In heterogametic species there is an imbalance in the number of sex-linked genes between males and females. Therefore, it is essential that transcription is equalized between the sexes and between the sex chromosomes and autosomes, a process called dosage compensation. Though different dosage compensation mechanisms have evolved, they all share a common initial step of distinguishing the X-chromosome from the autosomes. The model organism Drosophila melanogaster has evolved a single-step mechanism for dosage compensation, providing a simple model for studying how a single chromosome is discriminated from the rest of the genome. In Drosophila, selective two-fold upregulation of the single male X-chromosome equalizes transcription between the sexes. The selective identification of the X-chromosome involves the recruitment of a ribonucleoprotein complex, Male-Specific Lethal (MSL) that is expressed only in males. The MSL complex initially localizes to GA-rich sequences within 'seed sites' on the X-chromosome, termed MSL Recognition Elements (MREs). However, MSL complex cannot bind directly to MREs and these sequences are distributed throughout the genome. The Larschan laboratory has recently identified a previously unstudied zinc finger protein, Chromatin-Linked Adapter for MSL Proteins (CLAMP), that links the MSL complex to MRE sequences. Furthermore, CLAMP is highly enriched on the X-chromosome independent of MSL complex and therefore is likely to be involved in the earliest step of X-identification. Therefore, the identification of CLAMP provides the first opportunity to define the X-chromosome features that promote its identification for dosage compensation. Based on strong preliminary data, I hypothesize that identification of the X-chromosome requires contributions from primary genome sequence and higher-order nuclear organization mediated by the CLAMP protein. I will test my hypothesis by defining the role of linear and three-dimensional genomic organization in enrichment of CLAMP on the X-chromosome. First, using Chromatin Immunoprecipitation- qPCR (ChIP-qPCR), I will determine whether enrichment of CLAMP on the X-chromosome occurs cooperatively or additively as the number of tandem MRE sequences increases. Second, I will determine if putative initial seed sites cluster together in three-dimensional nuclear space using the Chromatin Interaction Analysis by Paired-End Tag Sequencing (ChIA-pet) method. The proposed experiments will use state-of-the- art approaches to provide key insight into how initial identification of the X-chromosome is achieved.

Public Health Relevance

Dosage compensation is a conserved process that corrects for the imbalance in sex chromosome number in heterogametic species. A key obstacle in all species that perform dosage compensation is the selective identification of the X-chromosome from the autosomes. Using a combination of genetic and genomic techniques; this project will provide insight into understanding how this evolutionarily conserved mechanism is initiated.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Predoctoral Individual National Research Service Award (F31)
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Special Emphasis Panel (ZRG1-F08-A (20))
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Gaillard, Shawn R
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Brown University
Schools of Medicine
United States
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