Faithful maintenance of the genome is crucial to the individual and to species. DNA damage arises from both endogenous sources such as water and oxygen and exogenous sources such as sunlight and tobacco smoke. In human cells, base alterations are generally removed by excision repair pathways that counteract the mutagenic effects of DNA lesions. This serves to maintain the integrity of the genetic information, although not all of the pathways are absolutely error-free. In some cases, DNA damage is not repaired but is instead bypassed by specialized DNA polymerases.

https://science.sciencemag.org/content/sci/286/5446/1897.full.pdf

Quality control by DNA repair.

Xeroderma pigmentosum variant (XP-V) is an inherited disorder which is associated with increased incidence of sunlight-induced skin cancers. Unlike other xeroderma pigmentosum cells (belonging to groups XP-A to XP-G), XP-V cells carry out normal nucleotide-excision repair processes but are defective in their replication of ultraviolet-damaged DNA. It has been suspected for some time that the XPV gene encodes a protein that is involved in trans-lesion DNA synthesis, but the gene product has never been isolated. Using an improved cell-free assay for trans-lesion DNA synthesis, we have recently isolated a DNA polymerase from HeLa cells that continues replication on damaged DNA by bypassing ultraviolet-induced thymine dimers in XP-V cell extracts. Here we show that this polymerase is a human homologue of the yeast Rad30 protein, recently identified as DNA polymerase eta. This polymerase and yeast Rad30 are members of a family of damage-bypass replication proteins which comprises the Escherichia coli proteins UmuC and DinB and the yeast Rev1 protein. We found that all XP-V cells examined carry mutations in their DNA polymerase eta gene. Recombinant human DNA polymerase eta corrects the inability of XP-V cell extracts to carry out DNA replication by bypassing thymine dimers on damaged DNA. Together, these results indicate that DNA polymerase eta could be the XPV gene product.

The XPV (xeroderma pigmentosum variant) gene encodes human DNA polymerase eta.

• Quantitative frequencies of clinical abnormalities in xeroderma pigmentosum were estimated by abstracting published descriptions of 830 patients in 297 articles obtained from a survey of the medical literature from 1874 to 1982. The median patient age was 12 years with nearly equal numbers of male and female patients. Cutaneous symptoms (sun sensitivity or freckling) had a median age of onset of between 1 and 2 years. Forty-five percent of the patients described had basal cell carcinoma or squamous cell carcinoma of the skin. The median age of first nonmelanoma skin cancer among patients with xeroderma pigmentosum was 8 years, more than 50 years less than that among patients with skin cancer in the United States. Melanomas were reported in 5% of patients. Ninety-seven percent of the reported basal and squamous cell carcinomas and 65% of the melanomas in patients with xeroderma pigmentosum occurred on the face, head, or neck. Seventy percent probability of survival was attained at age 40 years, a 28-year reduction in comparison with the US general population. Ocular abnormalities were reported in 40% of the patients described and were restricted to tissues exposed to ultraviolet radiation (lid, conjunctiva, and cornea) and included ectropion, corneal opacity leading to blindness, and neoplasms. Neurologic abnormalities were found in 18% of the cases reported, consisting of progressive mental deterioration, hyporeflexia or areflexia, and progressive deafness in some patients in association with dwarfism and immature sexual development. There was scant information concerning the efficacy of any therapeutic regimen. (Arch Dermatol1987;123:241-250)

https://jamanetwork.com/HttpHandlers/ArticlePdfHandler.ashx?journal=derm&articleId=548003&pdfFileName=archderm_123_2_026.pdf

Xeroderma Pigmentosum: Cutaneous, Ocular, and Neurologic Abnormalities in 830 Published Cases

The primary immunodeficiency disorders reflect abnormalities in the development and maturation of cells of the immune system. These defects result in an increased susceptibility to infection; recurrent pyogenic infections occur with defects of humoral immunity, and opportunistic infections with defects of cell-mediated immunity. These two broad categories of illness correspond roughly to defects in the two principal types of immunocompetent cells, B lymphocytes and T lymphocytes. Defective development of B cells results in abnormalities in humoral immunity, whereas defects in the development of T cells cause problems with cellular immunity. When pathogens are taken up by macrophages or dendritic cells, . . .

