CCUS — Clonal Cytopenia of Undetermined Significance

Understanding clonal hematopoiesis with unexplained cytopenias — distinguishing CCUS from MDS, surveillance protocols, and cardiovascular implications

Key Points

• CCUS (Clonal Cytopenia of Undetermined Significance) is defined by the co-occurrence of clonal hematopoiesis (somatic mutation at VAF ≥2%) and cytopenia (Hgb <13 g/dL men/<12 g/dL women, or platelets <150,000/μL, or ANC <1,800/μL) that does not meet diagnostic criteria for MDS, MPN, or other myeloid neoplasm.
• CCUS exists on a spectrum with CHIP (Clonal Hematopoiesis of Indeterminate Potential, cytopenia-free) and represents a stage of clonal evolution where the bone marrow shows insufficient dysplasia or blast elevation to meet WHO MDS criteria despite molecular evidence of a myeloid clone.
• The annual risk of progression to MDS, AML, or MPN from CCUS is approximately 5–10%/year — substantially higher than CHIP (~0.5–1%/year) — because cytopenia suggests that the clone is already suppressing normal hematopoiesis.
• High-risk CCUS features that predict more rapid progression include: multiple somatic mutations, high variant allele frequency (VAF >10%), adverse mutations (RUNX1, TP53, SRSF2, IDH1/2, ASXL1), cytogenetic abnormalities, and severe or worsening cytopenias.
• No FDA-approved therapy exists for CCUS; management is observation, investigation of reversible causes of cytopenia, monitoring for progression, and cardiovascular risk reduction (clonal hematopoiesis independently increases atherosclerotic cardiovascular disease risk).
• Bone marrow biopsy is required to exclude MDS — the diagnosis of CCUS rests on demonstrating <10% dysplasia per lineage and <5% blasts in the context of somatic mutations and cytopenia.

Definition, Context, and Relationship to CHIP and MDS

**The clonal hematopoiesis spectrum:** With age, somatic mutations accumulate in hematopoietic stem cells. When a mutant clone expands to constitute ≥2% of blood cells (variant allele frequency ≥2% by sensitive NGS), this is detectable as *clonal hematopoiesis*. The clinical implications depend entirely on whether cytopenias or dysplasia are present: - **CHIP (Clonal Hematopoiesis of Indeterminate Potential):** Somatic mutation at VAF ≥2%, normal blood counts, no MDS/MPN/AML, no cytopenia. Prevalence increases dramatically with age: ~3–5% at age 60, ~10–15% at age 70, >20% at age 80. Annual…

Bone Marrow Evaluation and Molecular Testing

**When to perform bone marrow biopsy:** Bone marrow evaluation is required in patients with: - Unexplained cytopenia(s) persisting >3 months after correction of reversible causes - Known clonal hematopoiesis (positive NGS) with new or worsening cytopenia - Rising LDH, increasing splenomegaly, or constitutional symptoms with cytopenia - Pre-transplant evaluation for any indication The biopsy distinguishes CCUS (insufficient dysplasia, 10–20%) suggests a more dominant clone with greater hematopoietic displacement; lower VAF (<5%) more likely represents benign CHIP - **Number of mutations:**…

Surveillance, Management, and Cardiovascular Implications

**No approved therapy for CCUS:** There are no FDA-approved treatments for CCUS itself. Clinical trials are exploring early intervention with luspatercept, hypomethylating agents, or targeted agents in high-risk CCUS — results are pending. **Surveillance schedule (based on risk):** *Low-risk CCUS (single low-risk mutation, VAF 10%, abnormal cytogenetics, or significant cytopenias):* - CBC every 1–3 months - Repeat bone marrow biopsy at 6–12 months (even without clinical change) to assess for emerging dysplasia or blast increase - Earlier repeat NGS to track clonal evolution - Consider…