Clinical Focus


  • Dermatology
  • Genetic Skin Diseases
  • Genetic Predisposition to Disease

Academic Appointments


Honors & Awards


  • F32 NRSA Postdoctoral Fellow, National Institutes of Health
  • Alfred Kopf Research Award, Skin Cancer Foundation
  • Dermatologist Investigator Fellowship, Dermatology Foundation
  • James Shipley Research Prize, Harvard Medical School

Professional Education


  • Board Certification: Dermatology, American Board of Dermatology (2012)
  • Residency:Stanford University - Dept of Dermatology (2012) CA
  • Internship:Stanford University (2009) CA
  • Medical Education:Harvard Medical School (2008) MA
  • Doctor of Medicine, Harvard University (2008)
  • Doctor of Philosophy, Harvard University (2006)

Stanford Advisors


Journal Articles


  • Advances in skin grafting and treatment of cutaneous wounds SCIENCE Sun, B. K., Siprashvili, Z., Khavari, P. A. 2014; 346 (6212): 941-945
  • Activating HRAS mutation in agminated Spitz nevi arising in a nevus spilus. JAMA dermatology Sarin, K. Y., Sun, B. K., Bangs, C. D., Cherry, A., Swetter, S. M., Kim, J., Khavari, P. A. 2013; 149 (9): 1077-1081

    Abstract

    IMPORTANCE Spitz nevi are benign melanocytic proliferations that can sometimes be clinically and histopathologically difficult to distinguish from melanoma. Agminated Spitz nevi have been reported to arise spontaneously, in association with an underlying nevus spilus, or after radiation or chemotherapy. However, to our knowledge, the genetic mechanism for this eruption has not been described. OBSERVATIONS We report a case of agminated Spitz nevi arising in a nevus spilus and use exome sequencing to identify a clonal activating point mutation in HRAS (GenBank 3265) (c.37G→C) in the Spitz nevi and underlying nevus spilus. We also identify a secondary copy number increase involving HRAS on chromosome 11p, which occurs during the development of the Spitz nevi. CONCLUSIONS AND RELEVANCE Our results reveal an activating HRAS mutation in a nevus spilus that predisposes to the formation of Spitz nevi. In addition, we demonstrate a copy number increase in HRAS as a "second hit" during the formation of agminated Spitz nevi, which suggests that both multiple Spitz nevi and solitary Spitz nevi may arise through similar molecular pathways. In addition, we describe a unique investigative approach for the discovery of genetic alterations in Spitz nevi.

    View details for DOI 10.1001/jamadermatol.2013.4745

    View details for PubMedID 23884457

  • Mosaic Activating RAS Mutations in Nevus Sebaceus and Nevus Sebaceus Syndrome JOURNAL OF INVESTIGATIVE DERMATOLOGY Sun, B. K., Saggini, A., Sarin, K. Y., Kim, J., Benjamin, L., LeBoit, P. E., Khavari, P. A. 2013; 133 (3): 824-827

    View details for DOI 10.1038/jid.2012.377

    View details for Web of Science ID 000315008500032

    View details for PubMedID 23096709

  • X-Chromosome Inactivation and Skin Disease JOURNAL OF INVESTIGATIVE DERMATOLOGY Sun, B. K., Tsao, H. 2008; 128 (12): 2753-2759

    Abstract

    X-chromosome inactivation (XCI) is the process in which females transcriptionally silence one of their two X chromosomes in early embryonic development, equalizing X chromosome gene expression between males and females. XCI depends on a gene called XIST, a functional RNA molecule that does not code for a protein. Recent studies indicate abundant intergenic transcription and nonprotein coding RNAs in the human genome, which are suspected to function in modulating gene expression. XCI may therefore serve as a useful model to learn and understand the potential function of these elements, as well as their effects on human disease. Here, we review the genetic and molecular basis of XCI and describe how the mechanistics of this process lead to the phenotypes of X-linked skin diseases, most notably in the pattern of lines, swirls, and whorls first noted by the dermatologist Alfred Blaschko. We suggest that XCI, and other epigenetic phenomena, will continue to impact our understanding of the genetic mechanisms of disease.Journal of Investigative Dermatology (2008) 128, 2753-2759; doi:10.1038/jid.2008.145; published online 29 May 2008.

