5.1 Myelodysplastic Syndromes (MDS) & Overlap Syndromes

Myelodysplastic Syndromes (MDS) & Overlap Syndromes

Myelodysplastic Syndromes (MDS) represent a heterogeneous group of clonal hematopoietic stem cell malignancies characterized by ineffective hematopoiesis, progressive peripheral blood cytopenias, qualitative morphologic abnormalities (dysplasia) in one or more hematopoietic cell lineages, and an inherent propensity for clonal evolution and transformation into Acute Myeloid Leukemia (AML).

In classic MDS, the bone marrow is typically hypercellular (or occasionally normocellular) due to active clonal expansion, yet the peripheral blood demonstrates severe cytopenias (anemia, bicytopenia, or pancytopenia). This apparent paradox is driven by accelerated intramedullary apoptosis of developing erythroid, myeloid, and megakaryocytic precursors within the marrow microenvironment before they can mature and enter the peripheral circulation.

                        CLONAL STEM CELL INITIATION
         (Acquired Somatic Mutations: SF3B1, TET2, ASXL1, TP53, etc.)
                                     │
                                     ▼
                      INEFFECTIVE HEMATOPOIESIS
             ┌───────────────────────┴───────────────────────┐
             ▼                                               ▼
   BONE MARROW COMPARTMENT                         PERIPHERAL BLOOD
  • Hypercellular Marrow                          • Progressive Cytopenias
  • Accelerated Intramedullary Apoptosis            (Anemia, Neutropenia, Thrombocytopenia)
  • Trilineage Morphologic Dysplasia              • Circulating Dysplastic Cells
  • Accumulation of Myeloblasts                   • Risk of Leukemic Transformation (AML)

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1. Molecular Pathophysiology & Clonal Architecture

MDS arises from the sequential acquisition of somatic driver mutations in a multipotent hematopoietic stem cell. These genetic lesions disrupt core cellular pathways:

1. RNA Splicing Machinery: Mutations in spliceosome complex genes occur in over 50% of MDS patients. $SF3B1$ (splicing factor 3b subunit 1) is strongly associated with mitochondrial iron handling abnormalities and the formation of ring sideroblasts. Other spliceosome mutations include $SRSF2$, $U2AF1$, and $ZRSR2$.
2. Epigenetic Regulation & DNA Methylation: Mutations in $TET2$, $DNMT3A$, and $IDH1/IDH2$ alter DNA methylation patterns, while mutations in $ASXL1$ and $EZH2$ disrupt histone modification and chromatin remodeling, suppressing regular hematopoietic differentiation.
3. Transcription Factors & Signal Transduction: Alterations in $RUNX1$, $GATA2$, $ETV6$, $NRAS$, and $KRAS$ impair maturation programs and confer proliferative advantages.
4. DNA Damage Response: Inactivating mutations or deletions of $TP53$ (chromosome 17p13) are associated with complex cytogenetics, resistance to alkylating agents and hypomethylating therapies, and rapid transformation to secondary AML with dismal overall survival.

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2. Morphologic Hallmarks of Trilineage Dysplasia

Definitive diagnosis of MDS requires meticulous microscopic evaluation of well-prepared Wright-Giemsa-stained peripheral blood smears, bone marrow aspirates, and Prussian blue-stained iron preparations. Dysplasia must be unequivocally identified in $\ge 10%$ of cells within a specific hematopoietic lineage to qualify as dysplastic for classification purposes.

                             TRILINEAGE DYSPLASIA
                                      │
     ┌────────────────────────────────┼────────────────────────────────┐
     ▼                                ▼                                ▼
[ DYSERYTHROPOIESIS ]        [ DYSGRANULOPOIESIS ]        [ DYSMEGAKARYOCYTOPOIESIS ]
• Ring Sideroblasts (≥5 Fe   • Pseudo-Pelger-Huët         • Micromegakaryocytes
  granules encircling ≥1/3     (bilobed/hypolobated         (dwarf megakaryocytes)
  of nuclear ring)             pince-nez nuclei)          • Multiple discrete small
• Internuclear Bridging      • Hypogranulation /            round "pawn-broker" nuclei
• Multinuclearity & Budding    Agranular neutrophils      • Hypolobated / Mononuclear
• Megaloblastoid Chromatin   • Hypersegmentation /          megakaryocytes
• PAS-Positive Normoblasts     abnormal clefting          • Giant agranular platelets

