Insights into genotype-phenotype correlation in pachyonychia congenita from the human intermediate filament mutation database

Genotype-structurotype-phenotype correlations in pachyonychia congenita patients

Pachyonychia congenita (PC) is a genetic disorder of keratin that presents with nail dystrophy, painful palmoplantar keratoderma, and other clinical manifestations. We investigated genotype-structurotype-phenotype correlations seen with mutations in keratin genes (KRT6A, KRT6B, KRT6C, KRT16, KRT17) and utilized protein structure modeling of high frequency mutations to examine the functional importance of keratin structural domains in PC pathogenesis. Participants of the International PC Research Registry underwent genetic testing and completed a standardized survey on their symptoms. Our results support prior reports associating oral leukokeratosis with KRT6A mutations, and cutaneous cysts, follicular hyperkeratosis, and natal teeth with KRT17 mutations. Painful keratoderma was prominent with KRT6A and KRT16 mutations. Nail involvement was most common in KRT6A and least common in KRT6C patients. Across keratin subtypes, patients with coil 2B mutations had greatest impairment in ambulation, and patients with coil 1A mutations reported more emotional issues. Molecular modeling demonstrated that hotspot missense mutations in PC largely disrupted hydrophobic interactions or surface charge. The former may destabilize keratin dimers/tetramers, while the latter likely interferes with higher-order keratin filament formation. Understanding pathologic alterations in keratin structure improves our knowledge of how PC genotype correlates with clinical phenotype, advancing insight into disease pathogenesis and therapeutic development.

Identification of clinically useful predictive genetic variants in pachyonychia congenita

BACKGROUND

Pachyonychia congenita (PC) refers to a group of autosomal dominant disorders caused by mutations in five keratin genes (KRT16,KRT6A,KRT17,KRT6B or KRT6C). Current disease classification is based on the gene harbouring disease-causing variants.

AIMS

We harnessed the International Pachyonychia Congenita Research Registry (IPCRR) containing both clinical and molecular data on patients with PC worldwide, to identify genetic variants predicting disease severity.

METHODS

We ascertained 815 individuals harbouring keratin mutations registered in the IPCRR. We looked for statistically significant associations between genetic variants and clinical manifestations in a subgroup of patients carrying mutations found in at least 10% of the cohort. Data were analysed using χ² and Kruskal-Wallis tests.

RESULTS

We identified five mutations occurring in at least 10% of the patients registered in the IPCRR. The KRT16 p.L132P mutation was significantly associated with younger age of onset, presence of palmar keratoderma oral leucokeratosis and a higher number of involved nails. By contrast, the KRT16 p.N125S and p.R127C mutations resulted in a milder phenotype featuring a decreased number of involved nails and older age of onset. Patients carrying the p.N125S mutation were less likely to develop palmar keratoderma while p.R127C was associated with an older age of palmoplantar keratoderma onset. Moreover, the KRT17 p.L99P mutation resulted in an increased number of involved fingernails and patients demonstrating 20-nail dystrophy, while the opposite findings were observed with KRT17 p.N92S mutation.

CONCLUSIONS

We have identified novel and clinically useful genetic predictive variants in the largest cohort of patients with PC described to date.

Laryngeal manifestations of pachyonychia congenita: a clinical case and discussion on management for the otolaryngologist

BACKGROUND

Pachyonychia congenita is a rare keratinising disorder, which typically presents during the first three years of life and usually affects the nails and palmoplantar surfaces. It can involve the larynx and potentially result in life-threatening airway obstruction.

METHODS

A case report is presented and the findings of a literature review are reported. The review involved a PubMed search using the keywords 'pachyonychia congenita' together with 'larynx', 'laryngeal involvement', 'laryngeal obstruction', 'airway obstruction', 'hoarseness' and/or 'stridor'.

