Suhwan Chang

Publications (9)84.48 Total impact

• Article: The role of epigenetic transcriptional regulation in BRCA1-mediated tumor suppression.

  Suhwan Chang, Shyam K Sharan
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  ABSTRACT: Mutation in BRCA1 accounts for half of all familial breast cancer cases. Recent studies have shown its role in the epigenetic regulation of an oncogenic microRNA, revealing a novel mode of tumor suppression that is independent of its well-established role in DNA repair and cell cycle regulation.
  Transcription. 11/2012; 4(1).
• Article: BRCA1 and MicroRNAs: Emerging networks and potential therapeutic targets.

  Suhwan Chang, Shyam K Sharan
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  ABSTRACT: BRCA1 is a well-known tumor suppressor implicated in familial breast and ovarian cancer. Since its cloning in 1994, numerous studies have established BRCA1's role in diverse cellular and biochemical processes, such as DNA damage repair, cell cycle control, and transcriptional regulation as well as ubiquitination. In addition, a number of recent studies have functionally linked this tumor suppressor to another important cellular regulator, microRNAs, which are short (19-22 nt) RNAs that were discovered in the nematode in 1993. Soon their presence and function were validated in mammals, and since then, the role of microRNAs has been actively investigated in almost all biological processes, including cancer. In this review, we will describe recent progress in the understanding of the BRCA1 function through microRNAs and the role of microRNAs in regulating BRCA1, with emphasis on the implication of these processes on the development and progression of cancer. We will also discuss the therapeutic potential of microRNA mimics or inhibitors of microRNAs to affect BRCA1 function.
  Molecules and Cells 07/2012; · 2.18 Impact Factor
• Article: Using recombineering to generate point mutations: the oligonucleotide-based "hit and fix" method.

  Suhwan Chang, Stacey Stauffer, Shyam K Sharan
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  ABSTRACT: Ability to manipulate the genome or design genes with desired mutation is critical for functional studies. Recombineering has made genetic manipulation of large genomic fragments very feasible and efficient. In the bacteriophage lambda-based recombineering system, three prophage genes, exo, bet, and gam, under the control of a temperature-sensitive lambda cI-repressor, provide the recombination function. The high efficiency of recombineering by oligonucleotides allows generation of subtle alterations in the bacterial chromosomal DNA as well as episomal DNA. We describe here a two-step "Hit and Fix" method, in which a short heterologous sequence is inserted to the target site first (Hit) and this sequence is replaced with the desired mutation in the second step (Fix). Insertion and replacement of the heterologous sequence allows screening of the recombinant clones by PCR or colony hybridization.
  Methods in molecular biology (Clifton, N.J.) 01/2012; 852:111-20.
• Article: Tumor suppressor BRCA1 epigenetically controls oncogenic microRNA-155.

  Suhwan Chang, Rui-Hong Wang, Keiko Akagi, Kyung-Ae Kim, Betty K Martin, Luca Cavallone, Diana C Haines, Mark Basik, Phuong Mai, Elizabeth Poggi, Claudine Isaacs, Lai M Looi, Kein S Mun, Mark H Greene, Stephen W Byers, Soo H Teo, Chu-Xia Deng, Shyam K Sharan
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  ABSTRACT: BRCA1, a well-known tumor suppressor with multiple interacting partners, is predicted to have diverse biological functions. However, so far its only well-established role is in the repair of damaged DNA and cell cycle regulation. In this regard, the etiopathological study of low-penetrant variants of BRCA1 provides an opportunity to uncover its other physiologically important functions. Using this rationale, we studied the R1699Q variant of BRCA1, a potentially moderate-risk variant, and found that it does not impair DNA damage repair but abrogates the repression of microRNA-155 (miR-155), a bona fide oncomir. Mechanistically, we found that BRCA1 epigenetically represses miR-155 expression via its association with HDAC2, which deacetylates histones H2A and H3 on the miR-155 promoter. We show that overexpression of miR-155 accelerates but the knockdown of miR-155 attenuates the growth of tumor cell lines in vivo. Our findings demonstrate a new mode of tumor suppression by BRCA1 and suggest that miR-155 is a potential therapeutic target for BRCA1-deficient tumors.
  Nature medicine 09/2011; 17(10):1275-82. · 27.14 Impact Factor
• Article: Tumor suppressor BRCA1 epigenetically controls oncogenic microRNA-155.

  Suhwan Chang, Rui-Hong Wang, Keiko Akagi, Kyung-Ae Kim, Betty K Martin, Luca Cavallone, Diana C Haines, Mark Basik, Phuong Mai, Elizabeth Poggi, Claudine Isaacs, Lai M Looi, Kein S Mun, Mark H Greene, Stephen W Byers, Soo H Teo, Chu-Xia Deng, Shyam K Sharan
  Nature medicine 01/2011; 17(11):1521. · 27.14 Impact Factor
• Article: Epigenetic control of an oncogenic microRNA, miR-155, by BRCA1.

