The unseen battle
The structure that inspired December's image in our 2020 calendar is that of the whole Human Papillomavirus viral capsid. Whilst most people will experience a harmless HPV infection during their life, for some the consequences can be fatal.
Human Papillomaviruses (HPV) cause diseases of the basal epithelium - the foundation layer of cells that line and protect our internal and external tissues. Infections commonly arise in tissues of the genital area, mouth, and throat. However, whilst most cases are asymptomatic and resolved by the immune system without intervention, persistent HPV infections are known to give rise to abnormal cell growth. At the lower end of this spectrum, so-called low-risk HPV genotypes can cause benign growths or warts, whereas high-risk genotypes are associated with cancerous lesions, such as adenocarcinomas.
The most common HPV-related cancer is cervical cancer, a condition that is almost always attributable to HPV. If caught early, cervical cancer is treatable, with good long term survival prospects. Healthcare providers now regularly offer gynaecological screening programmes to help catch asymptomatic HPV infections before cancerous lesions become dangerous. More recently (ca. 2008 in the UK), the availability of HPV vaccines has led to a dramatic decline in the rate of transmission.
A viral vaccine
The HPV virus, a member of the viral family Papillomaviridae, is a non-enveloped, double-stranded DNA virus. This description means that the naked viral capsid itself (Figure 1) represents the transmissible infectious HPV particle, and that the capsid contains circularised double stranded DNA for the production of viral proteins.
Figure 1. The HPV virus. Left: Isolated capsids observed by negative stain electron microscopy (M.H.B. Catroxo and A.M.C.R.P.F.). Center: The entire HPV capsid, PDB ID: 5KEP. Right: One of the 72 L1-protein pentamers comprising the HPV capsid protein (5KEP).
The HPV capsid has a soccer ball-like icosahedral symmetry, wherein the structure as a whole is built from 360 copies of the HPV L1 protein. Individual copies first form pentameric “capsomeres� (Figure 1 right), with 72 capsomeres forming a complete capsid. HPV capsids also contain one copy of another protein, L2, per capsomere. However whilst this is known to play an important role in infection and the packaging of viral DNA, the structure and arrangement of L2 is poorly understood.
As non-enveloped viruses, the immune response raised against simplified viral capsids comprising only L1 proteins is sufficient to generate protective immunity against HPV infection. Of the three HPV vaccines currently available, all use mixtures of L1 virus-like-particles (VLPs) from different HPV variants to provide the best coverage against the most high-risk HPV genotypes.
Figure 2. A cropped section of the HPV16 viral capsid, highlighting the binding between an L1 protein capsomere (purple) and antibody fragments (green/orange) (PDB ID: 6BT3).
Behind enemy lines
HPV infection occurs when the viral capsid reaches basal epithelium and binds to specific host-cell receptors, triggering the internalisation of the virus into the host cell. Weighing in at just ~8 kbp, the HPV genome is notably concise, coding for only eight proteins - which are classed as either Early-phase (E-proteins) or Late-phase proteins (L-proteins), depending on when they are expressed during the viral cycle.
Once the viral DNA is transferred to the nucleus after infection, proteins E1 and E2 are believed to be directly involved in the replication and transcription of the viral genome. Nonetheless, given the minimal size of the HPV genome, DNA replication is still dependent on host cell machinery (e.g. DNA polymerases) and without intervention, replication of the viral genome would therefore be dependent on the host cell’s mitotic cycle. To this end, the early-phase proteins E5-7 act to disrupt the host cell cycle and promote the necessary conditions for continued replication of the HPV genome. Unfortunately, disruption to cell-cycle checkpoints constitutes oncogenic behaviour, and uncontrolled and inescapable mitosis (i.e. apoptosis) often results in the development of cancerous cells.
A further complication arises from the fact that HPV genes regularly become integrated into the host genome. It is unclear whether this is directed by any of the E- proteins, or if it simply results from close-quarters processing within the nucleus. However, such integration has been found to result in the upregulation of E6 and E7, further compounding disruption to the cell cycle and increasing the potential for malignancy.
Figure 3. A crystal structure of the HPV protein E6 bound to ubiquitin ligase E6AP (PDB ID: 4GIZ). The work which described this structure helped illustrate the structural features of E6 that give rise to its oncogenicity.
All known HPV genotypes are thought to be able enter long latency periods, during which no significant viral protein expression occurs, with the E2 protein also acting as a negative transcriptional regulator. However, as long as host cell differentiation is able to take place, the ultimate transition of the infected host cells into the outermost epithelial layer serves as the trigger for late-phase viral processing. Here, the expression of L1 capsid proteins causes the spontaneous formation of capsomeres. L2 proteins are then thought to facilitate both the aggregation of capsomeres into mature viral capsids and the packing of the viral genome, ready for release. At this point, many viruses depend upon the lysis of host cells for release via the “viral-induced cytopathic effect�. However HPV is able to stealthily exploit the natural “desquamation� of epithelial cells, the shedding that marks the end of their 14 day transition from basal membrane to the outer layer. Following this natural cellular disruption, HPV virions are then released into the extracellular milieu and the viral life cycle begins again.
About the artwork
The inspiration for the development of this ceramic piece comes from the structures that we see in living organisms at a microscopic level. Nicholas Malasiotis (Stephen Perse Foundation) was interested in proteins that repair damaged DNA along with Human Papillomavirus (HPV), both of which have an impact in cancer. While most people will be infected with HPV during their lifetime, it is usually dealt with effectively by the immune system. HPV infection increases the replication rate of cells and, on rare occasions, can contribute to the development of cancers. This sculpture is inspired by the structure of HPV, with the perforations on the main body, a stylised simplification of the symmetrical building blocks of the viral capsid. The spike-like appendages reflect how viruses are often perceived.
Structures mentioned in this article
Human Papillomavirus capsid, 5KEP
Human Papillomavirus capsid with V5 antibody fragment, 6BT3
HPV oncoprotein E6 in complex with ubiquitin ligase peptide, 4GIZ
James Tolchard
