rearranged chromosome 12 segments, three different derived chromosome 12 regions were isolated from 64CP, using chromosomal microdissection. The microdissected fragments were labeled and hybridized by FISH to normal human chromosomes. All three segments localized to distal 12p; 12p12-->12pter, but with apparently different breakpoints for each segment. Furthermore, three-color FISH experiments with 12p band-specific probes demonstrated that the derivative chromosome 12 regions in 833K also originate from distal 12p (12p12-->p13). These sequences now can be evaluated for degree of overlap or common breakpoints which may be of significance in the development or progression of GCT.

12p rearrangement and DNA amplification mapped by comparative genomic hybridization in a patient with secondary myeloid leukemia

Rearrangements of the short arm of chromosome 12 (12p) are a common finding in hematologic malignancies. There has recently been considerable interest in chromosome 12 abnormalities in view of the mapping of the TEL gene to 12p13 and frequent 12p interstitial deletions. Overrepresentation of 12p sequences is, on the other hand, a consistent finding in testicular germ cell tumor (TGCT), and the 12p11.2-p12.1 subregion has been found to be specifically involved. We have studied a secondary leukemic patient whose cells contained 12p rearrangements with a view to clarifying the underlying molecular events. Fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) have revealed the presence of 12p11 breakpoints on both 12 homologs as well as amplification of 12p11-p12-derived sequences. Six YACs and a cosmid probe have been used in an attempt to map the amplification unit on 12p. The two YACs contigs WC-1468 and WC-985 were not amplified, and our results suggested a small amplicon localized in the 12p11.2-p12 subregion. We speculate that this region harbors gene(s) which are critical in tumor formation and could be involved in both TGCT and our patient. Whether the same gene(s) are involved in both amplification and translocation is unknown.

Beyond ribosomal DNA: on towards the telomere

We have sequenced and analyzed 8.3 kb of sequence adjacent and distal to the human ribosomal DNA (rDNA); this distal sequence connects to the rDNA cluster just 4 kb upstream of the first promoter and is shared among the acrocentric chromosomes and, at least in part, it is also present in other primates. The sequence differs in character from that of the rDNA intergenic spacer (IGS) in that it does not contain long stretches of either polypyrimidine or polypurine. However, just like the IGS, it contains numerous repetitive elements, including retroposed fragments of 28S rRNA and large pieces of the IGS. In addition, we show that the rDNA clusters are not interrupted by other sequences and do not recombine with this distal segment.

Bicolor fluorescence in situ hybridization on nuclei from formalin-fixed, paraffin-embedded testicular germ cell tumors: comparison with standard metaphase analysis

Rearrangements of chromosome 12, especially i(12p), are common in testicular germ cell tumors (TGCT). We have developed 12p and 12q chromosome arm-specific painting probes for fluorescence in situ hybridization (FISH) through the use of chromosome microdissection. We developed a method to hybridize these probes to interphase nuclei released from formalin-fixed, paraffin-embedded tumors (PET). In this study, we compared simultaneous bicolor PET FISH painting with metaphase chromosome analysis of fresh tissue from the same tumor. Bicolor PET FISH produced patterns of 12p and 12q hybridization consistent with expectations based on metaphase chromosome analysis; adjoined 12p and 12q regions appeared to detect "normal" chromosome 12s, whereas relatively large isolated regions of 12p were suggestive of i(12p), and small 12p regions probably represent cryptic rearrangements of 12p. Fourteen tumors with successful cytogenetic analyses and available archival material from the same tumor source were selected for study. In a blinded analysis, PET FISH painting assessment was in very close agreement with karyotypic findings in seven subjects, in close agreement in five, and showed less agreement in two. Differences may be due in part to clonal selection during culture for metaphase studies, or regional selection within the tumor. Although PET FISH painting should not replace standard chromosome analysis, this study shows that it can reliably predict chromosome 12 constitution in TGCT, can serve as a useful adjunct to standard cytogenetics when such analysis is unsuccessful, and can provide limited karyotyping of archival materials.

Investigation of carcinoma in situ cells of testis by quantification of argyrophilic nucleolar organizer region associated proteins (AgNORs)

The silver staining which specifically stains argyrophilic proteins (AgNORs) in interphase nuclei was applied to paraffin sections of 24 testicular specimens with carcinoma in situ (CIS). AgNOR area per nucleus was quantified by a computerized image analyser. Significant quantitative differences were found between CIS, Sertoli cells, and spermatogonia (P = 0.0001), with median values of 10.3, 2.8, and 1.4 microns2 in the three cell types, respectively. A Sertoli cell index (SCI), defined as the ratio between AgNORs in CIS or spermatogonia and Sertoli cells, was shown to be significant in the differential diagnosis of CIS cells from spermatogonia when 1.0 was used as the cut-off value (CIS > 1; spermatogonia < 1). Furthermore, CIS associated with non-seminoma was found to have a significantly higher level of AgNORs than CIS associated with pure seminoma (P < 0.01), indicating that subclonal variation in transformation potential might be present within morphologically identical CIS of the testis. It remains to be seen whether quantification of AgNORs in isolated CIS could be used to predict transformation of CIS into seminoma or non-seminoma.

Amplification of chromosome subregion 12p11.2-p12.1 in a metastasis of an i(12p)-negative seminoma: relationship to tumor progression?

Cytogenetic analysis of a metastasis of a human testicular germ cell tumor (seminoma) revealed multiple numerical and structural anomalies, including an abnormally banding region (ABR) present on the short arm of one of the chromosome 12 homologs. Fluorescence in situ- and comparative genomic hybridization experiments revealed that the ABR results from the amplification of 12p11.2-p12.1 derived sequences. We speculate that this particular region may harbor gene(s) relevant for testicular germ cell tumor progression.