  Suhwan Chang, Shyam K Sharan
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  ABSTRACT: In summary, while our finding that BRCA1 epigenetically silences an oncogenic microRNA is quite significant, many questions remain to be answered. We hope that our efforts to answer these questions will lead to a better understanding of the tumor suppressor function of BRCA1 and allow us to develop novel therapeutic strategies for the breast cancer.
  Oncotarget 01/2011; 3(1):7-8. · 4.78 Impact Factor
• Article: Functional analysis of human BRCA2 variants using a mouse embryonic stem cell-based assay.

  Sergey G Kuznetsov, Suhwan Chang, Shyam K Sharan
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  ABSTRACT: We describe here a comprehensive and reliable assay to test the functional significance of variants of unknown clinical significance (VUS) identified in the human breast cancer susceptibility gene, BRCA2. The assay is based on the ability of human BRCA2 to complement the loss of endogenous Brca2 in mouse embryonic stem cells. The procedure involves generation of a desired mutation in BRCA2 present in a bacterial artificial chromosome (BAC) and the introduction of the BAC into ES cells engineered for the assay. These ES cells have one null and one conditional allele of Brca2. First, the effect of the BRCA2 variants on the viability of ES cells is tested by Cre-mediated deletion of the conditional allele. Subsequently, variants that result in viable ES cells are examined for their effect on known functions of BRCA2 using a variety of functional assays such as sensitivity to genotoxic agents, in vivo and in vitro proliferation, effect on homologous recombination and genomic stability. The method described herein allows for the analysis of three to five sequence variants within 2-3 months. This approach can also be used for functional analysis of variants identified in other human disease genes that result in a phenotype detectable in ES cells.
  Methods in molecular biology (Clifton, N.J.) 01/2010; 653:259-80.
• Article: Expression of human BRCA1 variants in mouse ES cells allows functional analysis of BRCA1 mutations.

  Suhwan Chang, Kajal Biswas, Betty K Martin, Stacey Stauffer, Shyam K Sharan
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  ABSTRACT: To date, inheritance of a mutant BRCA1 or BRCA2 gene is the best-established indicator of an increased risk of developing breast cancer. Sequence analysis of these genes is being used to identify BRCA1/2 mutation carriers, though these efforts are hampered by the high frequency of variants of unknown clinical significance (VUSs). Functional evaluation of such variants has been restricted due to lack of a physiologically relevant assay. In this study we developed a functional assay using mouse ES cells to study variants of BRCA1. We introduced BAC clones with human wild-type BRCA1 or variants into Brca1-null ES cells and confirmed that only wild-type and a known neutral variant rescued cell lethality. The same neutral variant was also able to rescue embryogenesis in Brca1-null mice. A test of several BRCT domain mutants revealed all to be deleterious, including a VUS. Furthermore, we used this assay to determine the effects of BRCA1 variants on cell cycle regulation, differentiation, and genomic stability. Importantly, we discovered that ES cells rescued by S1497A BRCA1 exhibited significant hypersensitivity after gamma-irradiation. Our results demonstrate that this ES cell-based assay is a powerful and reliable method for analyzing the functional impact of BRCA1 variants, which we believe could be used to determine which patients may require preventative treatments.
  The Journal of clinical investigation 09/2009; 119(10):3160-71. · 15.39 Impact Factor
• Article: Degradation of BRCA2 in alkyltransferase-mediated DNA repair and its clinical implications.

  Subha Philip, Srividya Swaminathan, Sergey G Kuznetsov, Sreenivas Kanugula, Kajal Biswas, Suhwan Chang, Natalia A Loktionova, Diana C Haines, Philipp Kaldis, Anthony E Pegg, Shyam K Sharan
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  ABSTRACT: Germ-line mutations in BRCA2 have been linked to early-onset familial breast cancer. BRCA2 is known to play a key role in repairing double-strand breaks. Here, we describe the involvement of BRCA2 in O6-alkylguanine DNA alkyltransferase (AGT)-mediated repair of O6-methylguanine adducts. We show that BRCA2 physically associates and undergoes repair-mediated degradation with AGT. In contrast, BRCA2 with a 29-amino-acid deletion in an evolutionarily conserved domain does not bind to alkylated AGT; the two proteins are not degraded; and mouse embryonic fibroblasts are specifically sensitive to alkylating agents that result in O6-methylguanine adducts. We show that O6-benzylguanine (O6BG), a nontoxic inhibitor of AGT, can also induce BRCA2 degradation. BRCA2 is a viable target for cancer therapy because BRCA2-deficient cells are hypersensitive to chemotherapeutic DNA-damaging agents. We show a marked effect of O6BG pretreatment on cell sensitivity to cisplatin. We also show the efficacy of this approach on a wide range of human tumor cell lines, which suggests that chemosensitization of tumors by targeted degradation of BRCA2 may be an important consideration when devising cancer therapeutics.
  Cancer Research 01/2009; 68(23):9973-81. · 7.86 Impact Factor