Dr Celia May's projects

Current projects in the laboratory are concerned with genome variability, meiotic recombination and genome instability, particularly within the pseudoautosomal regions of the human sex chromosomes.

The major human pseudoautosomal region (or PAR1), located on the short arms of the X and Y chromosomes, is the site of obligatory crossing-over in the male germline, such that this 2.7Mb interval supports one of the highest rates of recombination in the entire human genome. PAR1 is also characterised by a significantly higher GC content and a different suite of repeated DNA structures to the rest of the X chromosome.  

In contrast, the 0.33Mb minor pseudoautosomal region (or PAR2), found at the tips of the long arms, supports a modestly elevated rate of recombination in the male germline and is similar in sequence composition to the rest of the X, reflecting a recent X-to-Y transposition event in the human lineage. These regions are thus ideally suited to the exploration of DNA turnover processes. We predominantly use single gamete (sperm) DNA approaches to directly isolate and quantify de novo germline changes; subsequent characterisation of these events gives us insight into the dynamics of the process(es) involved.

Applications are invited from students wishing to pursue a PhD in this general area. Applicants may wish to explore the functional impact and mutational mechanisms associated with structural variation within PAR1, consider the relationship between recombination rate and sequence divergence, or develop their own ideas into an allied project.

E.g., Structural variation within PAR1
Cytogenetically-visible large-scale rearrangements of the human genome have long been associated with pathological disorders, but recently large international collaborative efforts (such as the HapMap project) have highlighted the ubiquity of kilobase-scaled structural variation even in the genomes of apparently healthy individuals. Indeed, such variation accounts for the majority of bases that differ between human genomes, and can also be linked to all classes of human genetic disease. Understanding how these forms of variation arise is therefore of fundamental evolutionary interest and is directly addressable using single-molecule PCR techniques.

Large-scale germline deletions involving the PAR1 gene SHOX, linked to idiopathic and syndromal short stature (Léri-Weill dyschondrosteosis, Langer mesomelic dysplasia, Turner syndrome), are inferred to occur at relatively high frequency. Patient analysis has identified a proximal deletion breakpoint hotspot, though in contrast to classic genomic disorders there appears to be no distal counterpart; this suggests either recombination repair by nonhomologous end-joining or a replication-based mechanism like Fork Stalling and Template Switching.

The relative contributions of these mechanisms can be determined from the direct analysis of large numbers of breakpoints from de novo events, efficiently obtained by screening batches of sperm DNA from healthy men. This could be complemented by establishing the allelic recombination profile of the breakpoint hotspot. Similar approaches may also be used to examine structural rearrangements in the CRLF2-P2RY8 interval, recently implicated in acute lymphoblastic leukemia (ALL).

The XG and MIC2 genes, encoding the Xg blood group and CD99 antigens, are located at the boundary of PAR1 with sex-specific sequences and appear to be co-regulated by a single locus speculated to map in between. Xga is a dominant character and CD99, which is detected on most human cells using monoclonal antibody, displays substantial red cell quantitative variation. 

Preliminary data indicate the X-borne boundary to be copy-number variable (some men carry at least three copies of a 18kb region of XG) but the true extent of the duplication remains to be established, as does the allele frequency distribution of this CNV and its functional impact on Xga/CD99 levels. These questions could be addressed by the development of a suite of Paralogue Ratio Tests and examining the genotype/phenotype relationship using blood samples from healthy volunteers.

Contact

Dr Celia May
+44 (0)116 252 3032
cam5@le.ac.uk