Characterisation of granulin and its role in neurodegeneration

The proportion of elderly people in our society is increasing. As a result there is an increase in the number of people with age related degenerative diseases that cause dementia. It is known that dementia, in certain circumstances, can be inherited and run in families. Approximately 750,000 people in the UK have some form of dementia, the most common and widely known is Alzheimer?s disease. Thankfully, a lot of progress has been made in understanding the genetic cause of Alzheimer?s disease. The second most common form of dementia is called frontotemporal dementia or FTD. It can affect people?s behaviour and ability to speak. About half of all people with FTD have other family members with this disease, therefore, there is a large inherited (genetic) reason for the development of this disease. We know around 10% of cases of FTD are caused by errors in a gene called tau, however, we know nothing about the remaining 90%. There are reports of families with FTD showing there is a gene problem on chromosome 17, where the tau gene lies. We have just identified this second gene on chromosome 17 that also causes FTD. We believe this will be a very common cause of FTD accounting for up to 20% of cases. This second gene is called granulin and is thought to be involved in wound healing in the skin. However, nothing is known about what granulin does or where it is found in the brain and we are unclear as to why mutations in it cause FTD. We propose to analyse granulin in the brain to see if it correlates with pathology in FTD. Also, we will put mutant copies of granulin into cells, and also reduce it?s levels in cells, to see if we can work out the pathways that are involved in brain cell loss in FTD. This work will help us to gain understanding to the biological reason of this disease. This knowledge will help in diagnosis and in the development of systems to produce a treatment, which will ultimately help people affected by this illness.

Frontotemporal dementia (FTD) is the second most common cause of dementia in people under 65 years. A large proportion of FTD patients (35-50%) have a family history of dementia, consistent with a strong genetic component to the disease. In 1998, mutations in the gene encoding the microtubule associated protein tau (MAPT) were shown to cause familial FTD with parkinsonism linked to chromosome 17q21 (FTDP-17). The neuropathology of patients with defined MAPT mutations is characterized by cytoplasmic neurofibrillary inclusions composed of hyperphosphorylated tau. However, in an increasing number of FTD families with significant evidence for linkage to the same region on chromosome 17q21 (D17S1787-D17S806), mutations in MAPT have not been found and the patients also consistently lack tau-immunoreactive inclusion pathology. In contrast, tau-negative FTD-17 patients have ubiquitin-immunoreactive (ub-ir) neuronal cytoplasmic inclusions and characteristic lentiform ub-ir neuronal intranuclear inclusions. We have demonstrated that in these families, tau-negative FTD is caused by mutations that result in premature termination of the coding sequence for Progranulin (PGRN) likely creating null alleles. PGRN is located 1.7 Mb from MAPT on chromosome 17q21.31 and encodes a 68.5 kDa secreted growth factor, involved in the regulation of multiple processes including wound repair, development and inflammation. However, knowledge of PGRN?s role in the brain in absent. In Manchester we have a unique collection of FTD brains (n=65), 60% of these have ub-ir pathology and 14 of these also possess intranuclear inclusions. We plan to use these in conjunction with tissue from cases with PGRN mutations and controls to investigate the state and levels of PGRN protein and its relationship to pathology. In addition, we will measure levels of PGRN mRNA and splice variants and also assess it?s relationship to pathology. By transfecting cells with appropriate constructs and using immunoprecipitation we will investigate whether binding of known PGRN interacting proteins is affected by PGRN mutations and polymorphisms. Furthermore, we will investigate the effect of reducing PGRN using siRNA on interacting protein levels. Finally, as PGRN is known to bind SLPI and SLPI binds tubulin we will investigate the microtubule network in cells with siRNA reduced levels of PGRN. Collectively, this work will provide the foundation knowledge required for our ultimate understanding of the relationship of PGRN to neurodegeneration in FTD.