RESULTS

A five-year-old boy, with confirmed pachyonychia congenita, presented with complications of laryngeal involvement over a four-year period. He required three intubations and a tracheostomy for acute airway obstruction. Treatment with potassium titanyl phosphate laser laryngoscopy stabilised the progression of laryngeal disease.

CONCLUSION

Patients with pachyonychia congenita and laryngeal involvement can have a varied presentation, ranging from hoarseness to acute airway obstruction. Management can be a challenge, requiring early evaluation, regular surveillance and aggressive treatment. This paper reports our experience in managing and treating the laryngeal complications of a child with pachyonychia congenita.

Gene expression profiling in pachyonychia congenita skin

BACKGROUND

Pachyonychia congenita (PC) is a skin disorder resulting from mutations in keratin (K) proteins including K6a, K6b, K16, and K17. One of the major symptoms is painful plantar keratoderma. The pathogenic sequelae resulting from the keratin mutations remain unclear.

OBJECTIVE

To better understand PC pathogenesis.

METHODS

RNA profiling was performed on biopsies taken from PC-involved and uninvolved plantar skin of seven genotyped PC patients (two K6a, one K6b, three K16, and one K17) as well as from control volunteers. Protein profiling was generated from tape-stripping samples.

RESULTS

A comparison of PC-involved skin biopsies to adjacent uninvolved plantar skin identified 112 differentially-expressed mRNAs common to patient groups harboring K6 (i.e., both K6a and K6b) and K16 mutations. Among these mRNAs, 25 encode structural proteins including keratins, small proline-rich and late cornified envelope proteins, 20 are related to metabolism and 16 encode proteases, peptidases, and their inhibitors including kallikrein-related peptidases (KLKs), and serine protease inhibitors (SERPINs). mRNAs were also identified to be differentially expressed only in K6 (81) or K16 (141) patient samples. Furthermore, 13 mRNAs were identified that may be involved in pain including nociception and neuropathy. Protein profiling, comparing three K6a plantar tape-stripping samples to non-PC controls, showed changes in the PC corneocytes similar, but not identical, to the mRNA analysis.

CONCLUSION

Many differentially-expressed genes identified in PC-involved skin encode components critical for skin barrier homeostasis including keratinocyte proliferation, differentiation, cornification, and desquamation. The profiling data provide a foundation for unraveling the pathogenesis of PC and identifying targets for developing effective PC therapeutics.

Comparative study of high-resolution multifrequency ultrasound of the plantar skin in patients with various types of hereditary palmoplantar keratoderma

BACKGROUND

High-variable-frequency ultrasound is used as an imaging tool for various cutaneous disorders. We utilized this tool in pachyonychia congenita (PC) patients, who typically present with plantar hyperkeratosis and often severely debilitating pain, compared to patients with epidermolytic palmoplantar keratoderma (EPPK) and mal de Meleda (MDM).

OBJECTIVE

To ascertain the feasibility of ultrasound technology for the diagnosis of PC.

METHODS

The study included a total of 16 patients, 7 with PC, 5 with EPPK and 4 with MDM, who underwent ultrasound examination of the plantar skin with high-resolution multifrequency ultrasound equipment.

RESULTS

Ultrasound scans performed over the proximal and distal plantar foot calluses in PC patients demonstrated hyperechoic dots and lines within the epidermis compatible with hyperkeratosis, engorged varicose veins in the dermis and an anechoic layer interposed between the epidermis and the dermis, corresponding to blister fluid below the calluses. In contrast to PC patients, patients with MDM and EPPK demonstrated no blisters.

CONCLUSION

PC patients, as opposed to a group of patients with MDM and EPPK, displayed subepidermal blistering beneath their calluses. This finding may help in the diagnosis of PC and in partially explaining plantar pain as part of PC symptomatology.

Best treatment practices for pachyonychia congenita

BACKGROUND

Numerous therapeutic modalities have been proposed to treat the manifestations of pachyonychia congenita (PC). While research hopes lie with molecular therapies, patients are in need of answers regarding the efficacy of conventional treatments.

