Introduction Germline ETV6 mutations (gETV6) are linked to mild-to-moderate thrombocytopenia, bleeding tendency and ~30% lifetime risk of hematological malignancies (HM). Patients with gETV6 are classified among myeloid neoplasms with germline predisposition and preexisting platelet disorders, along with RUNX1 and ANKRD26. However, unlike gRUNX1 and gANKRD26, the premalignant manifestations of gETV6, such as clonal hematopoiesis (CH) and bone marrow (BM) dysplasia, are less well defined. This knowledge gap limits the implementation of effective early detection and surveillance strategies

This study investigated the prevalence and features of CH and BM dysplasia in asymptomatic gETV6 carriers.

Methods We classified individuals with gETV6 without HM as “carriers without HM,” while those with gETV6 mutation and HM as “carriers with HM.” Family members without gETV6 (wild-type) served as controls.

Germline status was confirmed by identifying a pathogenic or likely pathogenic (P/LP) gETV6 variant in non-hematological tissue (hair follicles, fibroblasts, or buccal swabs) or peripheral blood (PB) in asymptomatic cases or by studying family segregation. Variants identified only in hematopoietic tissue (due to lack of germline tissue confirmation) were considered “presumed germline” if scoring ≥6 on a multi-criteria system adapted from Feurstein et al. (Blood, 2022).

All carriers without HM underwent baseline BM aspiration (BMA) and were prospectively followed with annual hematologic evaluations. Bleeding phenotype was assessed using the ISTH-SSC Bleeding Assessment Tool (BAT). CH was evaluated via targeted NGS of recurrent myeloid genes in PB or BM.

Results We enrolled 25 gETV6 carriers (23 confirmed, 2 presumed) and 18 wild-type controls from 12 families across 6 centers. Nine distinct ETV6 variants were identified: R55C (n=3), c.463+1_463+2dupGT (n=1), P214L (n=1), C338Y (n=1), F368L (n=1), R399L (n=1), R399C (n=1), Y401H (n=1), and R418M (n=2).

The median age at first clinical symptom (thrombocytopenia/bleeding) was 20 years (IQR: 5–53) while the median age at diagnosis of the inherited condition was 44 (IQR: 19–54), reflecting a 24-year diagnostic delay.

The overall incidence of HM was 28% (7/25), including one lymphoid neoplasm (age 20) and four myeloid (median age at onset 59 years, IQR: 36–72). One carrier developed MDS during follow-up (median 35 months, IQR: 20–90), after acquisition of somatic mutations identified in subsequent NGS.

Carriers without HM had significantly lower platelet counts than controls (127×10⁹/L, IQR: 117–194 vs. 264.5 ×10⁹/L, IQR: 239–303; p<0.001), with 61% (11/18) with values <150×10⁹/L. Other hematologic parameters were comparable between groups.

Bleeding symptoms were reported by 52% (13/25) of carriers, although only 36% (9/25) had abnormal ISTH-BAT scores, indicating discordance between reported symptoms and standardized tools. Both self-reported bleeding and abnormal ISTH-BAT were significantly more frequent than in controls (0/17 p<0.001). Hemorrhagic features were more frequent among individuals with P/LP variants affecting residue R399 in the ETS DNA-binding domain.

BMA were available from eleven adult carriers without HM. Despite being clinically healthy, 9/11 (82%) showed dysplasia in ≥1 hematopoietic lineage: dysgranulopoiesis in 8 (72%), dyserythropoiesis in 6 (55%) and occasional dysmegakaryopoiesis in 1 (9%). BM cellularity was normocellular (n=4), hypercellular (n=6), or hypocellular (n=1), with no correlation to dysplasia. Megakaryocyte number varied from normal (n=3), few (n=1), to abundant (n=7).

Although statistical association was limited by sample size, dysplasia was observed in most carriers with variants affecting R399 (5/7) and R55 (3/3), suggesting a genotype-phenotype correlation.

NGS in 14 adult carriers without HM revealed that 36% (5/14) harbored ≥1 acquired myeloid mutation, including EZH2 (Y579S, VAF 45.8%), KRAS (G12R, VAF 1.3%), TP53 (K132E, VAF 4.3%), CBL (C384R, VAF 2.4%), and KMT2A (VAF 46%). All individuals with CH had gETV6 at R399 (R399C or R399L).

Conclusions Our study, the largest comprehensive analysis of a gETV6 cohort to date, confirms the association with HM and reveals a high frequency of CH, dysgranulopoiesis, and bleeding tendency, particularly in carriers of the R399 variants, suggesting a potential genotype/phenotype correlation. These findings support early genetic testing and close monitoring, especially in carriers with recurrent ETS domain mutations.

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2025
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