What does a variant of uncertain significance (VUS) in the ENG (Endoglin) gene mean in the context of Hereditary Hemorrhagic Telangiectasia (HHT)?

What Does an ENG Mutation VUS Mean in HHT?

A variant of uncertain significance (VUS) in the ENG gene cannot be used to diagnose HHT, guide clinical management, or perform predictive testing in family members—the patient must be evaluated and managed based solely on the Curaçao clinical criteria, not the genetic finding. 1

Understanding VUS Classification

A VUS designation indicates insufficient or conflicting data regarding the mutation, preventing its assignment as either benign or pathogenic. 1 By definition, VUS results represent indeterminate findings that neither support nor refute an underlying genetic pathogenesis. 1

• VUS should never direct clinical decisions because they represent probabilistic uncertainty, not actionable genetic information 1
• Most VUS are eventually reclassified as benign, with 97% of reclassified VUS being downgraded to benign rather than upgraded to pathogenic 2
• Laboratories can differ in their interpretation of variant pathogenicity, and the ACMG has proposed standardized guidelines to address this variability 1

Clinical Diagnosis Takes Priority

The diagnosis of HHT must be established using the Curaçao clinical criteria, which requires 3 of 4 features: 3, 4

• Spontaneous and recurrent epistaxis
• Multiple telangiectasias at characteristic sites (lips, oral cavity, fingers, nose)
• Visceral arteriovenous malformations (pulmonary, hepatic, cerebral, or gastrointestinal)
• First-degree relative with HHT

The highest mutation detection rate (87%) occurs in probands meeting all four Curaçao criteria, but even in clinically definite HHT, approximately 13% of patients have no identifiable mutation in ENG or ACVRL1. 5 This means negative or uncertain genetic testing does not exclude HHT if clinical criteria are met. 3

Management Algorithm for ENG VUS

Step 1: Apply Clinical Diagnostic Criteria

• Assess whether the patient meets 3 of 4 Curaçao criteria for definite HHT, 2 of 4 for possible HHT, or fewer than 2 for unlikely HHT 3
• If clinically definite HHT (3+ criteria), proceed with full HHT screening and management regardless of the VUS 3, 4

Step 2: Comprehensive Genetic Testing

• Order simultaneous sequencing and deletion/duplication analysis of ACVRL1, SMAD4, and GDF2 genes, as these collectively identify 97% of clinically definite HHT cases 3, 6
• The ENG VUS may represent a false lead while the true pathogenic variant exists in another HHT gene 6

Step 3: Functional Characterization (If Available)

• Novel ENG variants can be evaluated through RNA analyses, functional assays, or structural modeling to determine if they affect protein function 1, 7
• Even intronic ENG variants can be pathogenic through transcriptional dysregulation by affecting regulatory elements like Sp1 binding sites 7
• Missense variants in ENG are particularly challenging because extracellular ENG amino acids are not strongly conserved, requiring crystal structure analysis for interpretation 6

Step 4: Segregation Analysis

• If other family members have clinically definite HHT, test them to determine whether the ENG VUS segregates with disease 1
• This process is nuanced and should be disease and family specific, ideally coordinated through genetic counseling 1

Screening Decisions with ENG VUS

Critical principle: Screen based on clinical phenotype, not the VUS. 1, 2

If Patient Meets Clinical Criteria for Definite HHT:

• Pulmonary screening: Perform contrast echocardiography or chest CT for pulmonary AVMs, as these can be treated presymptomatically to prevent stroke and cerebral abscess 3, 4
• Cerebral screening: Obtain brain MRI to detect cerebral vascular malformations 3, 4
• Liver screening: Perform Doppler ultrasonography as first-line imaging, but never perform liver biopsy due to catastrophic hemorrhage risk 3, 4
• GI evaluation: Consider upper endoscopy if unexplained anemia is disproportionate to epistaxis severity 3

If Patient Has Possible HHT (2 Criteria):

• Maintain clinical surveillance for development of additional manifestations 3
• Repeat genetic testing may be warranted if additional HHT genes become available or if variant reclassification occurs 1

Variant Reclassification Strategy

VUS can be reclassified over time as their status changes from uncertain to likely pathogenic or likely benign, and patient risk should be readdressed at that time. 1

• Establish recontact with the genetics laboratory every 1-2 years for variant reclassification updates 2
• Document clearly in the medical record that clinical decisions should not be based on the VUS to prevent mismanagement by other providers 2
• Refer the patient to research studies and variant reclassification programs to help resolve the variant's significance 2

Family Testing Considerations

Do not perform predictive genetic testing in family members using the VUS. 1, 2

• Family members should be evaluated clinically using Curaçao criteria 3
• If the VUS is later reclassified as pathogenic, cascade testing can then be offered 2
• Genetic counseling should emphasize that VUS results do not provide actionable information for family planning or risk assessment 2

Common Pitfalls to Avoid

• Do not withhold HHT screening in symptomatic patients just because genetic testing returned a VUS rather than a pathogenic variant 1, 2
• Do not assume the ENG VUS is the causal variant when the patient may have a pathogenic variant in ACVRL1, SMAD4, or GDF2 that was not tested 3, 6
• Avoid misclassification consequences: A wrongly classified variant can result in false reassurance for mutation-negative family members who may actually be at risk 1
• Recognize that more than 30% of identified HHT mutations are novel, but only 6% are VUS—most novel variants can be classified with appropriate analysis 5

Genotype-Phenotype Considerations

If the ENG VUS is eventually reclassified as pathogenic (HHT type 1), this has specific clinical implications: 4, 8

• Epistaxis typically starts earlier (mean age 11 years) in HHT1 4
• Pulmonary AVMs are more frequent and larger in HHT1 compared to HHT2 4, 8
• Cerebral AVMs occur more commonly in HHT1 4, 8
• Hepatic vascular malformations are less common in HHT1 than HHT2 4, 8
• Truncating ENG mutations are associated with more affected organs and more severe hemorrhaging than missense mutations 8

However, these genotype-phenotype correlations cannot be applied until the variant is definitively classified as pathogenic. 1