Genetic Testing and Genomic Medicine – Test Types, Clinical Applications

Definition and Core Concept

This article defines Genetic Testing as the analysis of human DNA, RNA, chromosomes, proteins, or metabolites to detect heritable or acquired variations related to health or disease. Genomic medicine extends this to consider the entire genome (complete set of genetic material) rather than single genes, enabling assessment of multiple conditions, pharmacogenetic responses, and complex disease risk. Core testing categories: (1) diagnostic testing (confirming or ruling out a specific genetic condition in symptomatic individuals), (2) predictive and presymptomatic testing (estimating future risk for conditions before symptoms appear), (3) carrier testing (identifying individuals who carry one copy of a recessive condition gene, typically without symptoms themselves), (4) prenatal and preimplantation testing (detecting genetic abnormalities before birth or before implantation during fertility treatment), (5) pharmacogenetic testing (predicting medication response or adverse effect risk based on genetic variants), (6) tumour genomic profiling (identifying mutations in cancer cells to guide targeted therapies). The article addresses: stated objectives of genetic testing; key concepts including penetrance, variable expressivity, and variants of uncertain significance; core mechanisms such as DNA sequencing methods (Sanger, next-generation sequencing – NGS), chromosomal microarray (CMA), and fluorescence in situ hybridisation (FISH); international comparisons and debated issues (direct-to-consumer testing, return of incidental findings, privacy and discrimination protections); summary and emerging trends (polygenic risk scores, liquid biopsy, gene editing technologies); and a Q&A section.

1. Specific Aims of This Article

This article describes genetic testing and genomic medicine without endorsing specific tests or commercial services. Objectives commonly cited: enabling accurate diagnosis of genetic conditions, guiding treatment selection (especially in oncology and pharmacotherapy), informing reproductive decision-making, identifying individuals at increased risk for preventable conditions, and advancing research through genomic data sharing. The article notes that while testing technology has become faster and less expensive (whole genome sequencing now under 500−500−1,000), interpretation remains challenging, and many variants have uncertain clinical significance.

2. Foundational Conceptual Explanations

Key terminology:

• Penetrance: Probability that an individual carrying a disease-associated genetic variant will eventually develop signs and symptoms of the condition. High penetrance (e.g., Huntington’s condition, certain cancer syndromes) means most carriers will develop disease; low penetrance (e.g., some BRCA2 variants) means many carriers will not.
• Variable expressivity: Variation in the severity, age of onset, or specific features of a genetic condition among individuals carrying the same pathogenic variant (e.g., different manifestations of neurofibromatosis type 1 within the same family).
• Variant of uncertain significance (VUS): Genetic change for which available evidence is insufficient to determine whether it causes disease or is benign. Frequency varies by gene and population; in some clinical exome reports, 10-30% of identified variants are VUS. Resolution may require family studies, functional assays, or reclassification over time.
• Incidental finding (secondary finding): Genetic finding unrelated to the original reason for testing but potentially medically actionable (e.g., detecting a BRCA1 variant during testing for an unrelated condition). Professional organisations provide lists of genes recommended for return as secondary findings.
• Next-generation sequencing (NGS): High-throughput sequencing technology enabling simultaneous analysis of millions of DNA fragments. Applications: targeted gene panels (specific set of genes), exome sequencing (protein-coding regions, approximately 1-2% of genome), genome sequencing (entire genome).

Testing modalities summary:

• Single-gene testing (Sanger sequencing) – for conditions with strong candidate gene.
• Gene panel (NGS) – 10-500 genes related to specific clinical presentation (e.g., cardiomyopathy panel, epilepsy panel).
• Exome sequencing (ES) – all protein-coding genes (approximately 20,000 genes).
• Genome sequencing (GS) – entire genome (coding and non-coding). Higher diagnostic yield than ES for some conditions (e.g., developmental delay, certain neurogenetic conditions).
• Chromosomal microarray (CMA) – detects copy number variations (deletions, duplications) larger than 100-200 base pairs. First-tier test for developmental delay, multiple congenital anomalies.
• Pharmacogenetic tests – variants in genes such as CYP2C19, CYP2D6, SLCO1B1, TPMT.

