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The Integration of Inherited Genetics into Clinical Practice¶

Chapter 480 | Part 16: Genes, the Environment, and Disease · Parts 15-16 – Genetics, Genomics & Precision Medicine · Chapter 480

Key Clinical Points¶

Family history is the cornerstone of genetic risk assessment; a cluster of related conditions suggests inherited risk.
Multiplex gene panels and whole exome/genome sequencing are replacing single-gene testing, increasing the risk of finding Variants of Uncertain Significance (VUS).
Direct-to-consumer (DTC) testing often lacks CLIA/CAP accreditation and should not be used for medical management without clinical validation.
Genetic testing for adult-onset disorders should generally not be offered to children unless management options are available (e.g., MEN2, FAP).
A negative genetic test result is often uninformative if the specific familial pathogenic variant is not known; management should still follow general population guidelines.
The Genetic Information Nondiscrimination Act (GINA) protects against health and employment discrimination but does not cover life, disability, or long-term care insurance.
Testing should ideally begin with an affected family member to identify the pathogenic variant before testing asymptomatic relatives.
Polygenic risk scores (PRS) combine small effect SNPs to assess disease risk, but their clinical utility for improving patient outcomes remains uncertain.
Population screening (e.g., Ashkenazi Jewish BRCA founder mutations) is a specific consideration for certain ethnic groups.
Management varies by disorder and includes surveillance, risk-reducing surgery, chemoprevention, and specific pharmacologic interventions.

1. DEFINITION & OVERVIEW¶

• Genetic Integration: The application of screening and prevention strategies based on genetic risk assessment. ◦ Germline Alterations: - Chromosomal abnormalities. - Specific gene pathogenic variants (mutations) with autosomal dominant or recessive patterns. - Single nucleotide polymorphisms (SNPs) with small relative risks associated with disease. ◦ Scope of Impact: - Germline alterations are responsible for disorders beyond classic Mendelian conditions. - Includes genetic susceptibility to common adult-onset diseases: asthma, hypertension, diabetes mellitus, macular degeneration, and various cancers. ◦ Complexity Factors: - Complex interplay of multiple genes and environmental factors affects lifetime risk, age of onset, disease severity, and treatment options. - Expansion of genetic knowledge is changing the understanding of pathophysiology and classification of diseases. ◦ Role of the Clinician: - Primary care physicians are essential in helping patients navigate testing and treatment. - Clinicians must incorporate personal and family history to determine risk for specific pathogenic variants and provide counseling. - Transition from single-gene testing to multigene panels, whole exome, and genome sequencing increases complexity of interpretation.

2. EPIDEMIOLOGY¶

• Ethnic Prevalence: - Ashkenazi Jewish: 2.5% of individuals carry one of three founder pathogenic variants in BRCA1 and BRCA2. - Factor V Leiden: More common in Caucasians than in Africans or Asians. ◦ Polygenic Risk vs. High-Penetrance: - Many adult-onset disorders are multifactorial. - Polygenic risk is conferred by multiple loci (SNPs) with small effects (typically relative risks <1.5). - Comparison of Risks: - SNP risk alleles are more common than high-penetrance germline mutations. - However, the cumulative lifetime risk from SNPs is modest, whereas risk from high-penetrance variants (e.g., BRCA1/2) is significant.

3. ETIOLOGY & PATHOPHYSIOLOGY¶

• Inheritance Patterns: - Autosomal Dominant (AD): - High penetrance, but incomplete. - Examples: Lynch syndrome (MLH1, MSH2, MSH6, PMS2, EPCAM) and Hereditary breast/ovarian cancer (BRCA1, BRCA2). - Lower Penetrance Example: CHEK2 (20–30% risk for breast cancer) vs. BRCA1/2 (50–70% risk). - Autosomal Recessive (AR): - Two mutant alleles required for full expression. - Examples: Hemochromatosis and MUTYH-associated polyposis. - Pediatric examples: Lysosomal storage diseases, cystic fibrosis. - Multifactorial/Polygenic: - Risk from multiple loci (SNPs) with small effects. - Locus Heterogeneity: - Multiple patterns of inheritance for a single disease. - Example: Colon cancer can be associated with Lynch syndrome (AD), MUTYH (AR), or multiple SNPs (polygenic). ◦ Gene-Environment Interaction: - Healthy lifestyles can mitigate risk in polygenic diseases, such as cardiovascular disease. ◦ Pedigree Analysis & Phenocopies: - Personal and family histories provide insight into inheritance patterns. - Phenocopies: Family members without the susceptibility allele may still develop the phenotype; these are confounding variables in pedigree analysis.

