The cervix-specific proteome

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Figure 1. The distribution of all genes across the five categories based on transcript abundance in cervix as well as in all other tissues.

154 genes show some level of elevated expression in the cervix compared to other tissues. The three categories of genes with elevated expression in cervix compared to other organs are shown in Table 1. In Table 2, the 5 enriched genes are defined.

Table 2. The 5 genes with the highest level of enriched expression in cervix. "Predicted localization" shows the classification of each gene into three main classes: Secreted, Membrane, and Intracellular, where the latter consists of genes without any predicted membrane and secreted features. "mRNA (tissue)" shows the transcript level as TPM values, TS-score (Tissue Specificity score) corresponds to the score calculated as the fold change to the second highest tissue.

Some of the proteins predicted to be membrane-spanning are intracellular, e.g. in the Golgi or mitochondrial membranes, and some of the proteins predicted to be secreted can potentially be retained in a compartment belonging to the secretory pathway, such as the ER, or remain attached to the outer surface of the cell membrane by a GPI anchor.

The cervix transcriptome

An analysis of the expression levels of each gene made it possible to calculate the relative mRNA pool for each of the categories. The analysis shows that 80% of the mRNA molecules derived from cervix correspond to housekeeping genes and only 9% of the mRNA pool corresponds to genes categorized to be either cervix enriched, group enriched or, cervix enhanced. Thus, most of the transcriptional activity in the cervix relates to proteins with presumed housekeeping functions as they are found in all tissues and cells analyzed.

Protein expression of genes elevated in cervix

In-depth analysis of the elevated genes in cervix using antibody-based protein profiling allowed us to visualize where these proteins are expressed within the cervix including glandular cells, stromal cells and squamous epithelial cells.

Proteins related to secreted expression in cervix

67 genes elevated in the cervix are predicted secreted proteins. The cervical mucus has several functions including acting as a protective barrier, and preventing bacteria from entering the uterine body. MUC16 is expressed in the cervix and is thought to be involved in forming a barrier protecting from pathogens. SLPI, antileukoproteinase, is another protein that is found in cervical mucus with antimicrobial properties. WFDC2, a protease inhibitor with antiviral properties, is highly expressed in cervical mucus.

Proteins related to expression in squamous epithelial cells in cervix

Examples of proteins with elevated expression in the squamous epithelium of cervix (ectocervix) include SERPINB3, which may act as a papain-like cysteine protease inhibitor, and ANKRD35, an undefined protein where expression is shared between cervix and other tissues with squamous epithelial cells.

Proteins related to expression in stromal cells in cervix

The cervical stroma is composed of smooth muscle fibers embedded in collagen. Examples of genes with elevated expression in cervical stroma cells include MITF, TGM2 and ESR1. MITF is a transcription factor that regulates the expression of genes involved in cell differentiation, proliferation and survival. TGM2, an enzyme that catalyses the crosslinking of proteins and the conjugation of polyamines to proteins. ESR1, estrogen receptor, is a nuclear hormone receptor important for sexual development and reproductive function. ESR1 is expressed in the cervix and other female tissues.

Proteins related to expression in glandular cells in cervix

Examples of proteins expressed in cervical glands (endocervix) include HTR2B, a G-protein coupled receptor for serotonin, MSX1, a transcriptional repressor, and PRR15, a poorly characterized protein that may be involved in proliferation and differentiation.

Genes shared between the cervix and other tissues

There are 42 group enriched genes expressed in the cervix. Group enriched genes are defined as genes showing a 5-fold higher average level of mRNA expression in a group of 2-7 tissues, including cervix, compared to all other tissues.

In order to illustrate the relation of cervix to other tissue types, a network plot was generated, displaying the number of genes shared between different tissue types.

Figure 2. An interactive network plot of the cervix enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of cervix enriched genes and orange nodes represent the number of genes that are group enriched. The sizes of the red and orange nodes are related to the number of genes displayed within the node. Each node is clickable and results in a list of all enriched genes connected to the highlighted edges. The network is limited to group enriched genes in combinations of up to 3 tissues, but the resulting lists show the complete set of group enriched genes in the particular tissue.

