X chromosome inactivation occurs early during mammalian development to transcriptionally silence one of the pair of X chromosomes in females, thereby achieving dosage equivalence with males who have a single X chromosome and the sex-determining Y chromosome.  Research in the lab is directed towards understanding both the mechanisms involved in the inactivation process and the clinical implications of X chromosome inactivation in females.

X chromosome inactivation is a truly remarkable example of both epigenetic determination (one of a pair of essentially identical chromosomes is chosen to be silenced) and of cell memory (the choice of the chromosome inactivated is stably inherited throughout subsequent somatic divisions).  Our research explores both the establishment and maintenance of inactivation through analysing gene expression from the active and inactive X chromosomes.  This unique gene encodes a 17 kb alternatively spliced, processed transcript that is not translated into a protein, but which is associated with the inactive X chromosome.  Lab research projects are directed to understanding all stages of the inactivation process.  As is also true in mice, we have identified an antisense transcript that is only expressed in embryonal carcinoma cells not somatic cells, and thus is a candidate regulator of early XIST expression.  We have isolated somatic cell hybrids that express localized XIST from the mouse active X chromosome and unlocalised XIST from the human X chromosome, allowing us to distinguish features resulting from XIST expression (localized or unlocalized) from inactivation.  We have also established a model of XIST function using an inducible XIST transgene in somatic cells.  We are using in vitro and in vivo methods to identify proteins that interact with the RNA.

While most of the several thousand X-linked genes are stably inactivated, over 10% of human X-linked genes ‘escape’ the inactive process and are expressed from both the active and the inactive X chromosome.  We are analysing the expression of the TIMP1 gene that is expressed from some inactive X chromosomes, but not others.  Determining the nature of this variable interaction will establish a hierarchy to the features involved in maintenance of X inactivation.

Mutations affecting X-linked genes cause relatively common and often serious medical disorders.  Insights into the underlying mechanisms of regulation unique to this chromosome is therefore of clinical interest.  We are collaborating with Dr. Wendy Robinson’s research group at BC Children’s Hospital to examine the sources of skewed inactivation in female embryonic and extraembryonic tissues.  We have observed that such skewed inactivation is more prevalent in women who have experienced recurrent spontaneous abortions.  Skewed inactivation is also often detected in association with X chromosome rearrangements, and we are interested in examining cases where non-random inactivation has failed to compensate for the imbalanced gene expression caused by the rearrangement.  We are also examining the role of X-linked gene expression in the development of lymphoma in collaboration with Drs. Doug Horsman and Randy Gascoyne at the BC Cancer Agency.  Our preliminary studies have shown evidence for both oncogenes and tumour suppressor genes on the X chromosome.

Thu KL, Vucic EA, Kennett JY, Heryet C, Brown CJ, Lam WL, Wilson IM.: Methylated DNA immunoprecipitation. J Vis Exp. 2009;(23). pii: 935. doi: 10.3791/935.

Johnson NA, Al-Tourah A, Brown CJ, Connors JM, Gascoyne RD, Horsman DE.: Prognostic significance of secondary cytogenetic alterations in follicular lymphomas. Genes Chromosomes Cancer. 2008 Dec;47(12):1038-48.

Minks J, Robinson WP, Brown CJ.  2008.  A skewed view of X chromosome inactivation.  J Clin Invest. Jan;118(1):20-3.  PMID:  18097476

Wong KK, deLeeuw RJ, Dosanjh NS, Kimm LR, Cheng Z, Horsman DE, MacAulay C, Ng RT, Brown CJ, Eichler EE, Lam WL. 2007. A comprehensive analysis of common copy-number variations in the human genome. Am J Hum Genet. Jan; 80(1):91-104. PMID: 17160897 

Chow JC, Hall LL, Baldry SE, Thorogood NP, Lawrence JB, Brown CJ.  2007.  Inductible XIST-dependent X-chromosome inactivation in human somatic cells is reversible.  Proc Natl Acad Sci USA Jun 12;104(24):10104-9. Epub 2007 May 3  0.  PMID: 17537922 (CIHR MOP 13690)

Bretherick KL, Metzger DL, Chanione J-P, Panagiotopoulos C, Watson SK, Lam WL, Fluker MR, Brown CJ, Robinson WP.  2007.  Skewed X-chromosome inactivation is associated with primary but not secondary ovarian failure.  Am J Med Genet. May 1;143(9):945-51.  PMID: 7431892

Yen ZC, Meyer IM, Karalic S, Brown CJ.  A cross-species comparison of X-chromosome inactivation in Eutheria. 2007.  Genomics Oct; 90(4):453-63.  PMID: 17728098 (NSERC RGPIN 184039)

Kuo ANC, Wilson IM, Vucic E, Lee EHL, Davies JJ, Brown CJ, Lam WL.  2007.  Comparative Cancer Epigenomics In Comparative Genomics Basic and Applied Research.  CRC Press. 

Chow JC, Yen Z, Ziesche SM, Brown CJ. (2005) Silencing the mammalian X chromosome. Annual Review of Genomics and Human Genetics. In press, available online.

Hatakeyama C, Anderson CL, Beever C, Hayden M, Casey B, Penaherrera M, Brown CJ, Robinson WP. (2004) Patterns of X-inactivation skewing as women age.  Clin. Genet. 66:327-332. (DOI; 10.1111/j.1399-0004.2004.00310.x)

Brown CJ, Greally JM. (2003) A stain upon the silence: Genes escaping X inactivation. Trends in Genet. 19:432-438.

Chow JC, Clemson CM, Hall L, Lawrence JB, Brown CJ. (2003) Characterization of expression at the human XIST locus in somatic, embryonal carcinoma and transgenic cell lines.  Genomics. 82: 309-322.