1. Home>
2. Specialties>
3. Dermatology>
4. Summary and Comment

Do Sunscreens Prevent Thymine Dimer Function in Vivo?

Only unflagging sunscreen protection prevented markers of DNA damage.

The incidence of skin cancer continues to increase. The strong link between nonmelanoma skin cancer and UV light exposure is widely accepted, and UV light appears to be the principal environmental cause of most melanomas. Sunscreen efficacy has been challenged: some findings suggest that inadequate coverage can actually increase UV-related risks. Investigators in Boston studied whether daily application of sunscreen protects against UV-induced DNA damage as indicated by the formation of thymine dimers, incomplete repair of which may cause mutation.

On 4 consecutive days, 18 healthy women (mean age, 31; skin types I-IV) applied physical/chemical sunscreen (zinc oxide and octyl methoxycinnamate) at recommended levels (2 mg/cm²) before receiving 2 minimal erythema doses (MED) of UV light. Five skin sites on each subject were observed: Sunscreen was applied daily to 1 site and omitted before the second, third, or fourth irradiation at 3 other sites; an unirradiated control site received no sunscreen. Punch biopsy samples of exposed and control skin were labeled with antithymine dimer antibodies and compared. Epidermal thymine dimers were evident in irradiated skin when sunscreen was omitted even once, but no significant difference was seen between control and irradiated skin after daily sunscreen use. However, in 7 of 18 volunteers, low levels of dimers were detected at the always-protected site, and in 1 subject, levels at this site resembled levels at the intermittently protected sites. The authors ascribe this finding to faulty sunscreen application. Intermittent sunscreen use did not prevent dimer formation.

Comment: Although these results show the value of daily sunscreen for preventing cancer-associated thymine dimers in vivo, it is troubling that the benefit was not universal, particularly as recommended amounts of sunscreen were applied. Sunscreen as the primary form of photoprotection has come under scrutiny. Consumers apply as little as 25% of recommended amounts. One can only imagine what these results would have been had similarly inadequate protection been applied by the study subjects. These findings underscore the need for careful sunscreen use, but even more strongly emphasize the importance of sun avoidance and protective clothing.

— James Ferguson, MD

Published in Journal Watch Dermatology December 31, 2002

Citation(s):

Al Mahroos M et al. Effect of sunscreen application on UV-induced thymine dimers. Arch Dermatol 2002 Nov; 138:1480-5.

• Original article (Subscription may be required)
• Medline abstract (Free)