Understanding HPV Strains: Types & Risks

Over 170 types of HPV have been identified, and they are designated by numbers. The most common types of HPV worldwide are 16 (3.2%), 18 (1.4%), 52 (0.9%), 31 (0.8%), and 58 (0.7%).
They may be divided into “low-risk” and “high-risk” types.  Low-risk types cause warts and high-risk types can cause lesions or cancer.

High Risk & Low Risk

• High-risk HPV types can infect cervical cells and cause cervical cancer. They can also infect certain other cells to cause anal cancer, penile cancer, vaginal cancer, vulvar cancer, and oropharyngeal cancer (cancer in the middle of the throat, including the tonsils and the back of the tongue).Cervical cancer is almost always caused by high-risk HPV. Types 16 and 18 two strains account for 70% of cases.

• Low-risk HPV types can cause genital warts. These are warts on the external and internal sex organs and glands. Genital warts do not turn into cancer.Types 6 and 11 are common causes of genital warts and laryngeal papillomatosis

HPV Genotyping: Identifying Specific Strains

Genotyping is a laboratory technique used to identify and distinguish between specific types or strains of the human papillomavirus (HPV).
Purpose of Genotyping: While a standard HPV test detects the presence of the virus, genotyping goes a step further by identifying the exact type of HPV.
Different HPV types have varying levels of risk for developing into cancer.

HPV tests – Variation between tests

Various types of HPV tests are currently available, and they use different technologies. The tests are broadly classified as those that detect the presence of DNA of high-risk HPV coding for L1 viral surface protein (HPV DNA tests) and those that detect the presence of messenger RNA (mRNA) coding for E6/E7 proteins of high-risk HPV types (HPV RNA tests).

Most of the currently available tests can detect the presence of any of the 12 high-risk HPV types. Some of the tests detect the presence of any of these HPV types in the sample without individually identifying the genotypes. Others may individually detect a limited number of genotypes (mostly HPV16 and HPV18) concurrently, with aggregate detection of the other high-risk HPV genotypes. The information on HPV16 and HPV18 received as part of the HPV test report (concurrent genotyping) is used for triage along with VIA, cytology, or colposcopy.

Several techniques are employed for HPV genotyping to identify specific strains of the virus. The choice of technique may depend on factors such as sensitivity, specificity, cost, and the number of HPV types being targeted. Here are some commonly used techniques for HPV genotyping:

Polymerase Chain Reaction (PCR):

• Principle: PCR is a molecular biology technique that amplifies DNA segments, allowing for the detection and identification of specific DNA sequences.
• Application: HPV DNA is amplified using specific primers for targeted regions. The resulting DNA products are then analyzed to identify the HPV types present.

Reverse Line Blot Hybridization (RLB):

• Principle: RLB is a technique that combines PCR amplification with DNA hybridization on a solid support (such as a membrane).
• Application: After PCR amplification of HPV DNA, the products are hybridized to specific probes immobilized on a membrane. The pattern of hybridization indicates the presence of specific HPV types.

DNA Microarray:

• Principle: DNA microarrays are solid supports containing a large number of unique DNA probes arranged in an array.
• Application: HPV DNA is amplified through PCR and then hybridized to the microarray. The pattern of hybridization helps identify the specific HPV types present.

Line Probe Assays (LPA):

• Principle: LPA is a molecular biology technique that combines PCR amplification with the detection of specific DNA sequences using labeled probes.
• Application: PCR-amplified HPV DNA is hybridized to specific probes fixed on a strip or membrane. Detection of labeled DNA indicates the presence of specific HPV types.

Next-Generation Sequencing (NGS):

• Principle: NGS allows for the high-throughput sequencing of DNA, providing detailed information about the entire genome.
• Application: HPV DNA is sequenced, and the obtained sequence data are analyzed to identify specific HPV types. NGS can provide information on multiple HPV types simultaneously.

Pyrosequencing:

• Principle: Pyrosequencing is a sequencing-by-synthesis method that detects the release of pyrophosphate during DNA synthesis.
• Application: After PCR amplification of HPV DNA, pyrosequencing is used to determine the sequence. Specific sequence patterns identify the HPV types.

Single-Strand Conformation Polymorphism (SSCP):

• Principle: SSCP is a technique that exploits the differences in mobility of single-stranded DNA fragments on a gel.
• Application: PCR-amplified HPV DNA is denatured and separated on a gel. Variations in the single-stranded DNA mobility pattern help identify specific HPV types.

The choice of genotyping technique depends on factors such as the level of resolution needed, the number of HPV types to be identified, and the available resources. Each technique has its advantages and limitations, and researchers or clinicians may choose the most suitable method based on the specific requirements of their study or clinical setting.

Prevalence of Each Type

The prevalence of the types of Human Papillomavirus (HPV) is significant throughout the world, being one of the most common sexually transmitted viruses. It is estimated that the vast majority of sexually active people will contract at least one form of HPV at some point in their lives.

HPV types vary in their oncogenic potential, and high-risk types, such as types 16 and 18, are linked to an increased risk of developing cervical and other related cancers.