AIM OF THE STUDY

To determine patients' experience and preferences regarding conventional treatments for PC.

METHODS

The study population included 120 PC patients from 20 countries. The study was based on a patient survey developed by physicians and researchers from the International Pachyonychia Congenita Consortium and conducted via the internet. Using an effectiveness scale of 1 to 5, the patients were asked to grade treatments for different manifestations, including keratoderma, cysts, follicular hyperkeratosis, fingernail and toenail involvement.

RESULTS

Patients reported surgical treatments being most effective for cysts and mechanical treatments the most effective conventional therapeutic approach for all other investigated manifestations. The other conventional medical treatments were found to be non-effective to only slightly effective. Among patients with keratoderma, older people were more likely to report beneficial effect from mechanical treatments (P = 0.04), topical retinoids (P = 0.04) and topical steroids (P = 0.02). Likewise, females were more inclined to report filing and grinding beneficial than males (P = 0.02). Finally, carriers of KRT16 and KRT6a were more likely to benefit from keratolytics than carriers of mutations in KRT17 (P = 0.04).

CONCLUSIONS

None of the currently available therapeutic options for PC are ideal, although they provide some relief, with mechanical/surgical options being preferred over medical therapies. These results emphasize the need for more efficient and targeted therapies.

Transgrediens pachyonychia congenita (PC): case series of a nonclassical PC presentation

BACKGROUND

Pachyonychia congenita (PC) is a rare keratin disorder that typically presents with nail dystrophy and focal plantar keratoderma. We present seven cases of PC with transgrediens involvement of the dorsal feet.

OBJECTIVES

To document the extension of their disease to the dorsum of the feet in patients with mutation-confirmed PC, to report the natural history of PC with such transgrediens involvement, to generate hypotheses regarding aetiology, and to suggest prevention and treatment modalities.

METHODS

Genetically confirmed cases of PC with transgrediens foot involvement were verified through the International Pachyonychia Congenita Research Registry (IPCRR) and characterized via telephone survey and photography.

RESULTS

Seven patients with PC in the IPCRR were confirmed to have transgrediens lesions on the dorsal feet (six KRT6A mutations; one KRT16 mutation). Six cases had pre-existing nontransgrediens keratoderma and all cases reported standing, wearing shoes, foot moisture, and/or infection as exacerbating or predisposing factors. Improvement, reported in six cases, was attributed to use of antibiotics or gentian violet, or improved footwear.

CONCLUSIONS

Transgrediens involvement of the dorsal feet is a rare manifestation of mutation-confirmed PC and may be more common in patients who carry a KRT6A mutation. Trauma, friction, infection and wound healing may exacerbate or predispose toward transgrediens lesions. It remains to be proven whether transgrediens-associated infection is causal or represents a primary or secondary process. Patients with PC who develop transgrediens lesions may benefit from fungal and bacterial cultures, followed by appropriate antimicrobial treatments. Efforts to decrease skin friction and moisture may also improve and/or prevent transgrediens spread.

Alexander disease causing mutations in the C-terminal domain of GFAP are deleterious both to assembly and network formation with the potential to both activate caspase 3 and decrease cell viability

