When most people hear the word 'cancer,' they think of tumours — solid masses growing in organs. But some of the most important cancers begin much more quietly, with a bone marrow that slowly stops doing its job properly. Myelodysplastic Syndromes, or MDS, are a group of disorders in which the bone marrow produces blood cells that are abnormal in shape, size, or maturation — a condition called dysplasia. These defective cells often die before they ever reach the bloodstream, leading to dangerously low blood counts despite a marrow that is, paradoxically, working overtime.
MDS is increasingly recognised as a form of blood cancer, though it behaves very differently from aggressive leukaemias. It is a disease of aging — the median age at diagnosis is 73 years, and 86% of patients are over 60. Between 10,000 and 15,000 new cases are diagnosed in the United States each year, with men affected roughly twice as often as women. And approximately 30% of patients with MDS will eventually progress to Acute Myeloid Leukaemia (AML), making early and accurate diagnosis critical.
This article explains what MDS is, how it is classified and diagnosed, what drives its behaviour at the genetic level, and how modern medicine approaches its treatment — from watchful waiting to stem cell transplantation.
What Goes Wrong in MDS
To understand MDS, it helps to recall how normal blood production works. Hematopoietic stem cells in the bone marrow divide and differentiate through a series of stages, eventually producing mature red blood cells, white blood cells, and platelets. In MDS, one or more of these stem cells acquires genetic mutations that disrupt this orderly process. The mutated cells can still divide — often quite actively — but the daughter cells they produce are abnormal. Many of these abnormal cells die in the marrow before reaching maturity, a phenomenon called ineffective hematopoiesis.
The result is a frustrating paradox: the bone marrow is often hypercellular (more cells than normal), yet the blood counts are low. The body is trying harder than ever to make blood cells, but the factory is producing defective products that do not survive quality control. This inefficiency is what drives the clinical features of MDS — persistent cytopenias (low blood counts) that do not respond to the usual nutritional fixes like iron or B12.
Signs and Symptoms
MDS often presents insidiously. Many patients are asymptomatic and are diagnosed only because a routine blood test reveals unexpectedly low counts. When symptoms do occur, they typically reflect one or more of the three major cytopenias:
- Anemia (low red blood cells): fatigue, weakness, shortness of breath on exertion, pale skin, palpitations. This is the most common presentation and affects the majority of MDS patients.
- Neutropenia (low neutrophils): recurrent or unusual infections — skin infections, sinusitis, pneumonia, urinary tract infections. Neutropenia can be life-threatening if severe.
- Thrombocytopenia (low platelets): easy bruising, prolonged bleeding from minor cuts, petechiae (tiny red spots on the skin), nosebleeds, and bleeding gums.
Because these symptoms overlap with many common conditions — and because MDS primarily affects older adults who may attribute fatigue or frequent infections to aging — the diagnosis can be significantly delayed. A high index of suspicion is needed when an older patient presents with persistent, unexplained cytopenias that do not respond to iron, B12, or folate supplementation.
Causes and Risk Factors
MDS is broadly divided into primary (de novo) and secondary (therapy-related) forms. Primary MDS arises without an identifiable external cause — the genetic mutations accumulate over decades, likely accelerated by aging itself. Secondary MDS, also called therapy-related MDS (t-MDS), develops after prior exposure to chemotherapy (particularly alkylating agents and topoisomerase II inhibitors), radiation therapy, or both. Patients treated for other cancers may develop t-MDS years or even a decade later. Secondary MDS generally carries a worse prognosis than primary MDS.
Other risk factors include chronic exposure to benzene and certain industrial chemicals, heavy tobacco use, and male sex. MDS is not inherited in the usual sense, though rare familial predisposition syndromes involving genes like GATA2, RUNX1, and DDX41 have been identified. GATA2 deficiency accounts for approximately 15% of MDS cases in children and young adults — a notably different population from the typical elderly patient.
Classification: From FAB to WHO 2022
The classification of MDS has evolved considerably. The original French-American-British (FAB) system, introduced in 1982, classified MDS based primarily on morphology and blast percentage. The WHO classification — now in its 5th edition (2022) — has refined this substantially by incorporating genetic information alongside morphology.
Key WHO 2022 subtypes include:
- MDS with Low Blasts (MDS-LB): Less than 5% blasts in the marrow. This replaces older terms like refractory anemia and refractory cytopenia with multilineage dysplasia.
- MDS with Increased Blasts-1 (MDS-IB1): 5-9% marrow blasts.
- MDS with Increased Blasts-2 (MDS-IB2): 10-19% marrow blasts. This subtype carries the highest risk of progression to AML.
- MDS with low blasts and isolated del(5q): A distinct subtype defined by a deletion on the long arm of chromosome 5. It typically affects women, presents with macrocytic anemia and normal or elevated platelets, and responds remarkably well to lenalidomide.
- MDS with low blasts and SF3B1 mutation: Characterised by ring sideroblasts in the marrow and a generally favourable prognosis.
- MDS with biallelic TP53 inactivation: A newly defined high-risk category with particularly poor prognosis.
The blast percentage is one of the most important numbers in MDS. When marrow blasts reach 20% or above, the WHO classification considers this transformation to AML — the disease has crossed a line from a smouldering bone marrow disorder to an outright aggressive leukaemia.
Risk Stratification: The IPSS and IPSS-R
Not all MDS is the same. Some patients live for a decade with minimal symptoms, while others progress to AML within months. To predict behaviour and guide treatment, hematologists use scoring systems — most commonly the Revised International Prognostic Scoring System (IPSS-R).
The IPSS-R scores patients based on five factors: cytogenetic category, bone marrow blast percentage, haemoglobin level, platelet count, and absolute neutrophil count. The resulting score places patients into one of five risk groups ranging from Very Low to Very High, with median survival ranging from approximately 8.8 years in the Very Low group to less than 1 year in the Very High group.