4. CLINICAL FEATURES¶

• Family History Assessment: - Cornerstone of risk assessment; clusters of related conditions suggest inherited risk. - Identification of specific pathogenic variants through pedigree analysis. ◦ Case Study Analysis (Fig 480-1): - Proband (36-year-old woman) with strong history of breast/ovarian cancer on paternal side. - Strategy: Test the affected living relative first (e.g., a cousin) to identify the specific variant before testing the proband or other relatives. - Outcome of Testing: - If cousin is positive for a known variant, it can be tested in the father; if he is positive, there is a 50:50 chance of transmission to the proband. - If father is negative (true-negative), the proband and brother are not at risk for that specific variant.

5. DIFFERENTIAL DIAGNOSIS¶

• Locus Heterogeneity: - Different genes can cause similar phenotypes, complicating assessment. - Example: Colon cancer may result from different genetic mechanisms (AD, AR, or polygenic).

6. INVESTIGATIONS & DIAGNOSIS¶

• Methodologic Approaches: - DNA sequence analysis is primary for identifying variants. - RNA/protein studies can provide additional information (e.g., immunohistochemistry to check protein expression in Lynch syndrome). ◦ Direct-to-Consumer (DTC) Testing: - Requirements for Clinical Labs: Must be accredited by CLIA, state, and/or other agencies; accreditation must be on the report. - Patient Information Requirements: - What the test can or cannot determine about health/risk. - Potential for results that neither confirm nor exclude disease. - Potential for unexpected results unrelated to the indication. - Potential impact of a result on relatives. - Clinical Warning: DTC tests often lack clinical validation and should not be used for medical management without confirmation.

7. MANAGEMENT & TREATMENT¶

Testing Strategy:
Identify candidates via pedigree analysis.
Determine if testing is indicated based on risk factors (e.g., young age of onset, multiple relatives).
Decision Pathways (Flowchart 480-2):
Pathway A (Specific Genes): Patient → Pretest counseling (specific genes) → Mutational analysis → [If result is negative → Consider next most likely genes] → Posttest counseling → Primary care.
Pathway B (Broad/Whole Genome): Patient → Pretest counseling (genes or whole genome) → Mutational analysis → Interpretation by physician → Posttest counseling → Primary care.
Intervention Strategies (Table 480-2):
Oncologic:
Lynch syndrome: Early endoscopic screening, risk-reducing surgery, aspirin.
Familial adenomatous polyposis: Early/frequent endoscopy, prophylactic colectomy, chemoprevention.
Hereditary breast and ovarian cancer: Risk-reducing salpingo-oophorectomy, intensified surveillance (MRI), mastectomy, PARP inhibitors.
Hereditary diffuse gastric cancer: Prophylactic gastrectomy, enhanced breast cancer surveillance.
Hematologic:
Factor V Leiden: Management of thrombotic risk.
Hemophilia A/B: Specific factor replacement/management.
G6PD deficiency: Clinical management of hemolysis.
HBB (Beta-thalassemia): Management of anemia.
Cardiovascular:
Hypertrophic cardiomyopathy: Echocardiographic screening, pharmacologic intervention, myomectomy.
Long QT syndrome: Electrocardiographic screening, pharmacologic intervention, implantable cardiac defibrillator devices.
Marfan’s syndrome: Echocardiographic screening, prophylactic beta blockers, aortic valve replacement as indicated.
Gastrointestinal:
MEFV (Familial Mediterranean Fever) and HFE (Hemochromatosis): Management as indicated.
Pulmonary:
α1-Antitrypsin deficiency: Avoidance of smoking/toxins.
Cystic fibrosis: Chest physiotherapy, airway clearance agents, CFTR modulators, lung transplantation.
Endocrine:
AVP, CASR, RET: Management as indicated.
Renal:
Polycystic kidney disease (PKD1, PKD2, PKHD1): Prevention of hypertension and UTI, kidney transplantation.
Nephrogenic diabetes insipidus (AVPR2, AQP2): Fluid replacement, thiazides with or without amiloride.
Neurologic:
RYR1, CACNA1S, SCN4A, DMD, ATP7B: Management as indicated.