Alexander disease is a primary genetic disorder of astrocyte caused by dominant mutations in the astrocyte-specific intermediate filament glial fibrillary acidic protein (GFAP). While most of the disease-causing mutations described to date have been found in the conserved α-helical rod domain, some mutations are found in the C-terminal non-α-helical tail domain. Here, we compare five different mutations (N386I, S393I, S398F, S398Y and D417M14X) located in the C-terminal domain of GFAP on filament assembly properties in vitro and in transiently transfected cultured cells. All the mutations disrupted in vitro filament assembly. The mutations also affected the solubility and promoted filament aggregation of GFAP in transiently transfected MCF7, SW13 and U343MG cells. This correlated with the activation of the p38 stress-activated protein kinase and an increased association with the small heat shock protein (sHSP) chaperone, αB-crystallin. Of the mutants studied, D417M14X GFAP caused the most significant effects both upon filament assembly in vitro and in transiently transfected cells. This mutant also caused extensive filament aggregation coinciding with the sequestration of αB-crystallin and HSP27 as well as inhibition of the proteosome and activation of p38 kinase. Associated with these changes were an activation of caspase 3 and a significant decrease in astrocyte viability. We conclude that some mutations in the C-terminus of GFAP correlate with caspase 3 cleavage and the loss of cell viability, suggesting that these could be contributory factors in the development of Alexander disease.

The phenotypic and molecular genetic features of pachyonychia congenita

Pachyonychia congenita (PC) is an autosomal dominant genodermatosis caused by heterozygous mutations in any one of the genes encoding the differentiation-specific keratins K6a, K6b, K16, or K17. The main clinical features of the condition include painful and highly debilitating plantar keratoderma, hypertrophic nail dystrophy, oral leukokeratosis, and a variety of epidermal cysts. Although the condition has previously been subdivided into PC-1 and PC-2 subtypes, the phenotypic characterization of 1,000 mutation-verified PC patients enrolled in the International PC Research Registry, coordinated by the patient advocacy group PC Project, shows that there is considerable overlap between these subtypes. Thus, a new genotypic nomenclature is proposed, in which PC-6a represents a patient carrying a mutation in the K6a gene, etc. Although a rare disorder, PC represents a good model for therapy development, and international efforts are ongoing to develop and deliver siRNA, gene, correction, small molecule, and other strategies to treat this painful, disabling skin condition. The special relationship between PC Project and the PC research community has greatly accelerated the development pathway from gene identification to clinical trials in only a few years and represents a paradigm of hope for other orphan diseases.

Keratin gene mutations in disorders of human skin and its appendages

Keratins, the major structural protein of all epithelia are a diverse group of cytoskeletal scaffolding proteins that form intermediate filament networks, providing structural support to keratinocytes that maintain the integrity of the skin. Expression of keratin genes is usually regulated by differentiation of the epidermal cells within the stratifying squamous epithelium. Amongst the 54 known functional keratin genes in humans, about 22 different genes including, the cornea, hair and hair follicle-specific keratins have been implicated in a wide range of hereditary diseases. The exact phenotype of each disease usually reflects the spatial expression level and the types of mutated keratin genes, the location of the mutations and their consequences at sub-cellular levels as well as other epigenetic and/or environmental factors. The identification of specific pathogenic mutations in keratin disorders formed the basis of our understanding that led to re-classification, improved diagnosis with prognostic implications, prenatal testing and genetic counseling in severe keratin genodermatoses. Molecular defects in cutaneous keratin genes encoding for keratin intermediate filaments (KIFs) causes keratinocytes and tissue-specific fragility, accounting for a large number of genetic disorders in human skin and its appendages. These diseases are characterized by keratinocytes fragility (cytolysis), intra-epidermal blistering, hyperkeratosis, and keratin filament aggregation in severely affected tissues. Examples include epidermolysis bullosa simplex (EBS; K5, K14), keratinopathic ichthyosis (KPI; K1, K2, K10) i.e. epidermolytic ichthyosis (EI; K1, K10) and ichthyosis bullosa of Siemens (IBS; K2), pachyonychia congenita (PC; K6a, K6b, K16, K17), epidermolytic palmo-plantar keratoderma (EPPK; K9, (K1)), monilethrix (K81, K83, K86), ectodermal dysplasia (ED; K85) and steatocystoma multiplex. These keratins also have been identified to have roles in apoptosis, cell proliferation, wound healing, tissue polarity and remodeling. This review summarizes and discusses the clinical, ultrastructural, molecular genetics and biochemical characteristics of a broad spectrum of keratin-related genodermatoses, with special clinical emphasis on EBS, EI and PC. We also highlight current and emerging model tools for prognostic future therapies. Hopefully, disease modeling and in-depth understanding of the molecular pathogenesis of the diseases may lead to the development of novel therapies for several hereditary cutaneous diseases.

