DNA修復是防禦細胞突變和細胞癌化的一個重要機制，但在此諸多因子參與影響之中，本研究想聚焦於釐清DNA修復基因人類hHR23A和hHR23B的功能。這兩個基因與酵母菌RAD23基因同源，均含有數個功能區域，包括N端的ubiquitin相似區域與兩個ubiquitin連結區域，以及和XPC鍵結區域。先前研究已顯示hHR23B蛋白質會與XPC蛋白質結合，並會參與整體基因核酸切除修復nucleotide excision repair (NER)的過程。此時，同存在於細胞中而與hHR23B極為相似的hHR23A，其在NER過程中的功能為我們所注目。

為了探討hHR23A和hHR23B的功能，我們於HtTA1細胞導入了RNAi技術，對這兩個基因建立knock-down(KD)模式，並各分別設計了三個干擾性RNA的建構物（hHR23Ai和hHR23Bi），以試圖釐清hHR23A和hHR23B的功能。此結果得知，在HtTA1細胞能抑制內源性的hHR23A或hHR23B蛋白質表現量約為50%，繼之也發現hHR23A或hHR23B之KD細胞對於紫外線的敏感度比正常細胞較高。其次，利用寄主細胞再活化法，仔細觀察了hHR23A和hHR23B之KD細胞中的DNA修復活性，結果發現hHR23A和hHR23B都是NER所必須的，此在西南方墨點法也得到相同的結果。

另外，也透過共免疫沉澱法之分析，觀察到活化NER的GADD45A蛋白質會與hHR23A鍵結，此乃表示hHR23A與GADD45A於體內應同在一個蛋白質複合體上。目前由文獻已得知GADD45A的表現有部份是受到腫瘤抑制者p53所調控，又本實驗室另一研究上也發現了hHR23A會與p53蛋白質鍵結之故，我們綜合之，推測hHR23A是與GADD45A和p53共同協調來執行它參與NER的功能。

DNA repair is an essential mechanism for cellular defense to mutations and cancers. This project studies the functions of human DNA repair genes hHR23A and hHR23B. The human hHR23A and hHR23B genes are two sequence homologues of the Saccharomyces cerevisiae rad23 gene. These two factors both contain a few functional domains, including an ubiquitin-like (UBL) domain at the N-termini, two ubiquitin-associated (UBA) domains and an XPC-binding domain. Previous studies showed that the hHR23B protein is complexed with the XP group C (XPC) protein, defective in the xeroderma pigmentosum C syndrome, and is involved in the global genome nucleotide excision repair (NER) pathway. Therefore, the hHR23B is involved in NER. However, the function of hHR23A is not clear so far.
To study the functions of hHR23A and B, we have employed the RNA interference (RNAi) techniques to construct the knock-down (KD) models for these two genes. The hHR23Ai and hHR23Bi constructs could reduce endogenous levels of the hHR23A or hHR23B in HtTA1 cells by around 50%. Either of the hHR23A or B KD cells were more sensitive to UV irradiation than the wild type cells were. By the host reactivation assay as well as the South-western blot to observe DNA repair activities in the hHR23A/B KD cells, we found that the hHR23A and B are required for NER. By co-immunoprecipitation analysis, we also found that NER activator GADD45 protein interacts with the hHR23A, indicating that the hHR23A is associated with the GADD45A in vivo. Other study in the lab also found that the hHR23A is associated with the tumor suppressor p53 protein.
In conclusion, the hHR23A and hHR23B proteins are NER factors in vivo. And the fact that the hHR23A is associated with GADD45A suggests that the hHR23A is a NER factor and performs its functional role in concert with GADD45A and p53.

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