| Summary: | Thesis (M.Sc.)--University of the Witwatersrand, Faculty of Health Sciences, 1998. === Retinoblastoma (RB) is a malignant intraocular tum our that affects
newborns and young children. The disease results from inactivation
of both alleles o f the Rb-1 tumour-suppressor gene within
chromosome band 1 3q 14 by deletion, mutation or gene silencing.
About 4 0 % of RB cases are of the hereditary type and about 6 0 % are
sporadic. The incidence in the Caucasoid population is 1 : 1 2 0 0 0 .
Various mutations have been found throughout the 180 kb gene,
ranging from point mutations to large deletions, but no "hot spot" or
specific region for mutations has been identified. However, many
mutations appear to occur as a result of C -» T transitions at CpG
dinucleotides.
The promoter region and exon 1 of the Rb-1 gene encompass a CpG
rich region. Although CpG islands are not methylated in normal
tissues, they have been demonstrated to become methylated in
various tumour types.
In this study, DNA from paraffin-embedded tum our specimens of
southern African Negroid RB individuals was analyzed using a
II
methylation-specific PCR-based method, targeting specific methylation
sites in order to ascertain differences of methylation patterns in RB
tum ours as opposed to normal retinal tissue. Methylation of certain
CpG sites in the 5 ' region of Rb-1 may result in transcriptional
silencing and thereby contribute to loss of function of the gene.
All of the tumours analysed were methylated at a minimum o f one
site, while DNA from the normal fetal retina was unmethylated at all
of the methylation sensitive CpG sites analyzed.
The study provided an effective means for screening methylation
changes in the Rb-1 gene that had occurred during tumorigenesis, as
well as the methylation pattern of normal retinal tissue.
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