Richard B. Setlow

Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know

High doses of ionizing radiation clearly produce deleterious consequences in humans, including, but not exclusively, cancer induction. At very low radiation doses the situation is much less clear, but the risks of low-dose radiation are of societal importance in relation to issues as varied as screening tests for cancer, the future of nuclear power, occupational radiation exposure, frequent-flyer risks, manned space exploration, and radiological terrorism. We review the difficulties involved in quantifying the risks of low-dose radiation and address two specific questions. First, what is the lowest dose of x- or γ-radiation for which good evidence exists of increased cancer risks in humans? The epidemiological data suggest that it is ≈10–50 mSv for an acute exposure and ≈50–100 mSv for a protracted exposure. Second, what is the most appropriate way to extrapolate such cancer risk estimates to still lower doses? Given that it is supported by experimentally grounded, quantifiable, biophysical arguments, a linear extrapolation of cancer risks from intermediate to very low doses currently appears to be the most appropriate methodology. This linearity assumption is not necessarily the most conservative approach, and it is likely that it will result in an underestimate of some radiation-induced cancer risks and an overestimate of others.

Quantitation of radiation-, chemical-, or enzyme-induced single strand breaks in nonradioactive DNA by alkaline gel electrophoresis: application to pyrimidine dimers

We have developed an alkaline agarose gel method for quantitating single strand breaks in nanogram quantities of nonradioactive DNA. After electrophoresis together with molecular length standards, the DNA is neutralized, stained with ethidium bromide, photographed, and the density profiles recorded with a computer controlled scanner. The median lengths, number average molecular lengths, and length average molecular lengths of the DNAs can be computed by using the mobilities of the molecular length standards. The frequency of single strand breaks can then be determined by comparison of the corresponding average molecular lengths of DNAs treated and not treated with single strand break-inducing agents (radiation, chemicals, or lesion-specific endonuclease). Single strand break yields (induced at pyrimidine dimer sites in uv-irradiated human fibroblasts DNA by the dimer-specific endonuclease from Micrococcus luteus) from our method agree with values obtained for the same DNAs from alkaline sucrose gradient analysis. The method has been used to determine pyrimidine dimer yields in DNA from biopsies of human skin irradiated in situ. It will be especially useful in determining the frequency of single strand breaks (or lesions convertible to single strand breaks by specific cleaving reagents or enzymes) in small quantities of DNA from cells or tissues not amenable to radioactive labeling.

Epidemiological support for an hypothesis for melanoma induction indicating a role for UVA radiation

Abstract. An hypothesis for melanoma induction is presented: UV radiation absorbed by melanin in melanocytes generates products that may activate the carcinogenic process. Products formed by UV absorption in the upper layers of the epidermis cannot diffuse down as far as to the melanocytes. Thus, melanin in the upper layer of the skin may be protective, while that in melanocytes may be pho‐tocarcinogenic. Observations that support this hypothesis include: (1) Africans with dark skin have a reduced risk of getting all types of skin cancer as compared with Caucasians, but the ratio of their incidence rates of cutaneous malignant melanoma to that of squamous cell carcinoma is larger than the corresponding ratio for Caucasians. (2) Albino Africans, as compared with normally pigmented Africans, seem to have a relatively small risk of getting cutaneous malignant melanomas compared to nonmela‐nomas. This is probably also true for albino and normally pigmented Caucasians. (3) Among sun‐sensitive, poorly tanning persons, frequent UV exposures are associated with increased risk of melanoma, wherease among sun‐resistant, well‐tanning persons, increased frequency of exposure is associated with decreased melanoma risk. (4) It is likely that UVA, being absorbed by melanin, might have a melanoma‐inducing effect. This is in agreement with some epidemiological investigations which indicate that sun‐screen lotions may not protect sufficiently against melanoma induction. The relative latitude gradient for UVA is much smaller than that for UVB. The same is true for the relative latitude gradient of cutaneous malignant melanoma as compared with squamous cell carcinoma and basal cell carcinoma. Under the assumption that the average slopes of the curves relating incidence rates with fluences of carcinogenic UV radiation are similar for melanomas and nonmelanomas, these facts are in agreement with the assumption that UVA plays a significant role in the induction of melano.

Addressing the health benefits and risks, involving vitamin D or skin cancer, of increased sun exposure

Solar radiation is the main cause of skin cancers. However, it also is a main source of vitamin D for humans. Because the optimal status of vitamin D protects against internal cancers and a number of other diseases, a controversy exists: Will increased sun exposure lead to net health benefits or risks? We calculated the relative yield of vitamin D photosynthesis as a function of latitude with a radiative transfer model and cylinder geometry for the human skin surface. The annual yield of vitamin D is 3.4 and 4.8 times larger below the equator than in the U.K. and Scandinavia, respectively. In populations with similar skin types, there are clear latitude gradients of all major forms of skin cancer, indicating a north–south gradient in real sun exposure. Surprisingly, the incidence rates of major internal cancers also increase from north to south. However, the survival prognosis also improves significantly from north to south. Reasons for these findings are discussed in view of the role of vitamin D. In Norway, melanoma rates increased by a factor of 6 from 1960 to 1990, while the prognosis improved in the same period. After 1990, melanoma rates have remained constant or even decreased in age groups <50 years, whereas the prognosis has not improved further. These data, together with those for internal cancers and the beneficial effects of an optimal vitamin D status, indicate that increased sun exposure may lead to improved cancer prognosis and, possibly, give more positive than adverse health effects.

