Hypercalcaemia of malignancy and basic research on mechanisms responsible for osteolytic and osteoblastic metastasis to bone

Calcium homeostasis is a tightly regulated process involving the co-ordinated efforts of the skeleton, kidney, parathyroid glands and intestine. Neoplasms can alter this homeostasis indirectly through the production of endocrine factors resulting in humoral hypercalcaemia of malignancy. Relatively common with breast and lung cancer, this paraneoplastic condition is most often due to tumour production of parathyroid hormone-related protein and ensuing increased osteoclastic bone resorption. Although control of hypercalcaemia is generally successful, the development of this complication is associated with a poor prognosis. The metastasis of tumour cells to bone represents another skeletal complication of malignancy. As explained in the 'seed and soil' hypothesis, bone represents a fertile ground for cancer cells to flourish. The molecular mechanisms of this mutually beneficial relationship between bone and cancer cells are beginning to be understood. In the case of osteolytic bone disease, tumour-produced parathyroid hormone-related protein stimulates osteoclasts that in turn secrete tumour-activating transforming growth factor-beta that further stimulates local cancer cells. This 'vicious cycle' of bone metastases represents reciprocal bone/cancer cellular signals that likely modulate osteoblastic bone metastatic lesions as well. The development of targeted therapies to either block initial cancer cell chemotaxis, invasion and adhesion or to break the 'vicious cycle' is dependent on a more complete understanding of bone metastases. Although bisphosphonates delay progression of skeletal metastases, it is clear that more effective therapies are needed. Cancer-associated bone morbidity remains a major public health problem, and to improve therapy and prevention it is important to understand the pathophysiology of the effects of cancer on bone. This review will detail scientific advances regarding this area.

Diminished levels of the putative tumor suppressor proteins EXT1 and EXT2 in exostosis chondrocytes

The EXT family of putative tumor suppressor genes affect endochondral bone growth, and mutations in EXT1 and EXT2 genes cause the autosomal dominant disorder Hereditary Multiple Exostoses (HME). Loss of heterozygosity (LOH) of these genes plays a role in the development of exostoses and chondrosarcomas. In this study, we characterized EXT genes in 11 exostosis chondrocyte strains using LOH and mutational analyses. We also determined subcellular localization and quantitation of EXT1 and EXT2 proteins by immunocytochemistry using antibodies raised against unique peptide epitopes. In an isolated non-HME exostosis, we detected three genetic hits: deletion of one EXT1 gene, a net 21-bp deletion within the other EXT1 gene and a deletion in intron 1 causing loss of gene product. Diminished levels of EXT1 and EXT2 protein were found in 9 (82%) and 5 (45%) exostosis chondrocyte strains, respectively, and 4 (36%) were deficient in levels of both proteins. Although we found mutations in exostosis chondrocytes, mutational analysis alone did not predict all the observed decreases in EXT gene products in exostosis chondrocytes, suggesting additional genetic mutations. Moreover, exostosis chondrocytes exhibit an unusual cellular phenotype characterized by abnormal actin bundles in the cytoplasm. These results suggest that multiple mutational steps are involved in exostosis development and that EXT genes play a role in cell signaling related to chondrocyte cytoskeleton regulation.

The structure of the human multiple exostoses 2 gene and characterization of homologs in mouse and Caenorhabditis elegans

Hereditary multiple exostoses (EXT) is an autosomal dominant disorder characterized by multiple cartilage-capped outgrowths from the epiphyses of long bones. In some cases, these osteochondromas progress to malignant chondrosarcomas. Alterations in at least three genes (EXT1, EXT2, and EXT3) can cause this disorder. Two of these have been isolated (EXT1 and EXT2) and encode related members of a putative tumor suppressor family. We report here the genomic structure of the human EXT2 gene consisting of 14 exons (plus 2 alternative exons) covering an estimated 108 kb of chromosome 11p11-13. We have derived the DNA sequences at all exon/intron boundaries throughout this gene-information that is important for the detailed study of mutations in EXT2. We have also characterized the mouse EXT2 cDNA and have mapped the mouse locus to chromosome 2 between D2Mit15 and Pax6. This mouse homolog should enable transgenic knockout experiments to be initiated to further elucidate gene function. Interestingly, sequence comparisons reveal that the human and mouse EXT genes have at least two homologs in the invertebrate Caenorhabditis elegans, indicating that they do not function exclusively as regulators of bone growth. This observation opens the way for a functional analysis of these genes in nematodes and other lower organisms.

Identification and localization of the gene for EXTL, a third member of the multiple exostoses gene family

Hereditary multiple exostoses (EXT) is an autosomal dominant disorder characterized by multiple bony outgrowths from the juxtaepiphyseal region of long bones. In a small proportion of cases, these exostoses progress to malignant chondrosarcomas. Genetic linkage of this disorder has been described to three independent loci on chromosomes 8q24.1 (EXT1), 11p11-13 (EXT2), and 19p (EXT-3). The EXT1 and EXT2 genes were isolated recently and show extensive sequence homology to each other. These genes are deleted in exostoses-derived tumors, supporting the hypothesis that they encode tumor suppressors. We have identified a third gene that shows striking sequence similarity to both EXT1 and EXT2 at the nucleotide and amino acid sequence levels, and have derived its entire coding sequence. Although the mRNA transcribed from this gene is similar in size to that from EXT1 and EXT2, its pattern of expression is quite different. We have localized this gene by fluorescence in situ hybridization to metaphase chromosomes and by whole genome radiation hybrid mapping to chromosome 1p36.1 between DIS458 and DIS511, region that frequently shows loss of heterozygosity in a variety of tumor types. This gene, EXTL (for EXT-like), is therefore a new member of the EXT gene family and is a potential candidate for several disease phenotypes.

Human chromosome-specific cDNA libraries: new tools for gene identification and genome annotation

To date, only a small percentage of human genes have been cloned and mapped. To facilitate more rapid gene mapping and disease gene isolation, chromosome 5-specific cDNA libraries have been constructed from five sources. DNA sequencing and regional mapping of 205 unique cDNAs indicates that 25 are from known chromosome 5 genes and 138 are from new chromosome 5 genes (a frequency of 79.5%). Sequence complexity estimates indicate that each library contains -20% of the approximately 5000 genes that are believed to reside on chromosome 5. This study more than doubles the number of genes mapped to chromosome 5 and describes an important new tool for disease gene isolation.

A single ataxia telangiectasia gene with a product similar to PI-3 kinase

A gene, ATM, that is mutated in the autosomal recessive disorder ataxia telangiectasia (AT) was identified by positional cloning on chromosome 11q22-23. AT is characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, cancer predisposition, radiation sensitivity, and cell cycle abnormalities. The disease is genetically heterogeneous, with four complementation groups that have been suspected to represent different genes. ATM, which has a transcript of 12 kilobases, was found to be mutated in AT patients from all complementation groups, indicating that it is probably the sole gene responsible for this disorder. A partial ATM complementary DNA clone of 5.9 kilobases encoded a putative protein that is similar to several yeast and mammalian phosphatidylinositol-3' kinases that are involved in mitogenic signal transduction, meiotic recombination, and cell cycle control. The discovery of ATM should enhance understanding of AT and related syndromes and may allow the identification of AT heterozygotes, who are at increased risk of cancer.