Clinical sensitivity of molecular genetic testing

in hypertrophic cardiomyopathy.

Kate ThomsonMolecular Genetics Laboratory,

Oxford

Overview

• Hypertrophic Cardiomyopathy– Clinical features– Genetics

• Clinical sensitivity in our cohort

• Factors affecting clinical sensitivity

• Characterised by thickening of the heart muscle, most commonly of the left ventricle, with no obvious cause (e.g. high blood pressure, athletes heart)

• Autosomal Dominant

• Prevalence of 1/500

• Most common cause of heart related sudden death in people under 35 and athletes

Hypertrophic cardiomyopathy

The hypertrophic heart

Clinical Features• Clinically heterogeneous

-No symptoms-Shortness of breath-Chest pain-Fainting-Dizziness-Palpitations-Exercise intolerance-Sudden death

• Variable presentation, age of onset and clinical course

• Differential diagnoses: -Cardiac amyloidosis-Hypertensive heart disease-Aortic stenosis-Athletes heart-Metabolic disease (Fabry’s disease, Danon disease)-Mitochondrial myopathy

Benefits of Genetic Diagnosis

• Confirm clinical diagnosis/familial disorder

• Offer testing to at risk family members to enable early diagnosis and treatment

• Future – Risk stratification and prognosis – Patient management

Genetics

• >20 genes known to be associated

• Majority of genes encode components of the sarcomere (contractile apparatus of the heart)

• Four genes commonly associated sarcomeric genes account for ~80% of mutations.

• Double/compound variants reported in 5-10%

Cardiac muscle cell & sarcomere

Gene Protein % of HCM

MYH7 Beta Myosin heavy chain 25-35%

MYBPC3 Myosin-binding protein C 20-30%

TNNT2 Troponin T 3-5%

TNNI3 Troponin I <5%

TPM1 Tropomyosin 1 alpha <2%

MYL3 Regulatory myosin light chain <1%

MYL2 Essential myosin light chain Rare

ACTC1 Actin Rare

Commonly associated sarcomeric genes

Clinical Sensitivity in HCM• HCM service introduced 2003

• Gene dossier submitted 2006

• Clinical sensitivity estimated to be 60%

• Review clinical sensitivity in cohort (2003-2008)– Determine clinical sensitivity in our cohort (>700 probands)– Comparison with published data– Identify factors affecting clinical sensitivity

Clinical Sensitivity in our cohort

• 737 probands screened

• 346/737 variant detected

• Clinical sensitivity 47%

Comparison with published data

• Yield ranged from 13-61%

• 8 most commonly associated genes ~47%

• MYBPC3,MYH7,TNNT2,TNNI3 ~44%

• ~3% increased sensitivity~30% more workload

• 62% family history vs. 29% sporadic

Van Driest et al Mayo Clin Proc 2005

Factors affecting clinical sensitivity

Clinical sensitivity

ClinicalDiagnosis

AnalysisStrategy

Results interpretation

Clinical Diagnosis• Exclusion of phenocopies

• Family History

• The future – Refining clinical criteria of “sarcomeric HCM”– Define frequency of phenocopies in HCM cohorts– Cost of clinical vs. genetic investigations

Analysis strategy• Analysis of less commonly associated genes

• Assay sensitivity and specificity

• New technology (Roche 454)– Expansion of screen– Faster throughput– Results interpretation– Cost implications

Interpretation of resultsClassification Family testingHighly likely /certain to be pathogenic. Testing available for unaffected family

members (FMs).

Likely to be pathogenic butcannot be formally proven.

Recommend testing affected FMsprior to analysis of unaffected FMs.

Intermediate-not possible todetermine neutral/pathogenic.

Recommend testing affected FMs. Testing unaffected FMs not indicated.

Unlikely to be pathogenic butcannot be formally proven.

Testing FMs not indicated.

Neutral polymorphism-certainly not pathogenic.

Testing FMs not indicated.

Issues with results interpretation

-the usual suspects……..• High number of private missense mutations

• Functional domains of proteins not defined

• Limited functional studies

• Segregation studies confounded by:– clinical heterogeneity– variable penetrance & age of onset– SCD of other affected FMs

• No clinically normal control cohort

Clinical sensitivity based on likely pathogenicity

• All 47%• Highly likely & Likely 37%• Highly likely only 27%

Intermediate21%

Likely21%

Highly Likely58%

In summary• Clinical sensitivity in our cohort 47%

• Several factors thought to impact clinical sensitivity: – Clinical criteria for testing– Analysis strategy chosen– Results interpretation

• Introducing new technology (Roche 454) and techniques (MLPA) to ensure comprehensive analysis

• Hope that future studies will refine clinical criteria and overcome some of the issues with results interpretation

Acknowledgements

Oxford SCD TeamDr Anneke SellerKaren McGuireMelanie ProvenOmer MohammedJessica ThistletonRia HipkissJohn TaylorSarah ReidPenny Clouston

NHS Department of Clinical GeneticsDr E. Blair