Genetic testing can establish a definitive, etiologically-based diagnosis for inherited cardiovascular diseases. Playing a vital role in early identification and treatment, it can save the lives of the patient and at-risk family members. Two studies at the National Heart Centre Singapore have led to a better understanding of inherited cardiovascular diseases in the local population.

Cardiovascular disease (CVD) has accounted for almost one out of three deaths in Singapore over the past few years, which equates to 17 people dying from either heart disease or stroke per day. It is currently ranked as one of the top three causes of hospitalisation and death.¹

OVERVIEW OF INHERITED CARDIOVASCULAR DISEASE

Progress over the past three decades has led to the discovery of an underlying genetic basis for many CVDs, resulting in the routine use of genetic testing in clinical practice. Some of these conditions are caused by disruption to a single gene that has a deleterious effect, known as monogenic conditions.

These inherited conditions can be divided into: ²

• Hypertrophic and dilated cardiomyopathy (associated with pathogenic variants in sarcomere and structural genes)
• Arrhythmogenic cardiomyopathy (associated with pathogenic variants in desmosomal genes)
• Inherited arrhythmias (associated with pathogenic variants in transmembrane ion channels genes)
• Marfan and related syndromes and thoracic aortic aneurysms (associated with pathogenic variants in genes encoding connective tissue elements)
• Other conditions such as familial hypercholesterolemia

Many inherited CVDs exhibit phenotypic overlay and genetic heterogeneity, with pathogenic variants in multiple genes causing the same condition.

Analysis can require sequencing the entire region of many genes to identify a genetic cause. Traditional genetic testing methods were time-consuming and expensive as single genes were tested sequentially.

Next generation sequencing (NGS) technologies have enabled large gene sequencing panels containing hundreds of genes of interest to be tested simultaneously at a much lower cost and with reduction in turnaround time. This has vastly increased the accessibility of genetic testing in clinical practice.

More recently, genome-wide association studies (GWAS) have identified a multitude of common genetic variants that underlie risk for the development of common CVDs such as coronary heart disease and atrial fibrillation. Individually these variants have a subtle effect, however collectively they can cause disease, referred to as polygenic risk.

A recent study found that more than 30% of coronary artery disease cases were attributed to genetic factors.³ The utility of genetic testing for polygenic CVD is progressing although not yet widely available for clinical integration.

WHY CONSIDER GENETIC TESTING?

The main utility of genetic testing is to establish a definitive, etiologically-based diagnosis.

1. It is particularly beneficial when the clinical phenotype can be shared by multiple conditions (known as phenocopies) which could each have different underlying genetic causes, prognoses, treatments and implications for family members.
   
   For example, left ventricular hypertrophy in hypertrophic cardiomyopathy could have an overlapping diagnosis with athlete’s heart, hypertensive heart disease, lysosomal storage cardiomyopathy (e.g., Fabry disease), metabolic storage cardiomyopathy (e.g., Danon disease), infiltrative process (e.g., cardiac amyloidosis), or be part of a phenotypic spectrum of Noonan syndrome and Friedreich ataxia.⁴
   
   
2. Likewise, genetic testing could distinguish long QT syndrome from rare multisystemic syndromic disorders with prolonged QTs including Timothy syndrome, Andersen-Tawil syndrome and recessive disorders Jervell and Lange-Nielson syndrome, as well as non-syndromic versus syndromic aortopathy.⁵
   
   Understanding the genetic etiology guides management and treatment for both cardiac and noncardiac manifestations.
   
   
3. Once the underlying genetic cause for an inherited CVD in the family is established by diagnostic genetic testing, then testing for the same pathogenic variant(s) can be offered to asymptomatic family members for preclinical diagnosis and prevention, which is referred to as cascade testing.
   
   Family members found to carry the familial pathogenic variant can be referred for ongoing specialist care. Relatives found not to carry the familial pathogenic variant do not require ongoing cardiac screening if the genetic etiology of CVD in the family has been well established.

