Xeroderma pigmentosum, XP, has become one the most important diseases in dermatology with major implication for a great deal of basic and clinical science associated with mechanisms of carcinogenesis. XP identifies as a rare recessive heterogeneous genetic disorder with an occurence rate of approx. 1 in 250,000 in the U.S. and 1 in 430,000 in Western Europe. The disorder entails defective UV-radiation damage repair, normally characterized by extreme photosensitivity, pigmentary alterations such as freckles, as well as evident eye damage and skin burning when the epidermal skin layer of the affected individual is exposed to minimal levels of ultraviolet (UV) radiation.
In some cases of XP, accelerated neurologic degeneration also occurs as a symptom due to increased neuronal death. XP is caused primarily by autosomal recessive genetic defects where by nucleotide excision repair (NER) enzymes are mutated, prompting a reduction in or destruction of NER. As more abnormalities form in our DNA, cells malfunction and eventually become cancerous or die. XP patients have more than a 10,000-fold increased risk of developing skin cancer.
XP is classified into seven complementation groups (XPA-XPG) that correspond to genetic alterations in one of seven genes involved in NER, the major mechanism able to repair UV-DNA damage in humans. For example, XPC, an XP gene, encodes a component of NER. It carries out a significant role in the premature steps of global genome NER (GG-NER), predominantly related to damage recognition and repair protein complex, XP-C, formation. Therefore, critical evaluation of how DNA damage can be repaired is required in order to examine how such diseases can be avoided or managed especially for patients who are diagnosed with the mutations caused by the inheritance of xeroderma pigmentosum disorders.
The human genome is frequently damaged and affected by an unending number of environmental aspects such as UV, ionizing agents and genotoxic chemicals such as benzene. UV-B and UV-C radiation develop two main types of photolesions created in DNA: pyrimidine (6-4), cyclobutane pyrimidine dimers (CPD) as well as pyrimidine photoproducts ((6-4) PP) (Cleaver, 1995: p.6154). These are the main types of premutagenic damage after sun contact. This indicates why individuals with genetic disorders in the NER course exhibit hypersensitivity to UV exposure. Defective repair also leads to genetic instability resulting in an increased number of chromosomal abnormalities (Menck & Munford, V., 2014; p.223). In the NER process, two subpathways exist: a quick transcription-coupled repair (TCR) pathway for the purpose of efficient elimination of lesions from the recorded thread of active genes which allows rapid continuation of the vital process of transcription and for some lesions a less effective global genome repair (GGR) subpathway exists that analyses the whole genome (Cleaver, 2000; p.4).The NER defect in the cells of most XP patients is located in the core of the NER mechanism and affects both transcription-coupled and global genome repair. In most cases, human mutations are a result of the existence of PTC (premature termination codons) which lead to the development of mutations in various genetic types. Basically, three types of genetic mutations exist which are caused by PTC, namely TGA, TAA, and TAG (Kuschal et al, 2013; p.19484). The existence of PTC decreases the quantity of mRNA and protein undergoing NMD (nonsense-mediated decay), an aspect which is responsible for the detection and degradation of the transcripts bearing the PTC (Sancar, 1994: p.1954). The process then leads to the prevention of the manifestation process of short proteins which can be nonfunctional and harmful in nature. NER-defective tissues from patients with XP are split into seven different groups that range from XP-A to XP-G. PTC genetic changes that are associated with the production of abnormal proteins, small protein amounts or no protein production at all due to NMD occur to 15% of the patients in the XP-C group (Khan et al, 2005; p.85). Moreover, human DNA is constantly damaged and this damage is normally associated with incidences of cancer at a rate ranging between 80 to 90% (Sancar, 1994; p.1954). One of the DNA repair mechanisms is known as molecular DNA repair (MDR). It is accepted that normal cells usually eliminate different DNA lesions that occur during MDR mechanism (Clancy, 2008; p.103). A study undertaken by Kelner on the different aspects of DNA discovered that microorganisms are normally protected against visible light, however with XP, adverse effects such as lesions known as pyrimidine dimers caused by UV exposure begin developing on the genetically deficient tissue. It was also discovered that the DNA repair process is known as photoreactivation, a process which is initiated by an enzyme called the photo-reactivating enzyme (Sancar, 1994: p.1954). This enzyme is perceived to convert UV lights into light chemical