A. Dyserythropoiesis

• Peripheral Blood: Macrocytic or dimorphic erythrocyte population, marked anisopoikilocytosis with prominent oval macrocytes, coarse basophilic stippling, Howell-Jolly bodies, Pappenheimer bodies (siderotic granules), and circulating dysplastic nucleated red blood cells (NRBCs).
• Bone Marrow Aspirate:
  • Nuclear Alterations: Multinuclearity, irregular nuclear contours, nuclear budding, lobulation, karyorrhexis, and internuclear chromatin bridging (thin chromatin strands connecting two distinct erythroblast nuclei).
  • Cytoplasmic-Nuclear Asynchrony: Megaloblastoid chromatin—fine, sieve-like, open nuclear chromatin persisting alongside advanced, hemoglobinized cytoplasm (resembling Vitamin $B_{12}$/folate deficiency but unresponsive to vitamin supplementation).
  • Ring Sideroblasts: Visualized on Prussian blue (Perls' acid ferrocyanide) stain as erythroid precursors with $\ge 5$ siderotic iron granules encircling at least one-third ($\ge 1/3$) of the nuclear circumference. These granules represent iron-overloaded mitochondria arranged around the nucleus.
  • Periodic Acid-Schiff (PAS) Reaction: Normal erythroblasts are completely PAS-negative. In MDS and erythroleukemia, dysplastic normoblasts demonstrate abnormal cytoplasmic glycogen accumulation, manifesting as diffuse cytoplasmic pink-red positivity or coarse, block-like granular PAS staining.

B. Dysgranulopoiesis

• Peripheral Blood and Bone Marrow:
  • Pseudo-Pelger-Huët Anomaly (Acquired Pelger-Huët): The most widely recognized granulocytic dysplastic feature. Mature neutrophils demonstrate hypolobation, presenting with bilobed nuclei connected by a thin filament ("pince-nez" or spectacle shape) or completely unsegmented, round/oval/peanut-shaped nuclei with heavily condensed, mature chromatin. Unlike the benign, hereditary Pelger-Huët anomaly (where 100% of neutrophils are bilobed with normal cellular function), acquired Pseudo-Pelger-Huët in MDS affects only a subpopulation of neutrophils and is accompanied by functional impairments and other dysplastic features.
  • Hypogranulation / Agranularity: Mature neutrophils and myelocytes exhibit pale, washed-out, or completely clear-to-pink cytoplasm due to a failure to synthesize secondary (specific) granules (lactoferrin and alkaline phosphatase deficiency).
  • Abnormal Nuclear Morphology: Ring-shaped nuclei (doughnut nuclei), hypersegmentation ($>5$ distinct lobes), nuclear clefting, and chromatin clumping anomalies.
  • Auer Rods: Fused primary azurophilic granules forming needle-like cytoplasmic crystalline structures inside myeloblasts; their presence in MDS automatically categorizes the disease as MDS with excess blasts-2 (MDS-EB-2) regardless of blast percentage.

C. Dysmegakaryocytopoiesis

• Peripheral Blood: Circulating giant, agranular, or hypogranular platelets; circulating bare megakaryocyte nuclei or micromegakaryocyte fragments.
• Bone Marrow Aspirate & Biopsy:
  • Micromegakaryocytes (Dwarf Megakaryocytes): Abnormally small megakaryocytes (roughly the size of a large myelocyte or monocyte, $12\text{--}30\ \mu\text{m}$ in diameter) with a single round or bilobed nucleus.
  • Multi-Nucleated / Disconnected Nuclei: Megakaryocytes containing multiple discrete, widely separated, small round nuclei arranged in a triangular or cluster pattern ("pawn-broker" nuclei).
  • Mononuclear / Hypolobated Forms: Large megakaryocytes containing a single non-lobated, round or oval nucleus instead of the normal multi-lobated, polyploid structure ($16\text{N}\text{ to }32\text{N}$).

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3. World Health Organization (WHO) Classification of MDS

The WHO classification stratifies MDS according to the number of dysplastic lineages, the percentage of blasts in peripheral blood and bone marrow, the presence of ring sideroblasts, and specific cytogenetic lesions:

                           MDS BLAST STRATIFICATION

 Peripheral Blood Blasts:   <1%          2 - 4%            5 - 19%          ≥ 20%
                            │              │                  │               │
 Bone Marrow Blasts:       <5%          5 - 9%            10 - 19%          ≥ 20%
                            │              │                  │               │
 Classification:       MDS-SLD / MLD    MDS-EB-1           MDS-EB-2          AML
                       MDS-RS / del(5q)             (or any Auer rods)  (Acute Myeloid)

MDS with Isolated del(5q)

MDS with isolated del(5q) represents a distinct, genetically defined clinicopathologic entity:

• Epidemiology: Predominantly affects elderly females (median age ~68 years; female-to-male ratio ~2:1).
• Clinical & Laboratory Profile: Severe, refractory macrocytic anemia (MCV often $>105\text{ fL}$), normal-to-leukopenic white count, and characteristically normal or elevated platelet count (thrombocytosis).
• Bone Marrow Morphology: Normal-to-increased cellularity with an abundance of pathognomonic small, hypolobated or non-lobated micromegakaryocytes containing single eccentric round nuclei.
• Molecular Mechanism & Therapy: Deletion of the critical 5q31-q33 chromosomal band results in haploinsufficiency of the RPS14 gene (causing p53 activation and erythroid arrest) and the microRNA-145/146a genes (upregulating TRAF6/TIRAP and driving megakaryocyte proliferation and thrombocytosis). Lenalidomide binds cereblon ($CRBN$) to induce ubiquitination and degradation of casein kinase 1A1 ($CSNK1A1$), selectively killing del(5q) clonal cells and producing transfusion independence in over $65%$ of patients.

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4. Myelodysplastic / Myeloproliferative Neoplasms (MDS/MPN Overlap Syndromes)

Overlap syndromes exhibit combined clinicopathologic features of both myelodysplasia (ineffective hematopoiesis, cytologic atypia, peripheral cytopenias) and myeloproliferation (autonomous effective overproduction of mature cells, marked leukocytosis or thrombocytosis, organomegaly).

                             MDS/MPN OVERLAP SYNDROMES
                                         │
        ┌────────────────────────────────┼────────────────────────────────┐
        ▼                                ▼                                ▼
     [ CMML ]                        [ aCML ]                     [ MDS/MPN-RS-T ]
• Persistent Monocytosis        • Severe Granulocytic Dysplasia • Refractory Anemia +
  ≥1.0 × 10^9/L (≥10% WBC)        with Leukocytosis               Thrombocytosis (≥450 × 10^9/L)
• Blasts <20%                   • BCR::ABL1 Negative            • Ring Sideroblasts ≥15%
• BCR::ABL1 Negative            • Basophils <2%                 • SF3B1 + JAK2/CALR/MPL
• Splenomegaly & Dysplasia      • SETBP1 / ETNK1 Mutations        Co-mutations

A. Chronic Myelomonocytic Leukemia (CMML)

CMML is the most prevalent MDS/MPN overlap neoplasm. Diagnostic criteria established by the WHO require:

1. Persistent Peripheral Blood Monocytosis: Absolute monocyte count $\ge 1.0 \times 10^9/\text{L}$, with monocytes accounting for $\ge 10%$ of the total leukocyte differential count.
2. Absence of the $BCR::ABL1$ Fusion Gene: Rules out CML presenting with monocytosis.
3. No Rearrangements of $PDGFRA$, $PDGFRB$, $FGFR1$, or $JAK2$.
4. Blasts $<20%$: Total myeloblasts, monoblasts, and promonocytes must comprise $<20%$ in blood and marrow.
   • CMML-0: Blasts $<2%$ in PB and $<5%$ in BM.
   • CMML-1: Blasts $2\text{--}4%$ in PB or $5\text{--}9%$ in BM.
   • CMML-2: Blasts $5\text{--}19%$ in PB, $10\text{--}19%$ in BM, or presence of Auer rods.
5. Genetic Mutations: Recurrent somatic mutations in $TET2$ (~60%), $SRSF2$ (~50%), $ASXL1$ (~40%), and $KRAS/NRAS$.

B. Atypical Chronic Myeloid Leukemia (aCML), $BCR::ABL1$-Negative

• Features: Marked leukocytosis with prominent dysgranulopoiesis (severe pseudo-Pelger-Huët, hypogranulation, abnormal nuclear shapes) and granulocytic precursors (promyelocytes, myelocytes, metamyelocytes) comprising $\ge 10%$ of total WBCs.
• Distinction from Classical CML: Lacks the Philadelphia chromosome / $BCR::ABL1$ fusion; basophilia is absent or minimal (basophils $<2%$ of WBCs, unlike the prominent basophilia of CML); frequently harbors $SETBP1$ and $ETNK1$ mutations.

C. MDS/MPN with Ring Sideroblasts and Thrombocytosis (MDS/MPN-RS-T)

• Features: Combines the dysplastic erythroid features of MDS-RS with the proliferative thrombocytosis of Essential Thrombocythemia.
• Diagnostic Criteria: Refractory anemia with $\ge 15%$ ring sideroblasts on marrow Prussian blue stain, sustained platelet count $\ge 450 \times 10^9/\text{L}$, and bone marrow proliferation of large, atypical megakaryocytes.
• Molecular Pathogenesis: Characterized by co-occurring mutations: $SF3B1$ mutation (driving ring sideroblast formation) combined with a $JAK2$ V617F mutation (or less commonly CALR or MPL, driving megakaryocyte hyperproliferation and thrombocytosis).