WHEN TO CONSIDER INHERITED CVD

Familial inheritance
Indications of familial inheritance are documented through the collection of over three generations of family medical history information, noting all affected and unaffected family members, and the age of onset and death of any cardiac-related events. The family history information can then be interpreted to assess the likelihood of a genetic condition being present and its inheritance pattern (Table 1).

Age of onset
Adult onset conditions are more often autosomal dominant, whereas childhood conditions can be autosomal dominant, autosomal recessive, X-linked or mitochondrial.⁶ Prior to genetic testing, screening advice for family members can be recommended according to the family history information.

Who should be tested
Patients who clearly meet diagnostic criteria for disease, have a younger age of onset and a family history of CVD have an increased chance of identifying a genetic cause. Therefore, it is recommended to commence testing with the youngest diagnosis in the family or a definitive phenotype.

If genetic testing is performed on individuals with an undefined or borderline diagnosis, the interpretation of the genetic variants and their implications in clinical practice can be inconclusive.

Testing with no family history
Genetic testing is also recommended for patients who do not present with a family history of CVD or sudden death to account for inaccuracies in the collection, incomplete penetrance or variable expression of disease, or the presence of a de novo pathogenic variant.

INDICATIONS FOR INHERITED CARDIOVASCULAR DISEASE

Table 1 Adapted from Moscarello, Tia, Chloe Reuter, and Euan A. Ashley. “Is Genetic Testing for Heart Disease Right for Me?.” JAMA Cardiol. 4.9 (2019): 956-956.

DIAGNOSING INHERITED CVD IN SINGAPORE

Until recently, the recognition of genetic factors predisposing to inherited CVDs has been derived largely from Western cohort studies, with a paucity of data from non-European populations. As genetic variation can be ethnic-specific, this renders challenges in interpreting genomic data from under-represented populations and has previously led to the misdiagnosis and mismanagement of cardiac disease.⁷ As such, genomic data from diverse populations is required for  accurate genetic diagnoses.

Since 2013, the National Heart Centre Singapore (NHCS) has initiated two prospective interlinked cohort studies called Biobank and SingHeart, led by Professor Stuart Cook and Associate Professor Yeo Khung Keong, which collect and store biological samples, health information and imaging data from Singaporeans with CVD (cases) and healthy volunteers (controls).

These studies are contributing to understanding of:

1. The genetic etiology of inherited CVD amongst Singaporeans
2. The prevalence of inherited CVD by identifying pathogenic genomic variants
3. Genotype-phenotype correlations

The genomic analysis involves screening a panel of 174 genes with known associations to 17 inherited cardiac conditions which was developed by NHCS.⁸ The Biobank study has partnered with the SingHealth Duke-NUS Institute of Precision Medicine (PRISM) to perform the genomic analysis and return clinically actionable findings to consenting participants involved.

ANALYSIS OF CARDIAC GENOMIC VARIANTS

Establishing a local control group
The identification of pathogenic variants is complex due to causative variants being rare and often unique to each family, and rare variants occur in individuals with and without disease.

With the creation of a Singaporean genomic control population database (the Singapore Exome Consortium) which comprises aggregated genomic data from over 3,000 “healthy individuals” to date of South East Asian ancestry,⁹ this enables the frequency of the variants amongst affected individuals to be compared. This local control genomic database aids in distinguishing rare pathogenic variants from benign variants that may be enriched amongst Singaporeans.

Classifying genomic variants
To classify each genomic variant, PRISM uses the American College of Medical Genetics and Genomics (ACMG) guidelines10 to characterise variants into five tiers of classification: pathogenic, likely pathogenic, variant of uncertain significance (VUS), likely benign and benign.

These classifications draw on evidence to include population prevalence data, the effect of the variant to the protein function, segregation of the variant amongst affected family members and presence of the same variant in other patients with the same phenotype.