Cervix shares the largest number of group enriched genes with endometrium. An example of a shared gene between cervix and endometrium is TRH, pro-thyrotropin-releasing hormone, which is involved in hormone-mediated signaling pathways, and produced by the hypothalamus. TRH regulates the biosynthesis of TSH in the anterior pituitary gland.

Cervix also shares several group enriched genes with epididymis. SCGB2A1, mammaglobin-B, bind androgens and other steroid hormones and is expressed in both cervix and epididymis.

Cervix function

The endocervix produces cervical mucus every day, with a peak around ovulation. The mucus viscosity changes during the menstrual cycle allowing it to act as either a barrier or a transport medium for sperm. The viscosity is determined by the hormones estrogen and progesterone. During ovulation, when estrogen levels are high, the mucus is thin, allowing sperm to enter the uterus. At other times in the cycle, the effect of progesterone makes the mucus thick, creating a barrier to sperm. The mucus also prevents pathogens from reaching the uterine body.

During pregnancy a cervical mucus plug is formed to protect against pathogens and prevent the leakage of fluids. The cervix also supports the fetal head as it descends in preparation for birth. The support gives way during labour when the uterus begins to contract. At this time the cervix, which is usually 2-3 cm, dilates to approximately 10 cm, by softening and increasing the elasticity in the cervical stroma.

Cervix histology

The cervix measures around 2–3 centimeters in length. It is part of the female reproductive system situated between the vagina and the uterine body. The cervix is composed of fibrous, muscular and elastic tissue lined with columnar and squamous epithelium. The lower part of the cervix is known as the ectocervix and is composed of squamous epithelial cells, resembling the vaginal epithelium. The cervical canal, connecting the vagina with the uterine body, is lined with columnar epithelial cells and contains underlying glandular structures. This is known as the endocervix. The mucosa covering the ectocervix is known as the exocervix. The junction between the endocervix and the ectocervix is called the squamocolumnar junction. This junction is where most cervical cancers arise. Human Papilloma Virus (HPV) are the cause of nearly all cervical cancers.

The histology of human cervix, uterine including detailed images and information about the different cell types can be viewed in the Protein Atlas Histology Dictionary.

Background

Here, the protein-coding genes expressed in the cervix are described and characterized, together with examples of immunohistochemically stained tissue sections that visualize protein expression patterns of proteins that correspond to genes with elevated expression in the cervix.

Transcript profiling and RNA-data analyses based on normal human tissues have been described previously (Fagerberg et al., 2013). Analyses of mRNA expression including over 99% of all human protein-coding genes was performed using deep RNA sequencing of 172 individual samples corresponding to 37 different human normal tissue types. RNA sequencing results of 2 fresh frozen tissues representing normal cervix was compared to 170 other tissue samples corresponding to 36 tissue types, in order to determine genes with elevated expression in cervix. A tissue-specific score, defined as the ratio between mRNA levels in cervix compared to the mRNA levels in all other tissues, was used to divide the genes into different categories of expression. These categories include: genes with elevated expression in cervix, genes expressed in all tissues, genes with a mixed expression pattern, genes not expressed in cervix, and genes not expressed in any tissue. Genes with elevated expression in cervix were further sub-categorized as i) genes with enriched expression in cervix, ii) genes with group enriched expression including cervix and iii) genes with enhanced expression in cervix.

Human tissue samples used for protein and mRNA expression analyses were collected and handled in accordance with Swedish laws and regulation and obtained from the Department of Pathology, Uppsala University Hospital, Uppsala, Sweden as part of the sample collection governed by the Uppsala Biobank. All human tissue samples used in the present study were anonymized in accordance with approval and advisory report from the Uppsala Ethical Review Board.

Relevant links and publications

Uhlén M et al, 2015. Tissue-based map of the human proteome. Science
PubMed: 25613900 DOI: 10.1126/science.1260419

Yu NY et al, 2015. Complementing tissue characterization by integrating transcriptome profiling from the Human Protein Atlas and from the FANTOM5 consortium. Nucleic Acids Res.
PubMed: 26117540 DOI: 10.1093/nar/gkv608

Fagerberg L et al, 2014. Analysis of the human tissue-specific expression by genome-wide integration of transcriptomics and antibody-based proteomics. Mol Cell Proteomics.
PubMed: 24309898 DOI: 10.1074/mcp.M113.035600

Histology dictionary - the cervix, uterine