More recently, the IPSS-Molecular (IPSS-M) has been introduced, incorporating 31 somatic gene mutations into the scoring. This molecular-enhanced model provides even more precise risk stratification — for instance, identifying that biallelic TP53 mutations and FLT3 mutations are strong adverse predictors, while SF3B1 mutations (without certain co-mutations) are favourable.
The Molecular Landscape of MDS
Modern genomic studies have revealed that MDS is driven by recurrent mutations in a relatively predictable set of genes. Understanding these mutations is increasingly important for diagnosis, prognosis, and treatment selection:
- SF3B1: Found in 20-30% of MDS patients, and in 60-80% of those with ring sideroblasts. SF3B1 mutations are generally associated with favourable prognosis and a good response to luspatercept.
- TP53: Mutations in TP53 — the so-called 'guardian of the genome' — occur in about 5-10% of MDS patients but are enriched in therapy-related MDS and in cases with complex karyotype. Biallelic TP53 inactivation (both copies of the gene affected) is now a distinct WHO category because of its uniformly poor prognosis and resistance to most therapies.
- TET2 and DNMT3A: These are among the most commonly mutated genes in MDS and are involved in epigenetic regulation (DNA methylation). They are also commonly found in age-related clonal hematopoiesis — meaning many elderly people carry these mutations without having MDS, blurring the line between aging and disease.
- ASXL1: Mutations in ASXL1 are associated with worse prognosis and are common in higher-risk MDS.
- IDH1 and IDH2: Found in 10-20% of MDS patients, these mutations confer a worse prognosis in lower-risk MDS and are now targetable with specific inhibitors (ivosidenib and enasidenib).
Treatment: A Risk-Adapted Approach
Treatment of MDS is guided almost entirely by risk category. The approach differs fundamentally between lower-risk and higher-risk disease.
Lower-Risk MDS (Very Low, Low, and some Intermediate)
For patients with lower-risk disease, the primary goals are managing symptoms, reducing transfusion dependence, and maintaining quality of life. Active surveillance (watchful waiting with regular blood counts) is appropriate for asymptomatic patients with mild cytopenias. When anemia becomes symptomatic or transfusion-dependent, options include erythropoiesis-stimulating agents (ESAs) such as epoetin alfa and darbepoetin; luspatercept (a first-in-class erythroid maturation agent that has shown significant reduction in transfusion burden, with median duration of response exceeding 30 weeks); and lenalidomide, which is particularly effective in the del(5q) subtype, achieving transfusion independence in approximately 67% of patients with a median duration of response exceeding 2 years.
Iron chelation therapy (with deferasirox or deferoxamine) is considered for chronically transfused patients to prevent iron overload, aiming to keep serum ferritin below 1,000 micrograms per litre.
Higher-Risk MDS (High and Very High)
For patients with higher-risk disease, the goals shift toward altering the disease course and preventing or delaying transformation to AML. The backbone of treatment is hypomethylating agents (HMAs) — drugs that reverse the abnormal DNA methylation that silences tumour suppressor genes in MDS cells. Azacitidine is the standard of care, having demonstrated in clinical trials an improvement in median overall survival to 24 months compared to 15 months with conventional care, along with higher response rates (29% vs 12%). Decitabine is an alternative HMA with response rates approaching 43% in some studies. An oral formulation (decitabine/cedazuridine) is now available, offering convenience.
However, HMAs are not curative. Responses are typically maintained only as long as treatment continues, and eventually most patients develop resistance. For eligible patients — generally those under 70 with adequate organ function and a suitable donor — allogeneic stem cell transplantation remains the only potentially curative option for higher-risk MDS. Transplant survival rates are approximately 50% at 3 years, though outcomes depend heavily on disease risk, patient fitness, and donor match quality.
The Progression to AML
Approximately 30% of patients with MDS will eventually progress to AML. This transformation occurs when the accumulating genetic mutations reach a tipping point — the abnormal cells stop dying in the marrow and instead begin proliferating aggressively as undifferentiated blasts. By WHO criteria, the threshold is 20% blasts in the marrow.
AML arising from MDS tends to be more resistant to treatment than de novo AML. It more commonly carries adverse cytogenetics and molecular features, and patients are often older with more comorbidities. This is why preventing or delaying progression is such an important goal in MDS management, and why early, accurate risk stratification matters — it identifies the patients most likely to benefit from aggressive intervention before transformation occurs.
Living with MDS: What Patients Should Know
MDS is a chronic condition that requires ongoing monitoring and a partnership between patient and hematologist. Here are the key messages for patients and families:
- Not all MDS is the same. The difference between very low-risk and very high-risk MDS is enormous — measured in years of expected survival and in treatment intensity. Know your risk category.
- Molecular testing is essential. A diagnosis made on morphology alone is incomplete. Cytogenetics and molecular profiling should be performed at diagnosis to accurately classify your disease and guide treatment.
- Regular monitoring matters. Even stable MDS can change over time. Regular blood counts and periodic marrow evaluations allow your doctor to detect progression early.
- Second opinions are valuable. MDS can be challenging to diagnose and classify accurately. Expert pathology review by a hematopathologist experienced in MDS can change the diagnosis in a meaningful percentage of cases.
- Clinical trials are worth considering. Novel agents — including targeted therapies for specific mutations, new combinations, and next-generation hypomethylating agents — are actively being tested and may offer benefits beyond standard treatment.
MDS is a disease that teaches patience and precision — patience because lower-risk disease may require only monitoring, and precision because higher-risk disease demands the right treatment at the right time. Understanding your diagnosis at the molecular level is no longer optional; it is the foundation on which every treatment decision should be built.