BBSRC EASTBIO DTP CASE - The biological basis of the genetic susceptibility to osteoporosis in laying hens

Laying hens produce 58 million tonnes of eggs annually, a highly digestible protein source that has done much to improve world nutrition. However the intense period of egg production can come with correlated problems of limb or keel bone fractures, making this a welfare challenge for the egg industry. The most recent UK survey (Sandilands, 2011) reported bone fracture in 30% of cage and 53% of extensive housed hens prior to depopulation. The biological reasons are a progressive osteoporosis caused by a cessation of structural bone formation and a switch to medullary bone formation under the influence of oestrogen when hens come into lay. Medullary bone is a labile source of calcium for shell formation but contributes less structural integrity. The osteoclasts that mobilise calcium from the medullary bone for shell formation also remain active on the structural cortical bone and both bone types are resorbed during the egg production cycle, leading to increased bone fragility. Genetics explains the largest portion of variation (40%) in osteoporosis and bone fracture (Dunn et al., 2007, Bishop et al., 2000). Breeding with a retrospective index improved tibial strength, without effects on production (Bishop et al., 2000). In an F2 cross a QTL for bone strength was found (Dunn et al., 2007) which we refined and confirmed in a commercial line with SNPs.

The best SNP explained a large difference in tibial breaking strength; 209 versus 276 Newtons. We have discovered a very strong candidate gene by comparing the bone transcriptome using NGS. However there is more genetic variation to account for. With collaborators we have completed a genome wide association study of the commercial population to capture more variation and identify markers explaining variation for bone strength. In funded projects we are advancing the use of these markers and other methods of delivering improvement in bone strength. However we do not currently have resources to probe the biological basis for the effect of the genetic loci on osteoporosis. What we would seek in the proposed project is a greater understanding of the mechanism by which the genetic basis of the QTL’s for osteoporosis in hens is mediated. We have a clear candidate in respect of the QTL on chromosome 1 and we have developed methods to characterise the genes function. We specifically wish to test hypothesis on how this gene influences bone strength in vivo and in vitro. From our new GWAS studies we have strong signals at other loci and these will also be available to exploit. As we develop new phenotypes in our funded project there may also be options to understand their application. The successful candidate would benefit from the considerable expertise that currently exists at the institute in bone biology, Heather McCormack and Bob Fleming have published many papers in this field, the Laboratory of bone expert Colin Farquharson and Barry Thorpe who is a partner in St David’s veterinary practice and specialised in his PhD in avian bone disorders and pathology and has comprehensive industrial experience. We aim to train the next generation of expert in avian bone biology and pathology. Techniques which will be used in understanding the biology of the gene on bone strength include compression testing, histo-morphometry, in vitro differentiation of osteoclasts from different genotypes, immunohistochemistry and histology, biochemical assays and protein expression. We also have a collaboration in our funded projects with a bio-mineralogist who is using a range of analytical techniques to characterize bone composition and structure; optical emission spectroscopy (Ca/P ratio, Mg content); infrared spectrometry for mineralization, collagen cross-linking and mineral crystallinity which opens further training options. This is a field in which there is a huge need for scientists that understand the unique nature of avian bone and who can continue to deliver the research and the solutions in the next 30 years to the industry. However we now have the advantage of access to the latest techniques and result from genetics to begin to get insights on the factors that determine bone strength. We have a strong relationship with Lohamnn Tierzucht as a primary breeder to ensure outputs are utilised.