The Primary Immunodeficiencies

▪ Abstract This review focuses on eukaryotic translesion synthesis (TLS) DNA polymerases, and the emphasis is on Saccharomyces cerevisiae and human Y-family polymerases (Pols) η, ι, κ, and Rev1, as well as on Polζ, which is a member of the B-family polymerases. The fidelity, mismatch extension ability, and lesion bypass efficiencies of these different polymerases are examined and evaluated in the context of their structures.One major conclusion is that, despite the overall similarity of basic structural features among the Y-family polymerases, there is a high degree of specificity in their lesion bypass properties. Some are able to bypass a particular DNA lesion, whereas others are efficient at only the insertion step or the extension step of lesion bypass. This functional divergence is related to the differences in their structures. Polζ is a highly specialized polymerase specifically adapted for extending primer termini opposite from a diverse array of DNA lesions, and depending upon the DNA lesion, it ...

Eukaryotic translesion synthesis DNA polymerases: Specificity of structure and function

Cells cultured from most patients suffering from the sunlight-sensitive hereditary disorder xeroderma pigmentosum are defective in the ability to excise ultraviolet light (UV)-induced pyrimidine dimers from their DNA. There is, however, one class of these patients whose cells are completely normal in this excision repair process. We have found that these cells have an abnormality in the manner in which DNA is synthesized after UV-irradiation. The time taken to convert initially low-molecular-weight DNA synthesized in UV-irradiated cells into high-molecular-weight DNA similar in size to that in untreated cells is much greater in these variants than in normal cells. Furthermore, this slow conversion of low to high-molecular-weight newly synthesized DNA is drastically inhibited by caffeine, which has no effect in normal cells. Two cell lines from classes of xeroderma pigmentosum that are defective in excision-repair show intermediate effects, with regard to both the time taken to convert newly synthesized DNA to high molecular weight and the inhibition of this process by caffeine.

https://www.pnas.org/content/pnas/72/1/219.full.pdf

Xeroderma pigmentosum cells with normal levels of excision repair have a defect in DNA synthesis after UV-irradiation.

ATAXIA telangiectasia (AT) (Louis–Bar syndrome) is a rare human neurovascular disease displaying an autosomal recessive pattern of inheritance1–3. Affected individuals, although clinically normal at birth, symptomatically develop cerebellar ataxia (loss of muscular coordination) and oculocutaneous telangiectasis (chronic dilation of the small blood vessels) in early childhood. The course of the disease follows a variable progression commonly leading to total neurological incapacitation before puberty. Accessory complications include lymphoreticular neoplasia2,3, bronchiectasis1,2, recurrent sinopulmonary infections1,2, decreased levels of serum immunoglobulins IgA and IgE (refs 3 and 4), impaired cellular immunity3,4, and widespread chromosomal instability5. AT patients, on receiving conventional radiotherapy for tumour treatment, tend to develop unusually severe complications often culminating in premature death6–8. Pronounced radiosenitivity is also observed at the cellular level in laboratory studies; the number of radiation-induced chromosomal aberrations is enhanced in leukocytes obtained from AT donors9. Moreover, diploid fibroblasts cultured from affected individuals exhibit a reduced ability to form colonies following exposure to γ radiation10 and radiomimetic chemicals11. Since the principal damage induced by both types of agents occurs in the DNA and seems to be acted on by the same enzymatic repair mechanisms12,13, it would seem probable that the molecular basis for the clinical radiosensitivity of AT patients stems from a deficient DNA repair mechanism. We therefore measured the DNA repair properties of AT fibroblasts after exposure to γ radiation. Data presented below provide direct biochemical evidence that diploid strains from AT donors are indeed impaired in DNA repair; in particular, these cell lines possess an enzymatic defect in an excision-type repair process operating on γ-modified nitrogenous base residues.