Single-Nucleotide-Specific siRNA Targeting in a Dominant-Negative Skin Model

RNA interference offers a novel approach for developing therapeutics for dominant-negative genetic disorders. The ability to inhibit expression of the mutant allele without affecting wild-type gene expression could be a powerful new treatment option. Targeting the single-nucleotide keratin 6a (K6a) N171K mutation responsible for the rare monogenic skin disorder pachyonychia congenita (PC), we demonstrate that small interfering RNAs (siRNAs) can potently and selectively block expression of mutant K6a. To test whether lead siRNAs could discriminate mutant mRNA in the presence of both wild-type and mutant forms, a dominant-negative PC cell culture model was developed. As predicted for a dominant-negative disease, simultaneous expression of both wild-type and mutant K6a resulted in defective keratin filament formation. Addition of mutant-specific siRNAs allowed normal filament formation, suggesting selective inhibition of mutant K6a. The effectiveness of our siRNA in skin was tested by co-delivering a firefly luciferase/mutant K6a bicistronic reporter construct and mutant-specific siRNAs to mouse footpads. Potent inhibition of the fluorescent reporter was demonstrated using the Xenogen IVIS200 in vivo imaging system. Additionally, wild type-specific siRNAs knocked down the expression of pre-existing endogenous K6a in human keratinocytes. These results suggest that efficient delivery of these "designer siRNAs" may allow effective treatment of numerous genetic disorders including PC.

A spectrum of mutations in keratins K6a, K16 and K17 causing pachyonychia congenita

BACKGROUND

Pachyonychia congenita (PC) is a rare autosomal dominant keratin disorder, subdivided into two major variants, PC-1 and PC-2. Predominant characteristics include hypertrophic nail dystrophy, focal palmoplantar keratoderma and oral leukokeratosis. Multiple steatocystomas that develop during puberty are a useful feature distinguishing PC-2 from PC-1. At the molecular level it has been shown that mutations in keratin K6a or K16 cause PC-1 whereas those in K6b or K17 lead to PC-2.

OBJECTIVE

To identify mutations in 22 families presenting with clinical symptoms of either PC-1/focal non-epidermolytic palmoplantar keratoderma (FNEPPK) or PC-2.

METHODS

Mutation analysis was performed on genomic DNA from PC patients by direct sequencing.

RESULTS

Here, we report four new missense and five known mutations in K6a; one new deletion and three previously identified missense mutations in K16; plus one known mutation in K17.

CONCLUSION

With one exception, all these heterozygous mutations are within the highly conserved helix boundary motif regions at either end of the keratin rod domain. In one sporadic case, a unique mutation in K16 resulting in deletion of 24bp was found within the central rod domain, in a child with a phenotype predominantly consisting of focal plantar keratoderma. The identification of mutations in cases of PC is prerequisite for future development of gene-specific and/or mutation-specific therapies.

Diseases of epidermal keratins and their linker proteins

Epidermal keratins, a diverse group of structural proteins, form intermediate filament networks responsible for the structural integrity of keratinocytes. The networks extend from the nucleus of the epidermal cells to the plasma membrane where the keratins attach to linker proteins which are part of desmosomal and hemidesmosomal attachment complexes. The expression of specific keratin genes is regulated by differentiation of the epidermal cells within the stratifying squamous epithelium. Progress in molecular characterization of the epidermal keratins and their linker proteins has formed the basis to identify mutations which are associated with distinct cutaneous manifestations in patients with genodermatoses. The precise phenotype of each disease apparently reflects the spatial level of expression of the mutated genes, as well as the types and positions of the mutations and their consequences at mRNA and protein levels. Identification of specific mutations in keratinization disorders has provided the basis for improved diagnosis and subclassification with prognostic implications and has formed the platform for prenatal testing and preimplantation genetic diagnosis. Finally, precise knowledge of the mutations is a prerequisite for development of gene therapy approaches to counteract, and potentially cure, these often devastating and currently intractable diseases.