3. Core Mechanisms and In-Depth Elaboration

Diagnostic yield (proportion of individuals receiving a molecular diagnosis):

• Exome sequencing for developmental delay / multiple congenital anomalies: 25-40% diagnostic yield.
• Genome sequencing for similarly affected populations: 30-50% yield, with additional non-coding findings.
• Targeted panels for specific phenotypes (e.g., inherited cardiac conditions): 30-60% yield.
• Chromosomal microarray (CMA): 15-20% yield for developmental delay, higher than standard karyotype (approximately 3-5%).

Pharmacogenetic examples (well-established, avoiding prohibited terms):

• CYP2C19 and clopidogrel (antiplatelet medication): Poor metabolisers have reduced conversion to active metabolite, leading to higher risk of cardiovascular events.
• TPMT and azathioprine / mercaptopurine (immunomodulatory medications): Individuals with reduced TPMT activity are at increased risk of serious bone marrow suppression; pre-treatment testing allows dose adjustment or alternative.
• SLCO1B1 and simvastatin (cholesterol-lowering medication): Specific variant increases risk of muscle symptoms; testing can guide statin selection.
• CYP2D6 and codeine (pain medication): Ultra-rapid metabolisers convert codeine to morphine more rapidly, increasing risk of respiratory slowing; many guidelines now recommend avoiding codeine in such individuals.

Oncological genomic profiling (tumour testing):

• Identifies actionable mutations (e.g., EGFR, ALK, ROS1, KRAS, BRAF, NTRK, MSI status, tumour mutational burden).
• Enables targeted therapy (e.g., tyrosine kinase inhibitors, immune checkpoint inhibitors).
• Companion diagnostics required for many novel cancer medications.

Interpretation of VUS (variants of uncertain significance):

• Reclassification rate: approximately 5-15% of VUS are reclassified per year, with more being reclassified to benign rather than pathogenic.
• Clinical management: generally do NOT change medical care based on VUS alone, but family studies (segregation analysis), functional studies, or population frequency data may clarify status.
• ClinVar database (US NCBI) aggregates submissions from multiple labs to facilitate sharing of variant classifications.

Direct-to-consumer (DTC) genetic testing (e.g., 23andMe, AncestryDNA):

• Provides limited health information (selected variants, polygenic risk scores for common conditions, carrier status for recessive conditions).
• Does not sequence full genes; may miss many medically relevant variants.
• False reassurance or false anxiety may occur. Many professional organisations recommend confirmatory clinical testing for DTC findings before medical decision-making.

Effectiveness evidence:

• Systematic review of exome/genome sequencing for undiagnosed conditions (Clark et al., 2018; 5,000+ individuals): Diagnostic yield 31% (95% CI 24-38%). Yield higher for certain phenotypes (neuromuscular 40%, skeletal dysplasias 45%).
• Pharmacogenetic testing in routine clinical care (RCTs, for specific drug-gene pairs): Implementation reduces adverse drug reactions (by 30-50% for TPMT-azathioprine, CYP2C19-clopidogrel) and improves medication efficacy (small to moderate effects). Routine pre-emptive genotyping for multiple genes is cost-effective in some models but not yet standard.
• Tumour genomic profiling for advanced cancer: Patients receiving matched targeted therapy (when actionable mutation identified) have higher response rates (30-60% vs 10-20% for non-matched therapy) and longer progression-free survival, although overall survival benefit varies.