Defective excision repair of γ-ray-damaged DNA in human (ataxia telangiectasia) fibroblasts

A sensitive enzymatic assay has been developed to follow the progress of NDA repair in human cells exposed to UV radiation. The assay employs an endonuclease selectively active at sites containing pyrimidine dimers in UV-damaged DNA. Primary fibroblasts are exposed to 254 nm radiation and incubated for specified times, their radioactivity labelled DNA is isolated and treated with a UV endonuclease extensively purified from Micrococcus luteus. Endonuclease-susceptible site remaining in the DNA are subsequently observed as single-strand scissions by sedimentation in alkaline sucrose gradients. In comparison to the situation with excision-proficient normal cells, those derived from patients suffering from either the classical or the De Sanctis-Cacchione clinical form of Xeroderma pigmentosum (XP) exhibit a marked diminution in the rate of disappearance of nuclease-susceptible lesions with time of post-UV incubation.

Use of a UV endonuclease from Micrococcus luteus to monitor the progress of DNA repair in UV-irradiated human cells

A unique feature of ataxia telangiectasia (AT), a hereditary multisystem disease, is extreme sensitivity to ionizing radiation, observed both clinically and in cell culture. Hence, we have measured the DNA repair capabilities of ten diploid fibroblast strains derived from unrelated AT donors, following anoxic 60Co γ-irradiation. Compared to two control strains, six of the ten mutant strains are markedly deficient in γ-induced repair replication. Two defective strains were defined further. While capable of rejoining single-strand breaks normally, both are impaired in the initial incision step in excision repair of base defects, assayed as γ-modified sites sensitive to a Micrococcus luteus endonuclease activity. Cell fusion studies assign three repair-deficient strains to two complementation groups; this result, coupled with a normal repair-replication ability in four of the ten AT strains, indicates genetic heterogeneity in the disease. AT strains appear otherwise normal, including their ability to repair UV damage. Aside from providing molecular insight into this complex disorder, our findings characterize AT as a γ-ray analogue of the UV-sensitive skin disease, xeroderma pigmentosum. Moreover, since AT patients are cancer-prone, faulty DNA repair is implicated in neoplastic transformation. Finally, given that (i) impaired embryonic differentiation best explains the clinical features of AT and (ii) defective DNA repair is of etiological relevance, we are led to conclude that DNA damage can lead to congenital malformations; thus, enzymatic DNA repair processes play a vital role in normal neonatal development.

Ataxia Telangiectasia: An Inherited Human Disease Involving Radiosensitivity, Malignancy and Defective DNA Repair

Ataxia telangiectasia (AT) is a rare autosomal recessive disorder in man characterized by progressive loss of muscular coordination (due to neurodegeneration) and permanent dilation of the small blood vessels of the eyes and skin1,2. AT patients also have repeated sinopulmonary infections, immune defects associated with thymus underdevelopment, and abnormal endocrine functions1,2. Affected patients are at high risk of developing malignancy, particularly lymphomas and lymphatic leukaemias1, and there are clinical indications3–5 that AT patients are hypersensitive to conventional radiotherapy administered for treatment of malignancy. Cultured diploid fibroblasts from AT donors are consistently hypersensitive to ionizing radiation2,6,7, apparently due to defective enzymatic repair of radiogenic DNA damage2,8. We have determined the survival responses and DNA repair abilities of AT cells exposed to 4-nitroquinoline-1-oxide (4NQO), a chemical carcinogen whose DNA-damaging properties partially mimic those of ionizing radiation9. We report here that certain AT cell strains show hypersensitivity to inactivation by 4NQO and defective repair of 4NQO-induced adducts in DNA.

M. C. Paterson

Papers

Xeroderma pigmentosum cells with normal levels of excision repair have a defect in DNA synthesis after UV-irradiation.

Defective excision repair of γ-ray-damaged DNA in human (ataxia telangiectasia) fibroblasts

Use of a UV endonuclease from Micrococcus luteus to monitor the progress of DNA repair in UV-irradiated human cells

Ataxia Telangiectasia: An Inherited Human Disease Involving Radiosensitivity, Malignancy and Defective DNA Repair

Defective DNA repair and increased lethality in ataxia telangiectasia cells exposed to 4-nitroquinoline-1-oxide