The data for each possible causative genetic variant is presented at a monthly multidisciplinary team meeting comprising cardiologists, clinical geneticists, genetic counsellors, scientists and bioinformatics experts from SingHealth, and is critically reviewed until consensus regarding pathogenicity is reached. A research report is then generated which documents each monogenic disease and/or carrier risk variant identified, and contains brief information about the associated genetic condition, inheritance and health risks.

DISCUSSING GENETIC RESULTS WITH PATIENTS

A monthly inherited cardiac clinic is held at NHCS where research participants who have consented to receive genetic findings can meet with the clinical genetics team from PRISM (a clinical geneticist and genetic counsellor) and a cardiologist. They are provided information regarding the outcome of genetic testing, genetic variants detected, associated health risk information, and the inheritance, penetrance and implications for family members.

Pathogenic variant detected
If a pathogenic variant associated with their clinical diagnosis is detected, research patients are given the opportunity to have the variant clinically validated at a certified laboratory to avoid false positives detected through sequencing, and this requires re-consent for a clinical test. In addition, a new blood sample is collected for variant analysis to avoid the possibility of any sample mix-up at recruitment.

During the consent process, the psychosocial impact of receiving such results, any possible genetic discrimination such as insurance, intention of sharing this information with family members and their possible responses are explored. Patients are then invited back to the clinic to receive the clinical validation results with further discussion regarding medical screening specific to their age and health history.

No pathogenic variant reported
If no pathogenic variants are reported, this could suggest that there is a variant which was either not detected with current technology, or that the presenting phenotype is caused by a combination of polygenic and environmental factors.

In this scenario, cardiac screening advice for family members is assessed according to family history information. When screening multiple genes, the detection of variants of uncertain significance is frequent. As the familial cause of CVD is uncertain, cascade testing is not available to family members. Information regarding variant interpretation does change over time as more knowledge becomes available and test results may be updated with new classifications.

IMPLICATIONS FOR CLINICAL MANAGEMENT

The detection of a pathogenic variant could have implications for lifestyle recommendations, such as exercise; medical therapies; risk to pregnancy; and surgical interventions such as an implantable cardioverter defibrillator, aortic root surgery or heart transplant6 (see Table 2 for examples experienced at the PRISM clinic). Pregnancy planning, if relevant, is also discussed.

Frequently, patients ask how the pathogenic variant will affect their family members, and in particular, their children.

Unfortunately, genetic testing for inherited CVD cannot predict age of onset or severity of disease. However, not everyone carrying a pathogenic variant expresses symptoms (known as reduced penetrance), and individuals within the same family or carrying the same pathogenic variant can be affected differently (known as variable expressivity). Clinical management for family members is advised with disease progression.

KEY POINTS FOR CVD GENETIC TESTING

• Genetic testing commences with a patient affected with CVD who is usually the youngest diagnosed or most severely affected in a family
• If a pathogenic variant is identified, then cascade testing is available to identify at-risk family members
• Most families have a unique pathogenic variant
• Not everyone carrying a pathogenic variant will express symptoms (reduced penetrance)
• Individuals within the same family can be affected differently (variable expressivity)
• In some families there may be more than one gene involved causing CVD (genetic heterogeneity)
• If a variant of uncertain significance (VUS) or no pathogenic variant is identified, then clinical management should not be changed and genetic testing is not available to family members
• Genomic research findings must be clinically validated before being used for clinical management of patients and their family members

EXPERIENCES IN THE INHERITED CARDIAC CLINIC

Table 2

CONCLUSION

Through the NHCS Biobank study, the aggregation of genomic data from CVD patients and the control group of healthy individuals is facilitating the differentiation between pathogenic and benign variants present in our local population. This analysis is leading to a further understanding of genomic variants and their association with CVD.

The identification of pathogenic variants has enabled the clinical management of CVD patients to be refined and their at-risk family members to be identified through cascade testing. Genetic investigations for CVD are rapidly evolving world-wide and will continue to play a vital role in guiding optimal treatment and risk stratification for patients and their families.

