**6. Discussion**

#### **6.1. L1 negative dysplastic lesions as proof of a non-productive, but deregulated life cycle**

As already described a tight communication between the virus and the host cell is of critical importance for the viral life cycle. On the one hand it is strictly linked to the epithelial cell differentiation, on the other hand HPV need to modulate the proliferation / differentiation status of the host cells to allow replication in ´non dividing cells´ and the maturation of new infectious virus particles.

*6.2.2. Loss of L1 in precancerous lesions with an episomal HPV genome*

deacetylases in a manner unfavourable to transcription [34].

differentiation and others.

of the HPV genome as well [35], [36].

respective parts of the viral genomes.

dinucleotides within the L1 gene [37].

**6.3. Post transcriptional control**

*6.3.1. Control of the stability of late mRNA*

*6.3.2. Nuclear export of late mRNAs,*

tiation only [38].

L1 expression.

from simple infected to transformed cells.

Control of gene expression by epigenetic modification of distinct DNA sequences is a funda‐ mental biological process, which affects for example embryonic development, cellular

HPV L1 Detection as a Prognostic Marker for Management of HPV High Risk Positive Abnormal Pap Smears

http://dx.doi.org/10.5772/55902

107

One important mechanism, affecting the chromatin conformation, is the methylation of DNA, specifically at cystidine-guanidine (CpG) dinucleotides. Methylated CpG dinucleotides bind repressors, which alter the conformation of nucleosomes through their interaction with histone

In the meantime it's known that epigenetic mechanisms play a major role in the transcription

Several reports showed that the HPV genome is differentially methylated during progression

Alterations were observed particularly in the control region, and the L1 and L2 gene in high grade precancer and invasive cancer. These observations lead to the suggestion that the lack of expression of these genes may be attributed at least in part to increasing methylation of the

Kalantari et al. for example reported that methylation exceeds 50% in the case of some CpG

In addition E2 expression seems to be strictly linked to the differentiation process from normal to malignant cells, indirectly affecting L1 expression as well. Vinokurova showed that E2 binding sites are highly methylated in undifferentiated cells, inhibiting E2-bind‐ ing, and demethylation at the E2 binding sites occurs in association with the cell differen‐

Once the transcription of the late mRNA was successful, additional mechanisms have been

Mori et al. [39] showed that RNA instability elements are within the L1 and L2 coding mRNAs of HPV16, which function in undifferentiated cells. Although the mechanism for RNA destabilization are still subject of further investigations this mechanism could be important for

Koffa et al. [40] reported that the L1 mRNA of HPV16 was retained in the nucleus in undif‐ ferentiated W12 epithelial cells, suggesting that the nuclear export of late mRNAs was

That means different mechanisms are existing to prevent L1 mRNA transcription.

reported that are able to control or block the L1 capsid protein expression.

As long as the L1 capsid protein can be detected within the nucleus of dysplastic cells the virus was successful in this ´walk on the edge´. Despite of all viral activities the cells are still in the condition to allow the normal, productive life cycle of the Human Papilloma Viruses.

L1 capsid protein negative dysplasias, however, are due to this virally induced cellular deregulation processes no longer capable to produce virions.

A shift from a productive HPV infection towards a non-productive or precancerous lesion has occurred.

The reasons for this event are multifarious since the differentiation dependent expression of L1 is controlled at multiple levels. A block at any of the following steps, such as transcription (integration and / or methylation of the DNA), post-transcriptional processing and translation, could be responsible for the loss of L1 capsid protein.

#### **6.2. Transcriptional control**

#### *6.2.1. Loss of L1 due to integration of the HPV genome*

Integration of the viral DNA is considered to be of critical importance for the progression from CIN to cancer, since the frequency of HPV-16 viral integration increase in parallel with the severity of cervical lesions.

During the integration process of the HPV genome into the host chromosome a linearization of the ring – shaped, episomal viral DNA is required. It's easy to imagine, that this non directed event is regularly associated with a deregulation of the strictly controlled episomal DNA. As a consequence of the integration process alterations of the control region and loss or disruption of HPV specific proteins like the early and late proteins can be observed [31].

Even if the L1 gene is not affected directly, the integration of the virus with loss of transcrip‐ tional control by E2 results in over expression of E6 and E7 leading to immortalization and transformation of the cells [32].

As a result, the epithelial host cell remains in the cell cycle and increasingly becomes genetically instable without being able to run its differentiating program.

L1 genes can functionally be inactivated afterwards too, as a result of mutation, gene deletion and insertion as well as DNA methylation so that no capsid protein will be produced anymore (discussed later).

A dysmaturational autonomous tumor emerges in the host epithelium; a ´point of no return´ is crossed.

But for the background that many of the HPV-associated cancers do not even carry any integrated viral genome [31], [33] additional mechanisms have to exist to block L1 expression. In the meantime a discussion started if integration is the initial step towards cervical cancer, or maybe only the consequence of the E6/E7 induced genetic instability of the host cells.