    View details for DOI 10.1038/jid.2008.145

    View details for Web of Science ID 000261062500004

    View details for PubMedID 18509358

  • Small RNAs in development and disease JOURNAL OF THE AMERICAN ACADEMY OF DERMATOLOGY Sun, B. K., Tsao, H. 2008; 59 (5): 725-737

    Abstract

    MicroRNAs (miRNAs) and short interfering RNAs (siRNAs) are classes of regulatory small RNA molecules, ranging from 18 to 24 nucleotides in length, whose roles in development and disease are becoming increasingly recognized. They function by altering the stability or translational efficiency of messenger RNAs (mRNAs) with which they share sequence complementarity, and are predicted to affect up to one-third of all human genes. Computer algorithms and microarray data estimate the presence of nearly 1000 human miRNAs, and direct examination of candidate miRNAs has validated their involvement in various cancers, disorders of neuronal development, cardiac hypertrophy, and skin diseases such as psoriasis. This article reviews the history of miRNA and siRNA discovery, key aspects of their biogenesis and mechanism of action, and known connections to human health, with an emphasis on their roles in skin development and disease.

    View details for DOI 10.1016/j.jaad.2008.08.017

    View details for Web of Science ID 000260384200001

    View details for PubMedID 19119093

  • Polycomb Proteins Targeted by a Short Repeat RNA to the Mouse X Chromosome SCIENCE Zhao, J., Sun, B. K., Erwin, J. A., Song, J., Lee, J. T. 2008; 322 (5902): 750-756

    Abstract

    To equalize X-chromosome dosages between the sexes, the female mammal inactivates one of her two X chromosomes. X-chromosome inactivation (XCI) is initiated by expression of Xist, a 17-kb noncoding RNA (ncRNA) that accumulates on the X in cis. Because interacting factors have not been isolated, the mechanism by which Xist induces silencing remains unknown. We discovered a 1.6-kilobase ncRNA (RepA) within Xist and identified the Polycomb complex, PRC2, as its direct target. PRC2 is initially recruited to the X by RepA RNA, with Ezh2 serving as the RNA binding subunit. The antisense Tsix RNA inhibits this interaction. RepA depletion abolishes full-length Xist induction and trimethylation on lysine 27 of histone H3 of the X. Likewise, PRC2 deficiency compromises Xist up-regulation. Therefore, RepA, together with PRC2, is required for the initiation and spread of XCI. We conclude that a ncRNA cofactor recruits Polycomb complexes to their target locus.

    View details for DOI 10.1126/science.1163045

    View details for Web of Science ID 000260605200052

    View details for PubMedID 18974356

  • Intersection of the RNA interference and X-inactivation pathways SCIENCE Ogawa, Y., Sun, B. K., Lee, J. T. 2008; 320 (5881): 1336-1341

    Abstract

    In mammals, dosage compensation is achieved by X-chromosome inactivation (XCI) in the female. The noncoding Xist gene initiates silencing of the X chromosome, whereas its antisense partner Tsix blocks silencing. The complementarity of Xist and Tsix RNAs has long suggested a role for RNA interference (RNAi). Here, we report that murine Xist and Tsix form duplexes in vivo. During XCI, the duplexes are processed to small RNAs (sRNAs), most likely on the active X (Xa) in a Dicer-dependent manner. Deleting Dicer compromises sRNA production and derepresses Xist. Furthermore, without Dicer, Xist RNA cannot accumulate and histone 3 lysine 27 trimethylation is blocked on the inactive X (Xi). The defects are partially rescued by truncating Tsix. Thus, XCI and RNAi intersect, down-regulating Xist on Xa and spreading silencing on Xi.

    View details for DOI 10.1126/science.1157676

    View details for Web of Science ID 000256441100044

    View details for PubMedID 18535243

  • A transient heterochromatic state in Xist preempts X inactivation choice without RNA stabilization MOLECULAR CELL Sun, B. K., Deaton, A. M., Lee, J. T. 2006; 21 (5): 617-628

    Abstract

    X chromosome inactivation (XCI) depends on a noncoding sense-antisense transcript pair, Xist and Tsix. At the onset of XCI, Xist RNA accumulates on one of two Xs, coating and silencing the chromosome in cis. The molecular basis for monoallelic Xist upregulation is not known, though evidence predominantly supports a posttranscriptional mechanism through RNA stabilization. Here, we test whether Tsix RNA destabilizes Xist RNA. Unexpectedly, we find that Xist upregulation is not based on transcript stabilization at all but is instead controlled by transcription in a sex-specific manner. Tsix directly regulates its transcription. On the future inactive X, Tsix downregulation induces a transient heterochromatic state in Xist, followed paradoxically by high-level Xist expression. A Tsix-deficient X chromosome adopts the heterochromatic state in pre-XCI cells. This state persists through XCI establishment and "reverts" to a euchromatic state during XCI maintenance. We have therefore identified chromatin marks that preempt and predict asymmetric Xist expression.

