Programme: The Genetics of Paraganglioma
Non-chromaffin paragangliomas, or glomus tumors, are benign and slow growing tumors of the extra-adrenal paraganglion system, usually occurring at the carotid bifurcation in the head and neck region. Their incidence in the population is low, but a substantial proportion of them are due to a genetic defect that has been mapped to the long arm of chromosome 11. The inheritance pattern, however, is non-Mendelian and strongly suggests that the gene, PGL1, is subject to genomic imprinting. The purpose of this project is to identify PGL1, elucidate the mechanism of imprinting at this locus, and perform functional studies to understand why this gene causes only a benign type of tumor growth in a highly specific, almost segmented, area of the paraganglion system.
Meiotic recombinants in Dutch and North-American families narrowed the location of PGL1 on 11q22-q23.2 to an interval of 4-6 cM defined by the markers D11S1647 and D11S908 [1,2]. Sharing of a linked haplotype, defined by 3 markers and rare in the general population, in a large Dutch multigenerational family (FGT189) suggested that the PGL1 gene was most likely located in the distal 2-cM of this interval surrounding the PLZF gene. However, PLZF was excluded as a candidate gene on the absence of any coding mutations in 12 paraganglioma patients. In addition, two new markers developed from the intronic regions of PLZF did not show the haplotype-sharing in FGT189. We therefore re-analysed marker data on FGT189, and hypothesized that haplotype-sharing might also be present in the proximal part of the 6-cM interval. This led to the identification of an 8-allele haplotype, very rare in the general population, but completely shared among all patients in FGT189 . This haplotype is flanked by the markers D11S1986 and D11S897, which bracket an interval of about 2 cM. A new recombinant in a North-American family (PGL12) pushed the telomeric border to D11S1347. The current PGL1 critical region region is available in P1- and YAC-contigs , and the transcript-map is progressing rapidly. Several candidate genes have been excluded by us by direct sequence analysis. Further fine-mapping will also exploit the strong founder effect noted among Dutch paraganglioma families .
We have analysed the DNA-content of non-chromaffin paragangliomas by flow cytometry and have found that about one-third of them carry aneuploid stemlines. This suggested the presence of numerical chromosome aberrations, including parental imbalances in copy-number, and indicated that these tumors represent true clonal proliferations. In order to further study chromosome abnormalities, and to find out if PGL1 is subject to Knudson's two-hit inactivation mechanism, we typed 26 tumors from 22 patients for the presence of loss of heterozygosity on all chromosome arms. Intriguingly, LOH was found only on the long arm of chromosome 11 in a very high proportion (81%) of tumors. It always involved the maternal allele in 8 cases where the parental origin of the lost allele could be determined. Although only parts of the 11q-arm were affected by loss in a number of cases, very little definitive mapping information could be gleaned from the breakpoints in the allele loss patterns.
Many cases with loss often displayed only weak losses of allele-signal intensities. This suggested either that not all cells were carrying the event, or that the tumor was not clonally derived, with some fraction of the cells losing one allele, and another fraction of the cells losing the opposite allele. The DNA-ploidy findings, and X-inactivation patterns in at least one tumor, suggest that substantial clonal populations do exist in these tumors. The most likely explanation was cellular heterogeneity for the LOH events. To further investigate this, we sorted aneuploid DNA-peaks from fresh tumor samples, and we microdissected approximately 100 chief cells from two tumors also known to carry partial LOH. The sorted fractions demonstrated complete loss of the allele that showed only partial reduction in total tumor DNA preparations . In isolated chief cells as well, LOH was complete. These results show that the chief cells are true neoplastic cells, which are clonally derived and most likely carry the aneuploid stemlines as seen in DNA flow cytometry. As yet, the role of the sustentacular cells remains unclarified in this regard.
1.Van Schothorst, E.M., Jansen, J.C., Bardoel, A., Van der Mey, A.G.L., James, M.J., Sobol, H., Weissenbach, J., Van Ommen, G.-J.B., Cornelisse, C.J., and Devilee, P. (1996) Confinement of PGL, an imprinted gene causing hereditary paragangliomas, to a 2 cM interval on 11q22-q23 and exclusion of DRD2 and NCAM as candidate genes. Eur J Hum Genet, 4:267-273.
2.Baysal, B.E., Farr, J.E., Rubinstein, W.S., Galus, R.A., Johnson, K.A., Aston, C.E., Myers, E.N., Johnson, J.T., Carrau, R., Kirkpatrick, S.J., Myssiorek, D., Singh, D., Saha, S., Gollin, S.M., Evans, G.A., James, M.R., and Richard, C.W. (1997) Fine mapping of an imprinted gene for familial nonchromaffin paragangliomas, on chromosome 11q23. Am J Hum Genet, 60:121-132.
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4.Baysal, B.E., Van Schothorst, E.M., Farr, J.E., James, M.R., Devilee, P., and Richard III, C.W. (1997) A high resolution STS, EST, and gene-based physical map of the hereditary paraganglioma region on chromosome 11q23. Genomics, 44:214-221.
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6.Van der Mey, A., Cornelisse, C., Hermans, J., Terpstra, J., Schmidt, P., and Fleuren, G. (1991) DNA flow cytometry of hereditary and sporadic paragangliomas (glomus tumours). Br.J.Cancer., 63:298-302.
7.Devilee, P., Van Schothorst, E., Bardoel, A., Bonsing, B., Kuipers-Dijkshoorn, N., James, M., Van der Mey, A., and Cornelisse, C. (1994) Allelotype of head and neck paragangliomas: allelic imbalance is confined to the long arm of chromosome 11, the site of the predisposing locus PGL. Genes Chrom Cancer, 11:71-78.
8.Van Schothorst, E.M., Beekman, M., Torremans, P., Kuipers-Dijkshoorn, N.J., Wessels, H.W. , Bardoel, A.F.J., Van der Mey, A.G.L., Van Ommen, G.-J.B., Devilee, P., and Cornelisse, C.J. (1998) Paragangliomas of the head and neck region show complete loss of heterozygosity at 11q22-q23 in chief cells and the flow-sorted DNA aneuploid fraction. Hum Pathol, 29:1045-1049.