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Evidence from a cytogenetic study supports the idea that retinoblastoma may exist in two distinct forms which differ clinically. Eleven of 13 children in the study had chromosome abnormalities in their tumour—an array of gains /losses in various chromosomes—including those other than in chromosome 13q where the retinoblastoma gene maps. Frequency of the abnormalities was bimodal, with tumours showing low level chromosomal instability and 0–3 abnormal events (seven children) or high level instability and eight plus events (six). Children with low level chromosomal instability showed some similar traits. Their mean age was half that of the other children; there were fewer males (male to female ratio 1.3:1 v 5:1) and undifferentiated tumours (57% v 83%); and hereditary retinoblastoma was commoner (57% v 33%), as was disease in both eyes (57% v 17%). The control showed no chromosome abnormalities.
The study was performed on 13 retinoblastomas from 13 consecutive children aged 0–45 months. Chromosome instability was found by comparative genomic hybridisation of DNA extracted from frozen sections of tumour biopsy specimens and control lymphocyte DNA from a healthy donor with metaphase spreads in cultured donor lymphocytes.
Retinoblastoma is the commonest malignancy in the eye in children. About 40% of cases are hereditary, involving chromosome 13. Changes in other chromosomes—extra copies, deletions and gains, ring chromosomes—have been noted. Comparative gene hybridisation is useful way of testing for chromosomal instability but has not been used much in retinoblastoma.