How many oncogenic hits were required?
In the case of hematopoietic cells, a single cell containing a specific integrated provirus can replicate and expand to vast cell numbers, thereby undergoing spontaneous mutations that could result in additional oncogenic hits. It is possible that all leukemias linked to retroviral vector integrations contain these types of secondary, cooperating mutations. This is much harder to envision in the case of HCC, because normal, individual hepatocytes do not undergo a massive, multi-log expansion during which random mutations could occur in cooperating oncogenes. Even if we assume that multiple hepatocytes contained proviruses at the AAV-HCC locus, the number is still too small for a second spontaneous mutation to consistently occur during subsequent proliferation.Another possibility is that the transformed hepatocytes contained multiple AAV integrants. With an average integration frequency of 0.1 proviruses per hepatocyte, the Poisson distribution predicts that 0.45% of hepatocytes will co
