There may not yet be a cure for cancer, but for some human cancers, one thing comes pretty close: the HPV vaccine.
That’s why, last week, the 69 National Cancer Institute-designated cancer centers, including UW-Madison’s Carbone Cancer Center (UWCCC), issued a statement urging increased uptake of the three-dose vaccination. This week, the CDC updated its HPV vaccination recommendation to include one that protects against nearly all cancer-causing strains.
I won’t go into why HPV vaccination rates are so low in the U.S., but thought the recent news was a good time to take a look at HPV, how it causes cancer and how HPV-related disease can be prevented. A huge thanks to two of UWCCC’s HPV experts, Paul Harari, professor and chair of human oncology, and Paul Lambert, professor and chair of oncology and director of the McArdle Laboratory for Cancer Research, for their help with this post:
- HPV, or human papillomavirus, is, as its name suggests, a virus. HPV is an infectious agent, like the flu virus or the common cold virus. It can be spread through physical contact.
- Over 120 different strains of HPV exist. Most strains cause benign skin warts. Nine strains are spread almost exclusively through sexual contact: the two low-risk strains that can cause disfiguring genital warts (HPV 6 and 11) and the seven high-risk strains associated with cancers (most commonly HPV 16 and 18 but including five other strains).
- HPV causes 5 percent of all human cancers, and is the majority cause of certain types of cancer. Over 97 percent of all cervical cancers, over 80 percent of anal cancers and over 65 percent of oropharyngeal (tonsil and base of tongue) cancers are caused by HPV. It can also cause penile, anal, vulvar and vaginal cancers.
- Infection with a high-risk strain ≠ cancer. 99 percent of all infected individuals clear the virus through their immune system, but one percent of uncleared infections persist and can develop into cancer over several years/decades.
- High-risk strains can cause cancer by inducing uncontrolled cell growth. Cancer-causing strains of HPV encode for two proteins, E6 and E7, which inactivate the human proteins p53 and Rb, respectively. p53 and Rb are both known as tumor suppressor proteins, meaning they normally act to stop a cell from growing when it shouldn’t. Inactivation of both tumor suppressors means the cell continues to divide uncontrollably and produce lots of new HPV virus particles – but that growth is also a key hallmark of cancer. [All HPVs encode for E6 and E7, but the benign strains only weakly, if at all, inactivate the tumor suppressor proteins.]
- Like many other viruses, a vaccine for HPV exists. Infectious viral particles encase their genetic material in a protein shell, and the shell attaches to human cells to start the infection process. Immunizing an individual against just the shell is sufficient to have their immune system recognize and prevent the virus from attaching to cells. A key discovery leading to the vaccines was that simply expressing the shell proteins at high enough concentrations leads to intact, self-assembled shells that can be recognized by the human immune system and lead to the production of protective, virus-fighting antibodies. The vaccine itself is only protein – no viral DNA included and, therefore, there is no way for the vaccine to cause an HPV infection.
- Three HPV vaccines are FDA-approved. They range from protecting against only the two most common cancer-associated strains to all nine cancer- and genital-wart associated strains.
- Vaccination is most effective if given well before sexual activity commences in children and teens, but is still recommended in sexually active adults into their 20s.
- The vaccine does not work-post infection
- Recent evidence suggests that deep kissing with exchange of saliva, and not only genital contact, can spread the virus
- 70% of all non-vaccinated, sexually active Americans will be infected with a sexually-transmitted strain of HPV at some point (even though most infections are cleared)
I’ll leave with this final note from Dr. Lambert:
“There aren’t many things you can do, besides not smoking, that will cut your risk or your children’s risk of getting cancer more than being vaccinated against HPV. Get vaccinated! It’s a no brainer.”