Human cancer, causes

Most human cancer is caused by substances in the environment,... 

Since many cellular oncogenes were originally identified in transforming retroviruses, and since some retroviruses cause rapid malignant transformation in animals, notably chicken, it seemed for a while that most human cancers are caused by infection with retroviruses. But that is not the case. The number of human cancers which originate from retroviral infections is actually rather small (Table 15.2). In many cases, proto-oncogenes can become oncogenes by mutation or overexpression in human cells, without retroviral assistance. 

Normally, cell proliferation and cell death are in a steady state, where the rate of proliferation is matched by the rate of apoptosis. Colon cancer, like other cancers, is caused by mutations in key components of regulatory pathways in this case mutations of the tumour-suppressor gene APC, the adenomatous polyposis coli gene. When APC is inactivated and turned off, the Wnt pathway, normally operative only in the embryo, is turned on. The Wnt pathway also plays a role in other cancers, for example in mammary tumours in mice and humans (for further information and literature references, see ref. 14). 

WHO) of the United Nations, has developed three categories for evidence of carcinogenicity. These include (i) sufficient evidence of human cancer definitely caused by exposure, (ii) hmited evidence due to inadequate data, and (iii) inadequate evidence (i.e., no data available) (lARC, 1987). 

History of Cancer, Carcinogenicity Testing, and Human Causes of Cancer... 

HUMAN HEALTH RISKS Acute Risks irritation of eyes, mucous membranes and upper respiratory tract severe skin burns depression of central nervous system weeping and twitching of eyelids headaches Chronic Risks disturbances of liver functions may cause cancer may cause genetic damage pulmonary edema more susceptible to illness. 

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