The Alexander disease-causing glial fibrillary acidic protein mutant, R416W, accumulates into Rosenthal fibers by a pathway that involves filament aggregation and the association of alpha B-crystallin and HSP27

Here, we describe the early events in the disease pathogenesis of Alexander disease. This is a rare and usually fatal neurodegenerative disorder whose pathological hallmark is the abundance of protein aggregates in astrocytes. These aggregates, termed "Rosenthal fibers," contain the protein chaperones alpha B-crystallin and HSP27 as well as glial fibrillary acidic protein (GFAP), an intermediate filament (IF) protein found almost exclusively in astrocytes. Heterozygous, missense GFAP mutations that usually arise spontaneously during spermatogenesis have recently been found in the majority of patients with Alexander disease. In this study, we show that one of the more frequently observed mutations, R416W, significantly perturbs in vitro filament assembly. The filamentous structures formed resemble assembly intermediates but aggregate more strongly. Consistent with the heterozygosity of the mutation, this effect is dominant over wild-type GFAP in coassembly experiments. Transient transfection studies demonstrate that R416W GFAP induces the formation of GFAP-containing cytoplasmic aggregates in a wide range of different cell types, including astrocytes. The aggregates have several important features in common with Rosenthal fibers, including the association of alpha B-crystallin and HSP27. This association occurs simultaneously with the formation of protein aggregates containing R416W GFAP and is also specific, since HSP70 does not partition with them. Monoclonal antibodies specific for R416W GFAP reveal, for the first time for any IF-based disease, the presence of the mutant protein in the characteristic histopathological feature of the disease, namely Rosenthal fibers. Collectively, these data confirm that the effects of the R416W GFAP are dominant, changing the assembly process in a way that encourages aberrant filament-filament interactions that then lead to protein aggregation and chaperone sequestration as early events in Alexander disease.

Loss-of-function mutations in the keratin 5 gene lead to Dowling-Degos disease

Dowling-Degos disease (DDD) is an autosomal dominant genodermatosis characterized by progressive and disfiguring reticulate hyperpigmentation of the flexures. We performed a genomewide linkage analysis of two German families and mapped DDD to chromosome 12q, with a total LOD score of 4.42 ( theta =0.0) for marker D12S368. This region includes the keratin gene cluster, which we screened for mutations. We identified loss-of-function mutations in the keratin 5 gene (KRT5) in all affected family members and in six unrelated patients with DDD. These represent the first identified mutations that lead to haploinsufficiency in a keratin gene. The identification of loss-of-function mutations, along with the results from additional functional studies, suggest a crucial role for keratins in the organization of cell adhesion, melanosome uptake, organelle transport, and nuclear anchorage.

The genetic basis of pachyonychia congenita

In 1994, the molecular basis of pachyonychia congenita (PC) was elucidated. Four keratin genes are associated with the major subtypes of PC: K6a or K16 defects cause PC-1; and mutations in K6b or K17 cause PC-2. Mutations in keratins, the epithelial-specific intermediate filament proteins, result in aberrant cytoskeletal networks which present clinically as a variety of epithelial fragility phenotypes. To date, mutations in 20 keratin genes are associated with human disorders. Here, we review the genetic basis of PC and report 30 new PC mutations. Of these, 25 mutations were found in PC-1 families and five mutations were identified in PC-2 kindreds. All mutations identified were heterozygous amino acid substitutions or small in-frame deletion mutations with the exception of an unusual mutation in a sporadic case of PC-1. The latter carried a 117 bp duplication resulting in a 39 amino acid insertion in the 2B domain of K6a. Also of note was mutation L388P in K17, which is the first genetic defect identified in the helix termination motif of this protein. Understanding the genetic basis of these disorders allows better counseling for patients and paves the way for therapy development.

