LONDON ¿ Gene therapy could help to treat the swelling and disfigurement caused by lack of lymphatic vessels, tests on mice suggest. The therapy involves delivering a growth factor that stimulates the growth of lymphatic vessels in the skin.
An international team of researchers said the treatment has the potential to be used to help women who suffer swollen arms following surgery for breast cancer, and people who have the rare inherited disease called human hereditary lymphedema. Conversely, blocking the receptor for the growth factor concerned could help to prevent the growth of tumors, because there is evidence that some tumors overexpress it.
Marika Karkkainen, a Ph.D. student in the laboratory of Kari Alitalo, research professor at the Finnish Academy of Sciences in Helsinki, told BioWorld International, ¿Our latest results suggest that you might be able to stimulate the growth of lymphatic vessels in the skin and thereby enhance lymphatic fluid drainage in the skin, as poor drainage is the cause of lymphedema.¿
Karkkainen is the first author of a paper in the Proceedings of the National Academy of Sciences that describes these studies. Its title is, ¿A model for gene therapy of human hereditary lymphedema.¿
Alitalo¿s group is renowned for its work on the family of vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs). In recent years, he has collaborated with a group headed by Robert Ferrell of the department of human genetics at the University of Pittsburgh, which has collected a large number of families affected by human hereditary lymphedema. This condition, also known as Milroy¿s disease, causes swelling of the extremities due to an absence of lymphatic vessels under the skin.
Alitalo is supported by the Finnish Academy of Sciences and the State Technology Development Center in Helsinki. To protect the intellectual property rights of the team¿s findings, the university set up a company called Licentia Ltd. that is in partnership with the Ludwig Institute for Cancer Research, which has a network of research centers worldwide.
Earlier this year, Alitalo and his team published work showing that if it prevented stimulation of the receptor known as VEGFR-3 in transgenic mice, the mice developed without lymphatic vessels in internal organs.
It therefore seemed logical to screen the affected individuals in the families with human hereditary lymphedema collected by the Pittsburgh group, to see if there were any mutations in the genes encoding VEGFR-3 in affected individuals. ¿We found a linkage with the chromosomal region 5q, which was where we found the VEGFR-3 gene,¿ Karkkainen said. ¿When we sequenced the coding exons of this gene, we found five different nonsense mutations in it that are associated with lymphedema in these families.¿ Only some of the families had VEGFR-3 mutations associated with lymphedema, suggesting that other ¿lymphedema genes¿ exist.
Alitalo¿s group also decided to study a mutant mouse known as Chy, which accumulates lymphatic fluid in its abdomen. Sequencing of the murine gene encoding VEGFR-3 revealed a missense mutation similar to that found in humans, Karkkainen said. ¿This is a good mouse model for human hereditary lymphedema because, like humans, the mice are heterozygous for the mutation, and they have nonfunctional VEGFR-3,¿ she said. ¿Like affected humans, the mice have swollen limbs and lack lymphatic vessels in the skin. So we then wanted to know whether we could treat the lymphedema in these mice somehow.¿
They tried two strategies. They already had available transgenic mice that overexpress VEGFC, the molecule that stimulates VEGFR-3. The version of VEGFC overexpressed in these mice, called VEGF-C156S, binds only to VEGFR-3 and not to other receptors, so they could be sure about how the effect was mediated. These transgenic mice have hyperproliferation of lymphatic vessels. When the researchers crossed these mice with the Chy mice, they found that the resulting offspring had normal-looking lymphatic vessels.
Following this interesting result, they decided to try gene therapy. They infected the ears of the Chy mice with adenovirus that had been engineered to express VEGFC. Karkkainen said, ¿We saw growth of the lymphatic vessels in the ear. But because adenovirus causes inflammation when it infects the skin, and you don¿t get long-term expression, we also decided to try using adeno-associated virus as a vector. Again, we got a good response, with less inflammation.¿
These studies, Karkkainen said, suggest that it may be possible to deliver extra growth factor to the limbs of people affected by human hereditary lymphedema. ¿Once these grow, they would remain in the limbs, but we need to investigate what sort of long-term growth factor expression might be needed to maintain these vessels. Earlier studies by our group suggest that other growth factors may take over after a few weeks.¿
The group also is investigating how this type of strategy could be applied to treat secondary lymphedema, which sometimes follows surgery for breast cancer, especially where the lymph nodes in the armpit have been removed. Intensive studies also are continuing into ways of blocking vascular endothelial growth factors produced by tumors in order to inhibit growth of metastases.