The potential for gene therapy to cause cancer has been predicted for many years. This is now confirmed in a new study that highlights, yet again, the dangers of unregulated, profit-driven genetic engineering. Dr. Mae-Wan Ho and Prof. Joe Cummins report.
Adenovirus is one of the earliest virus to be genetically engineered as gene therapy vectors, on grounds that it was 'safe and without side effects'. But from the beginning, these vectors were found to cause severe immune reactions in both human subjects and mice [1]. Undaunted, researchers continued to engineer new versions of the vectors and to experiment with them, claiming again, that they are "generally considered safe", until disaster struck two years ago.
Teenager Jesse Gelsinger died three days after receiving gene therapy with adenoviral vector [2]. He had suffered liver injury and inappropriate blood coagulation within the first day. On the third day he had trouble breathing and his vital organs began to fail. He was taken off life support on the fourth day. The autopsy revealed further abnormalities. The researchers had directed the gene therapy vector to the liver, but significant amounts of the vector were found in the spleen, lymph nodes, bone marrow and other tissues.
Now, yet another side effect long predicted by many scientists [1, 2] has surfaced in a new study in mice [3].
Ironically, adeno-associated virus vectors (rAAVs) were widely used initially because they were thought not to integrate into the genome. But it soon transpired that the wild-type, unmodified adenovirus integrates at high frequency into a specific site on human chromosome 19. However, this ability for site-specific integration is lost in rAVV vectors. Instead, they either persist as independently replicated 'episomes' outside the genome of the cell, or they integrate randomly into the human chromo-somes. For that reason, "the potential for rAAV to have adverse side-effects due to insertional mutagenesis may exist", the researchers concede.
Newborn transgenic mice with the mucopolysaccharide storage disease MPSVII were treated with rAAVs carrying the enzyme that breaks down the mucopoly-saccharide. At 18 months of age, 3 of the 5 remaining treated, apparently healthy mice, selected at random, were killed in order to study enzyme activities in the internal organs. The enzyme activities were high, and identical to other treated mice that were killed at one year of age. But the signs of cancer were unmistakable.
Following this unexpected discovery, the rest of the treated and control mice were examined, together with others that have been killed or have died earlier. In all, 6 treated animals with tumours were found, 5 with hepatocellular carcinoma (liver cancer in the cells lining the organ) and the sixth with angiosarcoma (a rare malignant tumour of the lining of the blood vessels). One of the five with hepatocellular carcinoma also had angiocarcinoma. None of the 8 age-matched controls had tumours.
A further summary review of studies carried out by the same group revealed 33% hepatocellular carcinoma in one group of 12 treated animals at the age of 12 months, and 17% with angiocarcinoma at the age of 12-18 months.
The cancers were found to be specific to rAAV, as they were absent in mice with bone marrow transplant and in transgenic mice carrying the same enzyme cassette but without the rAAV.
Quantitative polymerase chain reaction (PCR) was carried out on the cancerous and non-cancerous tissues in affected and unaffected mice. This gave no evidence that cancerous tissues were associated with increased rAAV, as might be expected if insertion of rAAV had caused the cell to grow uncontrollably into the cancer tumour. As the authors state, "these data do not exclude rAAV as the causative agent for the tumours in the rAAV-treated mice."
One possibility, not mentioned by the authors of the mice study, is that the initial insertion may have destabilised the genome, leading to large scale genome scrambling that is increasingly thought to underlie the cancerous state [4]. Under those circumstances, the cancerous cells may have lost most of the original rAAV insert.
Article first published October 2001