REFER A PATIENT

If you have a patient with a suspected inherited CVD you may refer them to the NHCS Cardiogenetics Clinic by contacting:
Tel: +65 6704 2222
Fax: +65 6222 9258
Email: central.appt@nhcs.com.sg

ENROL AS A VOLUNTEER

For Biobank study enquiries or those interested in enrolling as a healthy volunteer, please contact:
NHCS Biobank Coordinators
Tel: 9159 7029 (office hours 8.30am – 5.30pm)
Email: biobanking_enqueries@nhcs.com.sg

Publications derived from the NHCS Biobank genomic data:

1. Bylstra, Yasmin, et al. "Population genomics in South East Asia captures unexpectedly high carrier frequency for treatable inherited disorders." Genetics in Medicine 21.1 (2019): 207-212.
2. Pua CJ et al. Development of a comprehensive sequencing assay for inherited cardiac condition genes. J Cardiovasc Trans Res. 2016;9:3.
3. Wu, Degang, et al. "Large-scale whole-genome sequencing of three diverse Asian populations in Singapore." Cell 179.3 (2019): 736-749.
4. Yap, Jonathan, et al. "Harnessing technology and molecular analysis to understand the development of cardiovascular diseases in Asia: a prospective cohort study (SingHEART)." BMC cardiovascular disorders 19.1 (2019): 259.

REFERENCES

1. Singapore Heart Foundation. https://www.myheart.org.sg/myheart/heart-statistics/singapore-statistics. Accessed 30 June 2021.

2. Ackerman MJ, Priori SG, Willems S, et al. HRS/EHRA expert consensus statement on the state of genetic testing for the channelopathies and cardiomyopathies: this document was developed as a partnership between the Heart Rhythm Society (HRS) and the European Heart Rhythm Association (EHRA). Europace. 2011;13(8):1077-1109.

3. Zeng L, Talukdar HA, Koplev S, et al. Contribution of Gene Regulatory Networks to Heritability of Coronary Artery Disease. J Am Coll Cardiol. 2019;73(23):2946-2957.

4. Cirino AL, Harris S, Lakdawala NK, et al. Role of Genetic Testing in Inherited Cardiovascular Disease: A Review. JAMA Cardiol. 2017;2(10):1153-1160.

5. Ingles J, Macciocca I, Morales A, Thomson K. Genetic Testing in Inherited Heart Diseases. Heart Lung Circ. 2020;29(4):505-511.

6. Otto CM, Savla JJ, Hisama FM. Cardiogenetics: a primer for the clinical cardiologist. Heart. 2020;106(12):938-947.

7. Manrai AK, Funke BH, Rehm HL, et al. Genetic Misdiagnoses and the Potential for Health Disparities. N Engl J Med. 2016;375(7):655- 665.

8. Pua CJ, Bhalshankar J, Miao K, et al. Development of a Comprehensive Sequencing Assay for Inherited Cardiac Condition Genes. J Cardiovasc Transl Res. 2016;9(1):3-11.

9. Bylstra Y, Kuan JL, Lim WK, et al. Population genomics in South East Asia captures unexpectedly high carrier frequency for treatable inherited disorders. Genet Med. 2019;21(1):207.

10. Green RC, Berg JS, Grody WW, et al. ACMG recommendations for reporting of incidental findings in clinical exome and genome sequencing. Genet Med. 2013;15(7):565-574.

Yasmin Bylstra is a Principal Genetic Counsellor at the SingHealth Duke-NUS Institute of Precision Medicine (PRISM). She has a key role in the provision of genetic counselling to individuals with or at risk of developing genetic conditions, genomic variant interpretation and population genomics research. She is board-certified with the Human Genetics Society of Australasia (HGSA).

GPs can call the SingHealth Duke-NUS Genomic Medicine Centre for appointments at the following hotlines:
KK Women's and Children's Hospital: 6692 2984
National Cancer Centre Singapore: 6436 8288