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The XPD helicase plays a significant role in the process of nucleotide repair (NER). The process of DNA unwinding enables the separation of DNA into single strands for easy copying and replication. Point mutations in the XPD helicase cause the Cockayne Syndrome (CS). The main signs of CS include premature aging and short stature. The mapping of mutations occurs in domains that adversely affect the integrity of the TFIIH complex. Other features evident in CS patients include poor weight gain and growth, an abnormally small head and an impaired nervous system. The successful repair of damaged DNA depends on the correctness of the instructions from the CBS and CSA genes. The failure to repair damaged DNA in CS patients leads to increased DNA abnormalities and the malfunctioning and death of cells, which causes different CS features.
XPD is one of the main subunits of the TFIIH complex, which engages in transcriptional activities and repair functions. The repair functions of the TFIIH complex help to reverse various DNA damages. A mutation in the XPD helicase hampers the ability of the TFIIH complex to execute its functions because of the increase in the rigidity in the protein. The phenomenon impairs the ability of the TFIIH complex to perform repair and transcription functions. XPD mutations lead to a phenomenon called lock in the repair mode, which makes the helicase cut DNA segments that serve the transcription process. The decline in the rate of interaction between proteins and identification of areas of damaged DNA increases the accumulation of damaged cells. The damage to DNA in active genes prevents the enzymes involved in gene transcription from performing their functions. The decline in the TFIIH complex transcriptional activity arises due to the alteration of the interaction between XPD-CAD complex and the TFIIH and changes in the TFIIH stoichiometric composition. Evidence shows that the defects in the XPD helicase interfere with the structure and flexibility of the protein leading to Cockayne syndrome.
The description of Xeroderma pigmentosum in 1874 introduced the first abnormality associated with XPB/D mutations. The discovery that all the gene mutations in trichothiodystrophy (TTD)had a relationship with the TFIIH complex paved the way for the identification of the relationship between DNA repair and transcription and the genotype-phenotype relationships attributed to XPB and XPD defects. The discovery led to the conclusion that mutations in the human helicase contribute to diverse genetic disorders that exhibit through chromosomal instability, which initiates age-related diseases and cancer. Genetic engineering can significantly enhance the study of the effects of XPD mutations on the functions of the TFIIH complex by allowing researchers to circumvent the XPD defects by overriding the lock in repair mode to open the promoter. Researchers can develop cell lines supplemented with fetal bovine serum and appropriate antibiotics to study the functioning of the DNA damaged by UV. Genetic engineering is essential in the isolation of genomic DNA from cells exposed to radiation and control cells. Controlled growth of cells would allow the creation of different helicase domains and the cloning of mutant genes to allow the study of the manifestation of the Cockayane syndrome in different environments. Genetic engineering allows the incubation of DNA and incorporation of elements such a geneticin to create a collection of cells that can resist alterations evident in the gene transcription process.