    View details for DOI 10.1016/j.molcel.2006.01.028

    View details for Web of Science ID 000236135000004

    View details for PubMedID 16507360

  • Differential methylation of Xite and CTCF sites in Tsix mirrors the pattern of X-inactivation choice in mice MOLECULAR AND CELLULAR BIOLOGY Boumil, R. M., Ogawa, Y., Sun, B. K., Huynh, K. D., Lee, J. T. 2006; 26 (6): 2109-2117

    Abstract

    During mammalian dosage compensation, one of two X-chromosomes in female cells is inactivated. The choice of which X is silenced can be imprinted or stochastic. Although genetic loci influencing the choice decision have been identified, the primary marks for imprinting and random selection remain undefined. Here, we examined the role of DNA methylation, a mechanism known to regulate imprinting in autosomal loci, and sought to determine whether differential methylation on the two Xs might predict their fates. To identify differentially methylated domains (DMDs) at the X-inactivation center, we used bisulfite sequencing and methylation-sensitive restriction enzyme analyses. We found DMDs in Tsix and Xite, two genes previously shown to influence choice. Interestingly, the DMDs in Tsix lie within CTCF binding sites. Allelic methylation differences occur in gametes and are erased in embryonic stem cells carrying two active Xs. Because the pattern of DNA methylation mirrors events of X-inactivation, we propose that differential methylation of DMDs in Tsix and Xite constitute a primary mark for epigenetic regulation. The discovery of DMDs in CTCF sites draws further parallels between X-inactivation and autosomal imprinting.

    View details for DOI 10.1128/MCB.26.6.2109-2117.2006

    View details for Web of Science ID 000235915400009

    View details for PubMedID 16507990

  • X-chromosome kiss and tell: How the Xs go their separate ways COLD SPRING HARBOR SYMPOSIA ON QUANTITATIVE BIOLOGY Anguera, M. C., Sun, B. K., Xu, N., Lee, J. T. 2006; 71: 429-437

    Abstract

    Loci associated with noncoding RNAs have important roles in X-chromosome inactivation (XCI), the dosage compensation mechanism by which one of two X chromosomes in female cells becomes transcriptionally silenced. The Xs start out as epigenetically equivalent chromosomes, but XCI requires a cell to treat two identical X chromosomes in completely different ways: One X chromosome must remain transcriptionally active while the other becomes repressed. In the embryo of eutherian mammals, the choice to inactivate the maternal or paternal X chromosome is random. The fact that the Xs always adopt opposite fates hints at the existence of a trans-sensing mechanism to ensure the mutually exclusive silencing of one of the two Xs. This paper highlights recent evidence supporting a model for mutually exclusive choice that involves homologous chromosome pairing and the placement of asymmetric chromatin marks on the two Xs.

    View details for Web of Science ID 000245962800054

    View details for PubMedID 17381325

  • Strategies for mutational analysis of the large multiexon ATM gene using high-density oligonucleotide arrays GENOME RESEARCH Hacia, J. C., Sun, B., Hunt, N., Edgemon, K., Mosbrook, D., Robbins, C., Fodor, S. P., Tagle, D. A., Collins, F. S. 1998; 8 (12): 1245-1258

    Abstract

    Mutational analysis of large genes with complex genomic structures plays an important role in medical genetics. Technical limitations associated with current mutation screening protocols have placed increased emphasis on the development of new technologies to simplify these procedures. High-density arrays of >90,000-oligonucleotide probes, 25 nucleotides in length, were designed to screen for all possible heterozygous germ-line mutations in the 9.17-kb coding region of the ATM gene. A strategy for rapidly developing multiexon PCR amplification protocols in DNA chip-based hybridization analysis was devised and implemented in preparing target for the 62 ATM coding exons. Improved algorithms for interpreting data from two-color experiments, where reference and test samples are cohybridized to the arrays, were developed. In a blinded study, 17 of 18 distinct heterozygous and 8 of 8 distinct homozygous sequence variants in the assayed region were detected accurately along with five false-positive calls while scanning >200 kb in 22 genomic DNA samples. Of eight heterozygous sequence changes found in more than one sample, six were detected in all cases. Five previously unreported sequence changes, not found by other mutational scanning methodologies on these same samples, were detected that led to either amino acid changes or premature truncation of the ATM protein. DNA chip-based assays should play a valuable role in high throughput sequence analysis of complex genes.

    View details for Web of Science ID 000077860700003

    View details for PubMedID 9872980