8. PROGNOSIS & COMPLICATIONS¶

• Limitations of Genetic Testing: - Risk of finding Variants of Uncertain Significance (VUS) with multigene panels/WES/WGS. - Negative results are often uninformative if the specific familial pathogenic variant is unknown.

9. SPECIAL CONSIDERATIONS¶

• Legal and Ethical Considerations: - GINA Protections: Protects against health and employment discrimination; does NOT cover life, disability, or long-term care insurance. - DTC Risks: Lack of accreditation and clinical validation in non-clinical settings. ◦ Pediatric Considerations: - Do not offer testing for adult-onset disorders to children unless management options are available (e.g., MEN2, FAP).

10. KEY PEARLS & CLINICAL TRAPS¶

• Counseling is Mandatory: Clinicians must provide counseling on risks, benefits, and limitations of testing. - DTC Warning: Do not use DTC results for medical management without clinical validation. - Sequence of Testing: Test affected family members first to identify the specific pathogenic variant before screening asymptomatic relatives. - Clinical Utility: Only findings with clear clinical utility (e.g., leading to surgery or specific drugs) are actionable.

Reference Tables¶

TABLE 480-1 Summary of American College of Medical Genetics and Genomics Position Statement on Direct-to-Consumer…¶

Harrison's 22e, p.3803

The clinical laboratory must:	• Be accredited by the CLIA (Clinical Laboratory Improvement Amendments) program, the state, and/or other applicable accrediting agencies with this accreditation indicated on test result reports
The individual undergoing testing should be adequately informed of:	• What the test can or cannot determine about their health or health risk • The potential for receiving results that neither confirm nor exclude the possibility of disease • Potential unexpected results, unrelated to the indication for testing • The potential impact of a given genetic test result on relatives

TABLE 480-2 Examples of Genetic Testing and Possible Interventions¶

Harrison's 22e, p.3804

GENETIC DISORDER	INHERITANCE	GENES	INTERVENTIONS
Oncologic
Lynch syndrome (HNPCC)	AD	MLH1, MSH2, MSH6, PMS2	Early endoscopic screening; risk-reducing surgery; aspirin
Familial adenomatous polyposis	AD	APC	Early and frequent endoscopy; prophylactic colectomy; chemoprevention
Hereditary breast and ovarian cancer	AD	BRCA1, BRCA2	Risk-reducing salpingo-oophorectomy; intensified breast surveillance including breast MRI; risk-reducing mastectomy; PARP inhibitors
Hereditary diffuse gastric cancer	AD	CDH1	Prophylactic gastrectomy; enhanced breast cancer surveillance
Hematologic
AD	F5
XL	F8
XL	F9
XL	G6PD
AR	HBB
Cardiovascular
Hypertrophic cardiomyopathy	AD	>10 genes including MYBPC3, MYH7, TNNT2, TPM1	Echocardiographic screening; pharmacologic intervention; myomectomy
Long QT syndrome	AD, AR	>10 genes including KCNQ1, SCN5A, KCNE1, KCNE2	Electrocardiographic screening; pharmacologic intervention; implantable cardiac defibrillator devices
Marfan’s syndrome	AD	FBN1	Echocardiographic screening; prophylactic beta blockers; aortic valve replacement as indicated
Gastrointestinal
AR	MEFV
AR	HFE
Pulmonary
α Antitrypsin deficiency 1	AR	SERPINA1	Avoidance of smoking and occupational and environmental toxins
Cystic fibrosis	AR	CFTR	Chest physiotherapy; agents to promote airway secretion clearance; CFTR modulators; lung transplantation
Endocrine
AD	AVP
AD	CASR
AD	RET
Renal
Polycystic kidney disease	AD, AR	PKD1, PKD2, PKHD1	Prevention of hypertension; prevention of urinary tract infections; kidney transplantation
Nephrogenic diabetes insipidus	XL, AR	AVPR2, AQP2	Fluid replacement; thiazides with or without amiloride
Neurologic
AD	RYR1, CACNA1S
AD	SCN4A
XL	DMD
AR	ATP7B