1031

CDKN2C

INK4C|p18|p18-INK4C;cyclin-dependent kinase inhibitor 2C (p18, inhibits CDK4);CDKN2C;cyclin-dependent kinase inhibitor 2C (p18, inhibits CDK4)

CDK6 inhibitor p18|cyclin-dependent inhibitor|cyclin-dependent kinase 4 inhibitor C|cyclin-dependent kinase 6 inhibitor p18|p18-INK6

1p32

protein-coding

Medulloblastoma is the most common malignant pediatric brain tumor. In mice, Ptc1 haploinsufficiency and disruption of DNA repair (DNA ligase IV inactivation) or cell cycle regulation (Kip1, Ink4d, or Ink4c inactivation), in conjunction with p53 dysfunction, predispose to medulloblastoma. To identify genes important for this tumor, we evaluated gene expression profiles in medulloblastomas from thesemice. Unexpectedly, medulloblastoma expression profiles were very similar among tumors and also to those of developing cerebellum. However, 21 genes were specifically up-regulated in medulloblastoma, including sFrp1, Ptc2, and Math1, members of signaling pathways that regulate cerebellar development. Coordinated deregulation of these same genes also occurred in a large subset of human medulloblastomas. These data identify a group of genes that is central to medulloblastoma tumorigenesis.

Medulloblastoma (MB) is the most common malignant pediatric brain tumor which isthought to originate from cerebellar granule cell precursors (CGNPs) that fail to properly exit the cell cycle and differentiate. Although mutations in the Sonic Hedgehog (Shh) signaling pathway occur in 30% of cases, genetic alterations thataccount for MB formation in most patients have not yet been identified. We recently determined that the cyclin D-dependent kinase inhibitor, p18(Ink4c), isexpressed as CGNPs exit the cell cycle, suggesting that this protein might play a central role in arresting the proliferation of these cells and in timing their subsequent migration and differentiation. In mice, disruption of Ink4c collaborates independently with loss of p53 or with inactivation of the gene (Ptc1) encoding the Shh receptor, Patched, to induce MB formation. Whereas loss of both Ink4c alleles is required for MB formation in a p53-null background, Ink4c is haplo-insufficient for tumor suppression in a Ptc(1+/-) background. Moreover, MBs derived from Ptc(1+/-) mice that lack one or two Ink4c alleles retain wild-type p53. Methylation of the INK4C (CDKN2C) promoter and complete loss of p18(INK4C) protein expression were detected in a significant fraction ofhuman MBs again pointing toward a role for INK4C in suppression of MB formation.

A newly recognized family of proteins that inhibit cyclin-dependent kinases (CDKs) termed cyclin-dependent kinase inhibitors (CDKI) have an important role in regulation of cell-cycle progression. A subfamily of these CDKIs (p15INK4B/MTS2,p16INK4/MTS1, and p18) have a high degree of structural and functional homology and are candidate tumor-suppressor genes. We evaluated the mutational status of the p15, p16, and p18 genes in 103 childhood acute lymphoblastic leukemia (ALL) samples and correlated these results with both their clinical data and additional results concerning their loss of heterozygosity in the region of the p15/p16 genes. Homozygous deletions of the p16 gene occurred extremely frequently in T-ALLs (17/22; 77%), and it was also frequent in precursor-B ALLs (12/81; 15%). Homozygous deletions of the p15 gene were also very frequent in T-ALLs (9/22; 41%), and it occurred in 5 of 81 (6%) precursor-B ALL samples. No deletions of p18 was found in any of the 103 ALL samples. Also, no point mutations of the p15, p16, and p18 genes were detected. We correlated p15/p16 alterations at diagnosiswith their clinical characteristics as compared with 2,927 other patients treated similarly. Those with p15/p16 alterations were older; had higher white blood cell counts, often with T-cell ALL phenotype; and more frequently had a mediastinal mass at presentation; but they had the same nonremission, relapse, and survival rates at 5 years as did those patients whose blast cells did not have a p15/p16 deletion. To better understand the extent of alterations affecting chromosome 9p21 (location of the p15/p16 genes), loss of heterozygosity (LOH) was examined at D9S171, which is about 1 megabase proximal to the p15/p16 genes. LOH was detected in 15 of 37 (41%) informative samples. Interestingly, of the 24 informative samples that had no detectable alteration of the p15/p16 genes, 7 samples (29%) had LOH at D9S171. In summary, we show in a very large study that p15 and p16, but not p18, CDKI genes are very frequently altered in ALL; those with p15/p16 alterations are more frequently older children, have higher white blood cells at presentation, and often have a T-cell ALL phenotype. The LOH analysis suggests that another tumor-suppressor gene important in ALL also is present on chromosome 9p21.

p16(INK4A) and p18 proteins are highly specific inhibitors of cyclin-dependent serine/threonine kinase activities required for the overcoming of the G1 checkpoint in the eukaryotic cell division cycle. The frequent cytogenetic aberrations occurring in several human neoplasms at the level of their codifyinggenes along with their molecular function strongly suggest that they might be important tumor suppressor genes. We looked for homozygous deletions of p16(INK4A) and p18 genes in 21 cases of childhood T cell lineage acute lymphoblastic leukemia (ALL). Twenty of 21 patients (95%) had homozygous deletions of p16(INK4A) gene while three out of 21 (14%) showed p18 gene biallelic deletion. Loss of heterozygosity studies were performed in 18 of the Tcell ALL investigated by means of two highly polymorphic 9p21 markers. The results obtained demonstrated that genetic deletions of different extension occur on the short arms of the 9 chromosome pair. Karyotypic analyses, performed in 13cases, failed to demonstrate 9p alterations in 12 samples, (92%) thus demonstrating that p16(INK4A) gene homozygous deletions are not restricted to cases with cytogenetically detectable 9p aberrations. The high incidence of p16(INK4A) gene deletions in pediatric T cell lineage ALL suggests that this genetic alteration could represent an early and key event in the development of such a malignancy but it should not have any prognostic value. Conversely, the inactivation of p18 gene, observed in a lower but significant number of cases, could participate in the progression of acute leukemias towards a more aggressive disease. Finally, our results may suggest that p16(INK4A) protein plays a key role in the control of proliferation and/or differentiation of human T lymphocytes.