Clinical and Pathological Features of Pachyonychia Congenita

Pachyonychia congenita (PC) is a rare genodermatosis affecting the nails, skin, oral mucosae, larynx, hair, and teeth. Pathogenic mutations in keratins K6a or K16 are associated with the PC-1 phenotype whereas K6b and K17 mutations are associated with the PC-2 phenotype. Analysis of clinical, pathological, and genetic data from the literature and two research registries reveal that >97% of PC cases exhibit fingernail and toenail thickening, and painful plantar keratoderma. Prospective evaluation of 57 PC patients from 41 families revealed variable clinical findings: hyperhidrosis (79%), oral leukokeratosis (75%), follicular keratosis (65%), palmar keratoderma (60%), cutaneous cysts (35%), hoarseness or laryngeal involvement (16%), coarse or twisted hair (26%), early primary tooth loss (14%), and presence of natal or prenatal teeth (2%). Stratification of these data by keratin mutation confirmed the increased incidence of cyst formation and natal teeth among PC-2 patients, although cysts were more commonly seen in PC-1 than previously reported (25%-33%). Previously unreported clinical features of PC include development of painful oral and nipple lesions during breastfeeding, copious production of waxy material in ears, and inability to walk without an ambulatory aid (50%). Possible pathogenic mechanisms are discussed with respect to the clinicopathologic and genetic correlations observed.

Psoriasis, PUVA, and skin cancer--molecular epidemiology: the curious question of T-->A transversions

Photochemotherapy with 8-methoxypsoralen and long wavelength ultraviolet radiation (PUVA) is commonly used to treat psoriasis and vitiligo. These vastly different diseases respond to the therapy by different mechanisms even though the immediate effects of the therapy - photoadduct formation - is the same for both. Because psoriasis is not cured by PUVA, patients receive many treatments over their lifetime and develop a significant risk for the development of skin cancers (primarily squamous cell carcinomas). In this review the basic aspects of psoralen photobiology are reviewed briefly. In addition the impact of the analysis of mutations in the tumor suppressor gene, p53, are summarized. An unexpected mutation spectrum (very few T-->A transversions and frequent UVB signature C-->T transitions) suggest that effects other than direct DNA photoadduct formation may be at play. The roles of reactive oxygen species-induced base changes as well as other clastogenic factors are discussed. This analysis suggests that it may be possible to improve the therapeutic efficacy of PUVA by a careful evaluation of the mode of delivery.

DNA repair, DNA replication, and UV mutagenesis

Cells that have been irradiated with ultraviolet light (UV) suffer damage to their DNA, primarily in the form of covalent linkage between adjacent pyrimidines. Such photoproducts represent blocks to RNA and DNA polymerases and are potentially mutagenic. Blockage of RNA polymerase II by a photoproduct in the transcribed strand of an active gene leads to induction of the p53 protein, which induces pleiotropic responses that may include apoptotic cell death. If a cell survives, the blocked polymerase targets the nucleotide excision repair machinery to the site of the lesion, which is repaired in an error-free manner. Repair coupled to transcription in this manner strongly influences the mutation spectrum induced by UV, reducing the proportion of base substitutions that arise from photoproducts on the transcribed strand. If the damage persists when the DNA is replicated in S-phase, either because the cell is unable to repair the damage or because there is insufficient time between the induction of damage and the onset of S-phase. To do so, the replicative DNA polymerase complex may be blocked. In this situation, lesion bypass can be accomplished using an error-free mechanism, or using an error-prone mechanism that involves the newly described, non-processive DNA polymerase zeta encoded by the human homolog of the yeast REV3 gene.