Frederick P. Li

Familial cancer syndromes have helped to define the role of tumor suppressor genes in the development of cancer. The dominantly inherited Li-Fraumeni syndrome (LFS) is of particular interest because of the diversity of childhood and adult tumors that occur in affected individuals. The rarity and high mortality of LFS precluded formal linkage analysis. The alternative approach was to select the most plausible candidate gene. The tumor suppressor gene, p53, was studied because of previous indications that this gene is inactivated in the sporadic (nonfamilial) forms of most cancers that are associated with LFS. Germ line p53 mutations have been detected in all five LFS families analyzed. These mutations do not produce amounts of mutant p53 protein expected to exert a trans-dominant loss of function effect on wild-type p53 protein. The frequency of germ line p53 mutations can now be examined in additional families with LFS, and in other cancer patients and families with clinical features that might be attributed to the mutation.

Four families were identified in which a pair of children had soft-tissue sarcomas: three sets of sibs and one set of cousins. One parent of each affected child developed cancer; carcinoma of the breast occurred in three mothers under 30 years of age. Other young adults in these families had a high frequency of cancer, with no evidence of underlying genetic disorders known to carry a high risk of neoplasia. The increased familial susceptibility to cancer was manifested not only by the large number of members affected but by a seeming excess of multiple primary neoplasms. These findings suggest a new "familial" syndrome of neoplastic diseases in which heredity or oncogenic agents, or both, may have a causal role.

A search of the Cancer Family Registry of the National Cancer Institute revealed 24 kindreds with the syndrome of sarcoma, breast carcinoma, and other neoplasms in young patients. Cancer developed in an autosomal dominant pattern in 151 blood relatives, 119 (79%) of whom were affected before 45 years of age. These young patients had a total of 50 bone and soft tissue sarcomas of diverse histological subtypes and 28 breast cancers. Additional features of the syndrome included an excess of brain tumors (14 cases), leukemia (9 cases), and adrenocortical carcinoma (4 cases) before age 45 years. These neoplasms also accounted for 73% of the multiple primary cancers occurring in 15 family members. Six of these patients had second cancers linked to radiotherapy. The diversity of tumor types in this syndrome suggests pathogenetic mechanisms which differ from hereditary cancers arising in single organs or tissues. The syndrome is presently diagnosed on clinical grounds; laboratory markers are needed to identify high-risk individuals and families and to provide insights into susceptibility mechanisms that may be shared by a wide variety of cancers.

Normal human cells exhibit a limited replicative life span in culture, eventually arresting growth by a process termed senescence. Progressive telomere shortening appears to trigger senescence in normal human fibroblasts and retinal pigment epithelial cells, as ectopic expression of the telomerase catalytic subunit, hTERT, immortalizes these cell types directly. Telomerase expression alone is insufficient to enable certain other cell types to evade senescence, however. Such cells, including keratinocytes and mammary epithelial cells, appear to require loss of the pRB/p16(INK4a) cell cycle control mechanism in addition to hTERT expression to achieve immortality. To investigate the relationships among telomerase activity, cell cycle control, senescence, and differentiation, we expressed hTERT in two epithelial cell types, keratinocytes and mesothelial cells, and determined the effect on proliferation potential and on the function of cell-type-specific growth control and differentiation systems. Ectopic hTERT expression immortalized normal mesothelial cells and a premalignant, p16(INK4a)-negative keratinocyte line. In contrast, when four keratinocyte strains cultured from normal tissue were transduced to express hTERT, they were incompletely rescued from senescence. After reaching the population doubling limit of their parent cell strains, hTERT(+) keratinocytes entered a slow growth phase of indefinite length, from which rare, rapidly dividing immortal cells emerged. These immortal cell lines frequently had sustained deletions of the CDK2NA/INK4A locus or otherwise were deficient in p16(INK4a) expression. They nevertheless typically retained other keratinocyte growth controls and differentiated normally in culture and in xenografts. Thus, keratinocyte replicative potential is limited by a p16(INK4a)-dependent mechanism, the activation of which can occur independent of telomere length. Abrogation of this mechanism together with telomerase expression immortalizes keratinocytes without affecting other major growth control or differentiation systems.