OC 77.3 - When is factor VIII deficiency not hemophilia A? Variant F8 c.1094 A>G, p.Tyr365Cys illustrates the utility of genotyping and the value of disease specific expertise in variant classification

Wednesday, June 28, 2023

10:45 – 11:00 ET

Room: 518

Session Available As: On-Demand

Presenting Author(s)

Dominika A. Bajguz, MS, CGC (she/her/hers)

Genetic Counselor
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States

Co-Author(s)

VT

Valerie Trapp-Stamborski

Director, Hematology Genetics
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States
Elizabeth L. Palmer, MS, CGC

Genetic Counselor
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States
CS

Candace C. Smith

Genetic Counseling Assistant
Versiti Diagnostic Laboratories
Milwaukee, Wisconsin, United States
CC

Curt D. Czajkowski

Senior Laboratory Technologist
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States
SM

Samantha J. Meyers

Laboratory Technologist
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States
AM

Anna P. Miller

Clinical Variant Scientist
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States
SD

Stefanie N. Dugan

Genetic Counselor
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States
KF

Kenneth D. Friedman

Medical Director, Hemostasis Reference Laboratory
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States
JP

Juliana Perez Botero

Medical Director
Versiti Diagnostic Laboratory
Milwaukee, Wisconsin, United States

Background: Discrepant plasma factor VIII activity as measured by one-stage clotting assays (OSA) versus chromogenic substrate assays (CSA) is observed with some variants causing mild/moderate hemophilia A. The F8 c.1094 A>G, p.Tyr365Cys variant, observed in the general population and people with decreased FVIII activity (many without bleeding symptoms), carries pathogenicity assertions from pathogenic to likely benign, posing a challenge in determining the clinical approach to individuals with this variant.

Aims: To illustrate the importance of disease specific knowledge (clinical and laboratory phenotype, disease prevalence) for variant classification using a cohort of individuals with the F8 c.1094 A>G variant.

Methods: With prior IRB approval, a retrospective analysis of our 2016 - 2022 laboratory cohort was performed to identify individuals hemizygous or heterozygous for F8 c.1094 A>G and correlate factor VIII activity levels measured by OSA and/or CSA with reported bleeding phenotypes.

Results: Ten individuals (six males, four females) with F8 c.1094 A>G were identified (Table 1). OSA levels ranged from 17% to 47%. Four of individuals with both OSA and CSA levels provided showed discrepant levels between assays with CSA:OSA ratios ranging from 2.29-2.88. All patients in the study cohort lacked a significant bleeding phenotype based on reported clinical history, which correlated more closely with the CSA. The minor allele frequency of F8 c.1094 A>G in the general population is 0.00006544, including six hemizygous alleles (GnomAD), which is more frequent than expected for hemophilia A.

Conclusion(s): This study adds further evidence for the likely benign classification of F8 c.1094 A>G with respect to hemophilia A and highlights the importance of correlation of genetic findings with the clinical and laboratory phenotype (including OSA and CSA), for the accurate evaluation of patients with low FVIII activity.

Learning Objectives:

Describe the relevance of clinical and laboratory phenotype information, as well as disease prevalence, to F8 variant interpretation.
Explain why the F8 c.1094 A>G, p.Tyr365Cys variant carries conflicting pathogenicity assertions from pathogenic to likely benign.
Summarize how genetic analysis of F8 can supplement functional testing when patients are noted to have discrepant plasma factor VIII activity as measured by one-stage clotting assays (OSA) versus chromogenic substrate assays (CSA).