The development of homozygosity or hemizygosity in the 13q14 region by deletion, mitotic recombination, or chromosomal loss has been interpreted as a primary event in
retinoblastoma. This finding is consistent with the hypothesis that inactivation of both alleles of a gene located at 13q14.11 is required for
tumorigenesis. Observations reported by Benedict and colleagues in one case of bilateral
retinoblastoma, LA-RB 69, provided early evidence in favor of this hypothesis. By examining levels of
esterase D, an
enzyme also mapping to 13q14.11, it was previously inferred that one chromosome 13 was lost. Using a rabbit anti-
esterase D antibody and the
esterase D cDNA probe, we have found that low but detectable quantities of
esterase D protein and enzymatic activity are present in
tumor cells from LA-RB 69; fibroblasts from this patient contain two copies of the
esterase D gene, indicated by heterozygosity at an Apa I polymorphic site within this gene; and
tumor cells from the same patient are homozygous at this site, indicating loss and reduplication of the
esterase D locus. These results demonstrate that one of the two
esterase D alleles in this patient acted as a "null" or silent allele--that is, was present in the genome with markedly decreased
protein expression. This mutant allele acted as a marker for
tumor-associated loss of chromosome 13 heterozygosity, in concordance with previous proposals.