AN ACTION SPECTRUM FOR CELL KILLING AND PYRIMIDINE DIMER FORMATION IN CHINESE HAMSTER V‐79 CELLS

We have determined action spectra for pyrimidine dimer formation and loss of colony‐forming ability in Chinese Hamster V‐79 cells and have found a very strong correlation between the two. These data are consistent with the notion that damage to DNA is the principle cause of cell death and that the most important type of damage is the pyrimidine dimer. While the shape of the V‐79 spectra mimics that of action spectra for bacteria. phage, and purified DNA, V‐79 cells are about twice as sensitive to radiation at long wavelengths, relative to the sensitivity at 265 nm. However, if the action spectra are normalized to 297 nm. a wavelength included in the solar spectrum, the two sets of action spectra would coincide at wavelengths relevant to human skin‐cancer. Thus an action spectrum based on microorganisms should be adequate for extrapolation to humans in terms of risk due to ozone depleteion.

Temporal changes in the incidence of malignant melanoma: explanation from action spectra.

The incidence of malignant cutaneous melanoma has been increasing for more than 50 years, and is now rising more rapidly than that of any other cancer. This increase is not explicable by changes in the physical environment, particularly by any observed increase in UVB radiation (290-320 nm). The distribution of melanomas on the body differs from the site distribution of nonmelanoma skin cancer (relatively many more melanomas occur on areas of the body not chronically exposed to sunlight, such as the back of the trunk in males, and the legs in females). This localization of melanoma, together with its epidemiology, suggest that a change in lifestyle has contributed to the fast-rising incidence in many countries. There is no convenient mammalian animal model for malignant melanoma. However, certain inter- and intra-specific hybrids of fish of the genus Xiphophorus are very sensitive to light-induced melanomas; we have used them to determine the wavelengths effective in melanoma induction. The action spectrum has a relatively very large component in the UVA region (320-400 nm) compared to human erythema. Hence, if the human and fish spectra were similar, the use of sunscreens that minimize erythema would have little effect in preventing the induction of melanoma, and if people using sunscreens expose themselves to sunlight for longer periods, they will be increasing dramatically their exposure to these melanoma-inducing wavelengths. Such considerations are sufficient to explain the rising incidence of malignant melanoma and its distribution on the body.

Abilities of extracts of human lymphocytes to remove O6-methylguanine from DNA.

O6 MeGua is a presumptive mutagenic and carcinogenic product in DNAs treated with methylating agents. The abilities of lymphocyte extracts from 34 apparently normal individuals to remove O6 MeGua from exogenous DNA have been measured. The activity in extracts is stable to freezing and so permits repeat determinations and hence high precision in the assays. The data on removal are consistent with the idea that the removal is accomplished by the transfer of a methyl group to a methyl-accepting protein and that the protein acts in a stoichiometric fashion. Extracts from lymphocytes stimulated with PHA show on the average more activity than from unstimulated ones, although some extracts show no increase as a result of PHA stimulation of cells. There are large variations in the abilities of human lymphocytes to remove O6 MeGua, but the differences are not correlated significantly with sex or age. Unstimulated lymphocytes show a bimodal distribution of removing activity, whereas stimulated ones show a predominant single peak of activity. Extracts of T lymphocytes are more proficient than those of B lymphocytes and of any other white cells. On the average the number of presumptive acceptor molecules per cell in unstimulated lymphocytes is between 14 000 and 110 000 and in stimulated lymphocytes between 40 000 and 140 000.

DNA repair and longevity in three species of cold-blooded vertebrates ☆

Abstract The “error catastrophe” mechanism of ageing proposes that senescence results from the progressive accumulation of unrepaired damage to DNA throughout the life span. Studies of the changes in DNA repair capability in ageing cells both in vivo and in vitro have given ambiguous results, but a clear relation has been demonstrated in mammals between the DNA repair capacity and potential longevity. We have found no difference in excision repair capacity in cultured cells from three species of cold-blooded vertebrates, the long-lived turtle, with a potential life span of 118 + yr, the rainbow trout, 8 yr, and Amazon molly, with 3 yr.

Photoreactivation of pyrimidine dimers in DNA from thyroid cells of the teleost, Poecilia formosa.

Abstract— We have developed and used a simple technique to estimate the quantity of pyrimidine dimers in unlabeled cellular DNA. DNA is extracted from cells, treated with an endonuclease specific for dimers, and its molecular weight estimated by its electrophoretic mobility on alkaline agarose slab gels. The technique is used to show that cells from thyroid tissue of the fish Poecilia formosa have photoreactivating activity towards dimers in the cellular DNA.

ACTION SPECTRA FOR KILLING NON-DIVIDING NORMAL HUMAN AND XERODERMA PIGMENTOSUM CELLS

Abstract— Non‐dividing human cells degenerate and eventually detach from a culture vessel surface when exposed to UV light. Action spectra for this kind of cell inactivation were determined using eight monochromatic wavelengths from 240 to 313 nm and both a normal DNA excision‐repair‐proficient strain and a repair‐deficient Xeroderma pigmentosum (XP12BE) strain. The action spectra for both strains have similar shapes with a broad peak between 254 and 280 nm followed by a steep decline at wavelengths greater than 280 nm. The relative action spectra are similar to those for inactivation of reproductive capacity and pyrimidine dimer formation in rodent cells suggesting that the critical target and critical damage for inactivation of non‐dividing human cells is DNA and damage to DNA, respectively. Normal repair‐proficient cells are 5–7 times more resistant at all wavelengths, based on a comparison of Do values, than repair‐deficient XP12BE cells, supporting the conclusion that the inactivating damage at all wavelengths is to DNA.