energy. Photolyase is considered non-essential in the survival of different organisms like humans which do not have the enzyme. Another method of DNA repair is known as the excision repair. This method was first discovered in 1964. This process is considered to be universal in the world of biology as it occurs in most of the free-living organisms including mycoplasmas (Sancar, 1994: p.1954). This process is associated with the existence of bacteria mutants which are considered to be defective during the process of excision DNA repair a situation which has been maintained in the laboratory for over 40 years. Continuous research on the bacteria mutants indicates that it is hard for a species to survive in its natural habitat without the occurrences of excision repair in their cells (Dupuy & Sarasin, A., 2015; p.3). Excision repair basically relies on the dismissed information which is transmitted in the duplex which is used to remove the destroyed DNA base or nucleotide ant it replaces it with a base which is normal through the use of a complementary strand. During this process, the removal of the lesion occurs in two different steps. In the first step, the damaged base is unconstrained by glycosylase in the DNA and then the basic sugar is then eliminated by the AP endonucleases (Helena et al, 2018; p.1148). This process is associated with inadequate substrate rates since the DNA glycosylases which is responsible for the initiation of the reaction are directly in connection with the lesion throughout the catalysis. In the process of nucleotide excision DNA repair, a collection of enzymes undertake the hydrolysis of two phosphodiester connections in either side of the lesion. This process leads to leads to the development of an oligonucleotide which is used to carry the damage (Sugasawa, 2008; p. 457). This oligonucleotide is then disconnected from the duplex and the gap that is created after the release is then occupied and ligated to bring the repair process to a conclusion. The incision pattern and the size of the fragments which are incised differ in prokaryotes as well as the eukaryotes (Kuschal et al, 2013; p.19484). In general, the pattern of the incision is precise and chronological depending on whether the lesion is monoadduct or dual duct in nature. All XP genes, excluding for XP-V, have been duplicated, and their roles in the NER mechanisms are identified and understood or in the course of being explained and through this examination, it lets us present a cohesive knowledge of how the scientific description of XP patients is generated (Clancy, 2008; p.103). Numerous unknowns, extending from the comprehensive understanding of molecular features of DNA protein identification and the processes of repair can be used to answer more universal questions concerning the alterations made during DNA repair in patients, tissues and cells, and the main cause of cancer and the ailments affecting CNS ailments where that these aspects are very tough for patients with XP (Menck & Munford, V., 2014; p.223). Disease-causing alterations, however, have been identified in most of the matching genes. Additionally, there are two other disorders which are related to XP. These include Cockayne syndrome (CS) as well as trichothiodystrophy (TTD) (Thoms, Kuschal & Emmert, 2007; p.534). Both conditions show an increase in sensitivity of the patients to UVB or UVC rays, and the contribution of CNS. Unlike to patients with XP, the two disorders do not indicate any consistent increases in the rate of developing skin cancer. However, both disorders have alterations in genes that control the repairing process in human DNA and the duplication that intersects with XP substances. CS intersects with XP across the sets B, D and G, while TTD overlays with B and D. The biggest role which is portrayed by some of these genes is to manage the appropriate structure of DNA repair based on the levels at which the genes are manifested (Li et al, 2006; p.2530). In conclusion, there are different forms of DNA repairs which take place in different organisms. These processes Take place normal except for different situations where an individual has inherited gene disorder such as the XP. The different types of repair undertaken so differ from one organism to another depending on the nature of reactions associated with every gene repair process. Also, the human genome is frequently damaged and affected by an unending variety of environmental aspects such as UV light, ionizing agents and toxic substances such as benzene. The reaction of these substances with the body can lead to varied reactions of the body based on the level of sensitivity towards them. In the case of XP, the reactions would lead to the development of skin cancer. This is due to the way in which the body of an individual is able to deal in the different mutations caused by the different aspects and how their bodies eliminate the harmful substances.
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