Research Interests: Human X-Chromosome Inactivation
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 Brown 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 chromosome inactivated is stably inherited through subsequent somatic divisions for the life of the individual). Our research focusses on two aspects of inactivation: (1) the establishment and maintenance of inactivation by a long non-coding RNA (lncRNA); and (2) how genes evade the silencing process.
(1) The XIST gene is the only gene that is expressed from the inactive but not from the active X chromosome. This unique gene encodes a 17 kb alternatively spliced, processed transcript which is not translated into a protein but which remains in the nucleus where it associates with the inactive X chromosome. XIST was one of the first lncRNAs to be discovered, and understanding how it functions can reveal parallels for the growing number of lncRNAs being described. We have established a model of XIST function using an inducible XIST transgene in human somatic cells, which allows us to determine how XIST is able to recruit the assembly of silent chromatin on the chromosome from which it is expressed. We are also studying how the RNA associates with the chromosome from which it is transcribed; and the impact in a female cell when XIST expression, or domains of XIST, are lost.
(2) Surprisingly, over 20% of human X-linked genes continue to show significant expression from the inactive X chromosome. Therefore, these genes continue to have dosage differences between males and females and are likely to contribute to the phenotype of X-chromosome aneuploidies and some portion of sexually dimorphic traits. In addition to comparison of male and female cells and published datasets, we utilize somatic cell hybrids that allow us to distinguish expression from the active and inactive X chromosome. In collaboration with the Simpson and Wasserman groups at the Centre for Molecular Medicine and Therapeutics at the B.C. Children’s Hospital we are incorporating human DNA into mouse cells to identify the DNA elements involved in spreading (or blocking) silencing along the chromosome. We are also collaborating with Dr. Wendy Robinson’s research group at the B.C. Children’s Hospital to examine the impact of the second X chromosome in female extra-embryonic tissues.
Overall, our goal is to understand the interplay between DNA, RNA and chromatin that underlies silencing of a chromosome and use this knowledge to understand epigenetic gene regulation and the gene expression differences between males and females.