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Xeroderma Pigmentosum ( Genetic Diagnosis)

Xeroderma Pigmentosum ( Genetic Diagnosis). BY: MOHAMMED ALSAIDAN. Nucleotide-Excision Repair (NER) disorders:. Xeroderma Pigmentosum Cockayne syndrome Thrichthiodystrophy. Diagnosis . 1/ functional assays of DNA repair. cellular ultraviolet ( UV) hypersensitivity

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Xeroderma Pigmentosum ( Genetic Diagnosis)

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  1. XerodermaPigmentosum(Genetic Diagnosis) BY: MOHAMMED ALSAIDAN

  2. Nucleotide-Excision Repair (NER) disorders: • XerodermaPigmentosum • Cockaynesyndrome • Thrichthiodystrophy

  3. Diagnosis 1/ functional assays of DNA repair. • cellular ultraviolet (UV) hypersensitivity • unscheduled DNA synthesis (UDS) • host-cell reactivation 2/ molecular genetic testing • Sequence analysis of XPA, XPC, ERCC2, ERCC5, ERCC1, and ERCC3

  4. Cellular hypersensitivity to UV radiation • In the cellular hypersensitivity to UV radiation and chromosomal breakage studies, the xerodermapigmentosum fibroblasts are stressed with different doses of UV radiation. Then, chromosomal breakage is evaluated in at least 100-200 cells. The cells from the patient are compared with those from the patient's parents (if possible, as they are obligate heterozygotes for xerodermapigmentosum). Cells from unrelated healthy individuals are used as controls. • Helpful in Prenatal diagnosis studies (amniocytes from at-risk fetuses)

  5. The unscheduled DNA synthesis (UDS) • Creating 6-4 photoproducts and pyrimidine dimers using UVC irradiation, then allowing for their repair. • Repair is quantified by the amount of radioactive thymidine incorporated after this insult, and the length of time for repair • Radioactivity is evaluated by grain counting after autoradiography.

  6. Prenatal Diagnosis • Prenatal diagnosis is possible by amniocentesis or chorionic villi sampling. • Unscheduled DNA synthesis is the classic method for diagnosis(4-5 weeks) • A faster technique is the alkaline comet assay (single-cell gel electrophoresis assay). This method, in addition to being faster(24h) , requires fewer cells and does not require radioactivity.

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