p16INK4A, p15INK4B, and p18 proteins are highly specific inhibitors of cyclin-dependent serine/threonine kinase (CDK) activities required for GI-S transition in the eukaryotic cell division cycle. mutations, mainly homozygous deletions, of the CDKN2A (p16INK4A/MTSI) gene have been recently found in tumor cell lines and in many primary tumors. We looked for homozygous deletions of CDKN2A, CDKN2B (p15INK4B), and CDKN2C (p18) in 12 primary rhabdomyosarcoma (RMS)specimens and in five cell lines established from this cancer type. By means of polymerase chain reaction (PCR) and PCR-single strand conformation polymorphism (PCR-SSCP), we analyzed the presence of biallelic gene deletion or point mutation causing gene function loss. ALL the examined tumor cell lines (100%) and three of 12 (25%) primary tumors showed homozygous deletion of CDKN2A. Furthermore, no aberrant bands in primary tumors were detected via SSCP, suggesting the absence of mutations in the coding region. In ALL cases the deleted area at 9p21 also involved the CDKN2B gene. Conversely, no homozygous deletion or point mutations were detected when CDKN2C was analyzed. Our results strongly indicate that the p16INK4A (and/or p15INK4B) protein plays a key role in the development and/or progression of childhood rhabdomyosarcoma and suggest that this CDK-inhibitor protein might control proliferation and/or differentiation of human muscle cells. Moreover, alteration of CDKN2C does not appear to be involved in the genesis of rhabdomyosarcoma.

The cyclin-dependent kinase inhibitors known as p15, p16, p18 and p19 have been suggested as candidates for tumor suppressor genes. The main genetic alterationsare deletions (bi- or monoallelic) or 5' CpG island methylation of p15 and p16; very few cases or cell lines had p18 or p19 deletions or hypermethylation. Hypermethylation and homozygous deletions of tumor suppressor genes establish a new paradigm of inactivation by lack of expression, in contrast to the previously identified tumor suppressors which are predominantly inactivated by point mutations followed by loss of the wild-type allele. Here, the literature data onalterations of this gene family in more than 4700 primary cases of leukemia or lymphoma and some 320 continuous leukemia-lymphoma cell lines are summarized. Among hematopoietic malignancies, the highest frequencies of p15del and p16del were seen in acute lymphoblastic leukemia (ALL) (>30%) with striking rates in T-ALL (>50%), but also high rates in B cell precursor (BCP)-ALL (>20%); the rates of deletions in chronic lymphoid leukemia (CLL), multiple myeloma, acute and chronic myeloid leukemia (AML and CML), and myelodysplastic syndromes (MDS) wererather low, only some B cell and T cell lymphomas showed increased frequencies. Results are quite different with regard to the second mode of inactivation, hypermethylation of the promoter region. Here, p15 is most often inactivated, atparticularly high frequencies in the disorders lacking any p15/p16 deletions: 40-80% p15met in AML, MDS and multiple myeloma. Also p15met rates in BCP- and T-ALL cases were high (c. 40%). There is controversy concerning the prognostic impact of p15 and p16 aberrations with some studies describing a significant correlation between inactivation of these genes and poor prognosis, while most others did not detect any prognostic relevance, at least in pediatric ALL; theremay be a worse prognosis for adults with B or T cell lymphomas. Despite the small number of cases studied, paired sequential analyses suggested that disease progression is associated with loss of p15/p16 activity in a certain percentage of adult patients. p15del/p16del and p15met/p16met were also detected in the large panel of leukemia-lymphoma cell lines studied. In general, the results in cell lines reproduce the data seen in primary cells with the important difference that the rates of p15/p16 inactivation are clearly higher in the cultured cells compared with the freshly explanted cells. Retrovirus- or electroporation-mediated ectopic gene transfer of p16 wild-type into p16-deficient cell lines led to growth inhibition, arrest in G1 (without apoptosis) and occasionally to differentiation, suggesting that the malignant phenotype of p16-/- cell lines can, at least partially, be reversed by restoringp16 gene expression. A striking inverse correlation between the absence of p16 (due to deletion) and presence of wild-type retinoblastoma gene was observed in cell lines confirming a common growth suppressor pathway; no comparable relationship of p16 inactivation with p53 was detected. Paired analysis of cell lines and corresponding primary cell material showed that in ALL instances tested both populations carried the same gene configuration of p15 and p16. Thus, p15del or p16del did not occur during establishment of the cell lines or during prolonged culture. It is likely that p15 or p16 deletions already acquired in vivo provide a dramatic growth advantage for the immortalization process in vitro, thus increasing the success rate for cell line establishment which is commonly extremely difficult. In conclusion, the present review suggests an involvement of the p15 and p16 tumor suppressor genes in leukemo- and lymphomagenesis. Future studies will determine their exact role in the development and progression of hematopoietic neoplasms. These genes may represent interesting targets for new therapeutic strategies.