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Title: Developing nanobodies to stabilise the tumour suppressor protein p16INK4a
Author: Burbidge, Owen David
ISNI:       0000 0004 7653 9678
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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The tumour suppressor protein p16INK4a (p16) is a cyclin-dependent kinase (CDK) inhibitor that plays a key role in the regulation of the cell cycle by controlling the progression of cells through the G1 to S phase transition. Dysregulation of the protein through deletion, silencing or mutation of the gene encoding p16 is implicated in a range of different cancers including melanoma, cervical and oesophageal to name a few. p16 is composed of four ankyrin repeats and it has a very low thermodynamic and kinetic stability and rapidly unfolds even in the absence of denaturants. This low stability means that the protein is highly vulnerable to point mutations, which can result in functional inactivation through a range of different mechanisms such as deletion of key binding contacts, disruption of secondary or tertiary structure and consequent destabilisation leading to unfolding or aggregation. Heavy-chain antibodies are a unique form of antibody devoid of light chains found in the serum of the Camelid family (camels and llamas). Despite the absence of light chains, heavy-chain antibodies have evolved to complement traditional antibodies and retain the full binding capacity seen in canonical IgG antibodies. The single variable domain, known as a nanobody, is, at 15 kDa, the smallest antigen binding fragment, a tenth the size of a standard IgG antibody. The small size and relative ease of production, coupled with an unusually high stability, makes nanobodies useful tools as biological reagents, crystallography chaperones and therapeutics. The research contained within this PhD looks at the development of nanobodies to target p16. By leveraging the high stability of selected nanobodies, the aim was to obtain binders that could stabilise and reactivate a range of unstable cancer-associated mutants. The initial stages of the project focused on generating and optimising the expression and purification of p16 constructs prior to immunisation of animals to raise nanobodies. A high-throughput approach was taken to generate forty-five different p16 constructs with a range of different solubility and purification tags. These constructs were assessed in a multi-factorial expression screen, which resulted in the identification of a p16 construct with a ten-fold improvement in soluble expression levels compared with previous studies. A range of biophysical techniques, including circular dichroism and chemical denaturation, were performed to characterise this protein fully prior to immunisation. The second part of this project utilised a phage display library of two immune nanobody libraries generated against p16 and a p16 variant stabilised by previously published second-site mutations. This process yielded a large number of diverse nanobodies. Biophysical characterisation of these nanobodies was first performed, and they were found to have a range of chemical and thermal stabilities. Assays were then developed to test the ability of the nanobodies to stabilise p16. Two nanobodies were found to dramatically stabilise wild-type p16, with an increase in stability of approximately 44 % and 60 %, respectively. Furthermore, these nanobodies were also able to stabilise a subset of cancer-associated point mutants. Although there are NMR structures of p16, as well as a crystal structure of p16 bound to CDK6, the resolution of is very low, most likely due to the high backbone flexibility of p16. The last part of the project aimed to obtain a higher-resolution structure of p16 by using the two stabilising nanobodies as crystallisation chaperones. The more stabilising of the two nanobodies resulted in crystals that diffracted to a resolution of less than 2 $\AA$, a significant improvement compared with the previously published structure. In conclusion, a number of nanobodies were generated against tumour-associated p16 and shown to be capable of stabilising p16, allowing structure determination to high resolution and restoration of the stability of cancer-associated mutants to wild-type levels. In the project, a range of different approaches for nanobody production were explored, and these will be important for future applications. Moreover, the crystal structure of the p16-nanobody complex showed that the nanobody binds on the opposite face of p16, to the face involved in binding to CDKs; thus, this nanobody could potentially be exploited as a pharmacological chaperone to stabilise and restore the activity of cancer-associated mutant p16 in the cell.
Supervisor: Itzhaki, Laura Sponsor: Cancer Research UK
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: heavy-chain antibody ; antibody ; nanobody ; tumour suppressor ; p16ink4a ; cdkn2a ; p16 ; camelid ; crystallography ; biophysics ; protein stability ; ankyrin repeat ; cell-cycle control