A new approach to the Localisation of genes involved in Paediatric Brain Tumours using comparative Genomic Hybridisation (Dr Janice Royds)
2 year project with grant funding of “£48,489.00
Dec. 1999 - 2001
Childhood brain tumours merit special consideration. Unfortunately, underlying causes of these tumours are poorly understood, hampering the development of more specific therapies. The aim of this study is to systematically search the human genome for genetic changes which give rise to these tumours by using the powerful tool of comparative genomic hybridization (CGH). This technique will enable us to identify tumour specific regions of genetic amplification and deletion. Consistently abnormal chromosome areas will then be studied further to identify the genes involved. This will eventually lead to a better understanding of the causes and behaviour of these tumours. The identification of tumour associated chromosomal abnormalities will also have clinical value in areas of diagnosis and treatment design, indicating which tumours are more resistant or proactive to conventional treatment.
Conclusion: We have now studied 20 cases of medulloblastoma by comparative genomic hybridization (CGH). Overall, our findings have substantiated those we reported last year. The most frequent loss was chromosome 17p, the site of the P53 gene. All current studies reflect this findings. Other frequently occurring losses were of 1p, 8q, 9q, 10q, 11p/q, 14q and 22q. The most common copy number gains were observed on chromosomes X, 3q, 5 and 6q.
In spite of the fact that a number of these observations are common to other studies, there is still no clear and consistent picture of copy number changes in medulloblastoma. For example, less than a third of current studies report loss of 22q associated with this malignancy. However, in those studies that do, it is a frequent finding, suggesting a significant role for a gene or genes on this chromosome in the development of the disease. It is noteworthy that neither loss of 14q, nor gain of 6q, has so far been reported as a significant finding in other series of medulloblastomas. We were able to study unselected cases, since any paraffin embedded sample made available could be analysed. Inclusion was not dependent on excess material being made available from a previous large tumour mass. Tumour cells were micro-dissected from paraffin sections for analysis, thus minimizing the degree of contamination by normal tissue. It is possible that this approach increases the likelihood of detecting medulloblastoma specific abnormalities.
We have also carried out an immunohistochemical analysis for the expression of cell lineage markers, in order to establish genotype-phenotype correlations. We are currently expanding our series of cases to determine with desmoplastic and medulloblastomas show differences in their genetics and in the expression of calbindin, a marker which may cast light on the cell of origin of these tumours. The pattern of expression of cell lineage markers is particularly relevant to those cases with losses from chromosome 22, as some studies suggest that this may be a marker of adverse prognosis and of tumours characterized by the expression of multiple lineage markers. However, our results to date have not demonstrated such an expression pattern, suggesting that chromosome 22 loss may occur in otherwise typical medulloblastomas.
We have also studied a small series of atypical teratoid / rhabdoid tumours (AT/RT). These are uncommon tumours of the central nervous system in children, with a particularly poor prognosis in comparison to medulloblastoma. These tumours often demonstrate losses from chromosome 22, but overall, the cytogenetic profile of the tumour is poorly defined. We carried out CGH on paraffin embedded material from three cases of atypical teratoid / rhabdoid tumours. Two cases showed losses from chromosome 22 together with other chromosome imbalances, including losses from 1p in both cases. The third case demonstrated loss of 8p as the sole detectable abnormality.
Whilst monosomy or partial deletion of chromosome 22 appears to be a useful diagnostic maker for AT/RT, it is not present in all cases, and we have shown that this marker may also occur in some cases of medulloblastoma. The variation in cytogenetic patterns reported for AT/RT suggests that different genetic pathways may underlie this tumour, which may in turn give rise to diverse prognostic and treatment groups.
These studies have identified regions of chromosome change, associated with two distinct paediatric brain tumours. These chromosome regions are likely to contain genes that play an important part in the genesis and subsequent prognosis of these malignancies.
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