Cancer Genetics

Genetic Variants and Cancer Risk

Genetic variants play a significant role in determining an individual's risk for developing certain cancers. These variants can be classified based on their frequency in the population and their impact (penetrance) on disease risk.

• Rare, Highly Penetrant Variants: These are uncommon in the population but have a large effect on disease risk. BRCA1 and BRCA2 mutations are classic examples, significantly increasing breast cancer risk from the general population risk (~12%) to as high as 60% or more.

• Common, Low Penetrant Variants: These variants are frequent but have minimal impact on disease risk. For example, a common variant may increase breast cancer risk from 12% to 12.5% or 13%.

• Intermediate Variants: Variants such as CHEK2 or PALB2 are less common and moderately increase risk, but not as much as BRCA1/2.

Penetrance refers to the proportion of individuals with a particular genetic variant who exhibit the associated phenotype or disease. High penetrance means most carriers are affected; low penetrance means few are.

Example: A woman with a pathogenic BRCA1 mutation may have a breast cancer risk of 60%, compared to 12% in the general population.

Molecular Genetic Tools

Direct-to-Consumer (DTC) Genetic Testing

Direct-to-consumer genetic testing allows individuals to access genetic information without a healthcare provider. These tests typically analyze specific single nucleotide polymorphisms (SNPs) and provide information about ancestry, disease risk, and traits.

• Reasons for DTC Testing: Entertainment, education, finding relatives (e.g., adoption, ART), identifying disease risk (especially when family history is unknown), insight into interventions, low cost, convenience, and research participation.

• Limitations: DTC tests are not comprehensive; they usually analyze selected SNPs rather than sequencing entire genes. They may not detect all relevant variants and do not account for non-genetic factors.

• Clinical vs. DTC Testing: Clinical-grade testing is more comprehensive, often involves blood draws, and includes interpretation by clinical geneticists. DTC testing is typically done at home, often using saliva samples.

Example: DTC tests may only screen for a few mutations in BRCA1/2, missing hundreds of other possible pathogenic variants.

Cancer Genetics

Risks and Limitations of DTC Genetic Testing

While DTC genetic testing offers accessibility, it comes with significant risks and limitations.

• False Positives and Negatives: DTC tests may incorrectly report the presence or absence of variants. Without clinical interpretation, consumers may misunderstand results.

• Misinterpretation: Both consumers and healthcare providers may misinterpret DTC results, leading to false reassurance or unnecessary anxiety.

• Selection Bias: Early studies overestimated risk by testing only affected families. Broader population screening has shown lower penetrance for some variants.

• Psychological Impact: Unexpected results can cause stress, anxiety, or identity issues (e.g., ancestry findings).

• Inappropriate Actions: Consumers may change health behaviors based on incomplete or inaccurate information.

• Legal and Consent Issues: DTC tests lack chain of custody and informed consent, limiting their use in legal or clinical contexts.

Example: A negative DTC test for BRCA mutations may falsely reassure a patient, even though only a few mutations were tested.

Cancer Genetics

Reliability of DTC Genetic Testing: Study Findings

A 2018 study evaluated the reliability of DTC genetic testing by comparing DTC results with clinical-grade confirmatory testing.

• False Positive Rate: 40% of variants reported by DTC tests were not confirmed by clinical-grade testing.

• Interpretation Errors: Of confirmed variants, 15% were not interpreted as pathogenic by clinical labs.

• Clinical Impact: 80% of variants found were in genes related to cancer or cardiac disorders, potentially causing unnecessary concern.

Example: A DTC test may report a cancer risk variant, but clinical testing may show the variant is absent or not pathogenic.

Quantitative Genetics

Penetrance and Disease Risk Calculation

Penetrance is a key concept in quantitative genetics, describing the likelihood that a genetic variant will result in disease. Disease risk can be calculated using penetrance values.

• General Population Risk: For breast cancer, the risk is approximately 12%.

• Risk with Pathogenic Variant: For BRCA1/2 mutations, risk may be 60% or higher.

Formula:

Example: If a variant has a penetrance of 0.6 and frequency of 0.01, the absolute risk increase is 0.6%.

Population Genetics

Variant Frequency and Population Impact

Population genetics examines how variant frequency affects disease risk across populations. Rare, highly penetrant variants have large effects on individuals but minimal impact on population-level disease rates. Common, low penetrant variants may slightly increase risk for many people.

• Rare Variants: High impact, low frequency.

• Common Variants: Low impact, high frequency.

Example: BRCA1/2 mutations are rare but significantly increase individual risk; common variants may only slightly increase risk.

Table: Comparison of Genetic Variant Types

Additional info: Table entries inferred from context and standard genetics knowledge.