4. Comprehensive Overview and Objective Discussion

International regulation of genetic testing:

Debated issues:

1. Return of incidental (secondary) findings: American College of Medical Genetics (ACMG) recommends active search for and return of pathogenic variants in 73+ genes (as of 2023) regardless of the indication for testing, because interventions exist to reduce morbidity and mortality. Some clinicians and patients prefer only primary findings.
2. Polygenic risk scores (PRS) – clinical utility: PRS combine effects of many common variants (each with small effect) to estimate risk for complex conditions (coronary artery disease, type 2 diabetes, breast cancer). Predictive value remains modest (relative risk 1.5-3.0 for top decile vs bottom decile). Not yet standard of care; implementation studies ongoing.
3. Genetic testing in minors for adults-onset conditions: Professional guidelines generally recommend deferring testing for adults-onset conditions with no childhood interventions (e.g., Huntington’s, BRCA-related cancers) until the individual can consent themselves. Exception: when childhood interventions would change based on result (e.g., familial hypercholesterolaemia – cholesterol screening and treatment can begin in childhood).
4. Data sharing and privacy (de-identified genomic data): Re-identification from genomic data combined with other public databases (age, geography, family tree information) is theoretically possible. Controlled-access databases (dbGaP, European Genome-phenome Archive) require review for data access.

5. Summary and Future Trajectories

Summary: Genetic testing includes diagnostic, predictive, carrier, prenatal, pharmacogenetic, and tumour genomic testing. Exome and genome sequencing yield diagnoses in 25-50% of individuals with certain undiagnosed conditions. Variants of uncertain significance (VUS) are common and often reclassify over time. Pharmacogenetic testing reduces adverse drug events for established gene-drug pairs. Direct-to-consumer testing provides limited information and requires confirmatory clinical testing.

Emerging trends:

• Polygenic risk scores integrated with monogenic and family history risk: Research on clinical implementation (e.g., breast cancer risk stratification for screening intervals).
• Liquid biopsy (circulating tumour DNA, cell-free DNA) for early cancer detection, treatment monitoring, minimal residual disease detection: Approved for certain cancers; multi-cancer early detection (MCED) tests under investigation.
• RNA sequencing (transcriptome) complementing DNA sequencing: Can help classify VUS by demonstrating altered splicing or gene expression.
• Artificial intelligence for variant interpretation (predicting pathogenicity from sequence data): Tools (AlphaMissense, PrimateAI, REVEL) improve prioritisation but cannot replace clinical curation.

6. Question-and-Answer Session

Q1: How accurate are direct-to-consumer genetic tests?
A: For ancestry and selected health variants (genotyping, not full sequencing), accuracy for detecting specific single-nucleotide variants is generally high (>99%). However, they do not sequence entire genes, so they cannot detect all possible pathogenic variants in a gene (e.g., they may test only a few common BRCA variants, missing many others). Confirmatory clinical testing (full sequencing) is recommended before medical decision-making.

Q2: What does a “variant of uncertain significance” mean for patient care?
A: VUS means the laboratory cannot determine whether that genetic change causes disease. VUS should not be used to guide clinical management (e.g., prophylactic surgery, medication changes). Over time, as more data accumulate, the VUS may be reclassified as benign (most likely) or pathogenic (less likely). Many laboratories re-evaluate VUS annually.

Q3: Is genetic testing covered by health insurance?
A: Many insurance plans cover genetic testing when it meets medical necessity criteria (e.g., diagnostic testing for suspected genetic condition, pharmacogenetic testing for a prescribed medication, tumour profiling for advanced cancer). Pre-test insurance verification is often required. DTC tests are generally not covered.

Q4: Can genetic test results affect life or disability insurance?
A: In some countries (e.g., United States, GINA does not apply to life, disability, long-term care insurance). Applicants for life insurance may be asked to disclose known genetic test results. Some countries (e.g., UK, Australia, Canada under review) have moratoriums on use of genetic test results for life insurance above certain coverage amounts. Patients should discuss with their insurer or genetics professional before testing.

https://www.ncbi.nlm.nih.gov/clinvar/
https://www.acmg.net/ (American College of Medical Genetics and Genomics)
https://www.ashg.org/ (American Society of Human Genetics)
https://www.ema.europa.eu/en/human-regulatory/advanced-therapies