Somatic events unmask recessive cancer genes to initiate malignancy

A heritable mutation predisposes an individual to certain childhood malignancies, such as retinoblastoma and Wilms' tumor. The chromosomal locations of the genes responsible for the predisposition are known by linkage with chromosomal deletions and enzyme markers. A study of these tumors in comparison to the normal constitutional cells of the patients, using enzyme and DNA markers near the predisposing genes, has shown that these genes are recessive to normal wild-type alleles at the cellular level. Expression of the recessive phenotype (malignancy) involves the same genetic events that were observed in Chinese hamster cell hybrids carrying recessive drug resistance genes. In both the experimental and clinical situations, the wild-type allele is most commonly eliminated by chromosome loss with duplication of the mutant chromosome. Simple chromosome loss and mitotic recombination have been documented in both systems. In the remaining 30% of cases, inactivation or microdeletion of the wild-type allele are assumed to be responsible for expression of the recessive phenotype. Osteosarcoma is a common second tumor in patients who have had retinoblastoma. Studies with markers in osteosarcoma show that these tumors also result from unmasking of the recessive phenotype by loss of the normal allele at the retinoblastoma locus, whether or not the patient had retinoblastoma. Subsequent chromosomal rearrangements and amplification of oncogenes that occur in these homozygous tumors provide progressive growth advantage. In other malignancies, in which studies have so far focused on oncogene amplification and chromosomal rearrangements, unmasking of recessive mutations may also be the critical initiating events.