Ch4.0&4.1 Flashcards
What initial theory did cancer biologists develop about tumor viruses in the 1970s?
Tumor viruses might infect susceptible tissues and transform cells, leading to the formation of large cell masses that become tumors.
How did the initial tumor virus theory become challenged?
Most human cancers did not spread like infectious diseases, significant cancer clusters were rare, and attempts to isolate viruses from tumors were mostly unsuccessful.
How many human tumor types could be clearly linked to viral agents?
Only two commonly occurring tumor types in the Western world - cervical carcinomas and hepatomas (liver carcinomas) - could be clearly tied to specific viral agents.
What were the two main responses to the failure of the tumor virus theory?
Some researchers argued that chemical and physical carcinogens activated latent viruses, while others began investigating cellular genomes as a source of cancer-causing genes.
Who pioneered early research on chemical cancer induction?
Katsusaburo Yamagiwa, who demonstrated in the early 20th century that repeated application of coal tars on rabbit ears could induce skin carcinomas.
What early evidence existed about non-viral cancer causes?
A PhD thesis in Paris documented over 100 human cancer cases, largely of the skin, in individuals who worked with X-ray tubes.
What unique characteristic do retroviruses have regarding genome integration?
Retrovirus genomes can become integrated into germ-line chromosomes and be transmitted as Mendelian alleles across generations.
How can endogenous proviruses be activated?
In some cases, they can be activated by specific conditions, such as culturing mouse fibroblasts with bromodeoxyuridine (BrdU).
What hypothetical mechanism did some researchers propose for human cancer formation?
They suggested that mutagenic carcinogens might activate latent endogenous retroviruses, which could then spread and cause cancer.
What percentage of the human genome derives from endogenous retroviral genomes?
Approximately 8% of the human genome comes from endogenous retroviral genomes.
How many retrovirus-derived segments in the human genome might be genetically intact?
Only several of the approximately 40,000 retrovirus-derived segments have been shown to be potentially capable of specifying infectious virus particles.
What is unique about the HERV-K virus subfamily?
It has entered the human germ line relatively recently, and several of its proviruses appear intact, though they have not been found to produce infectious viruses.
When did the ancient germ-line retroviral infections occur?
The infections occurred 5 million years ago and earlier in ancestral primates.
Why did the endogenous retrovirus cancer causation models collapse?
Supportive evidence was not forthcoming, and infectious retroviral particles could not be verified in human tumors.
What happened to most endogenous retroviral genomes in human evolution?
They progressively mutated into sequences no longer capable of specifying infectious retrovirus particles and became part of junk DNA.
