What Is a PNH Clone and What Does It Do to the Body?

A PNH clone is a population of blood cells descended from a single bone marrow stem cell that has acquired a mutation in a gene called PIGA, which is needed to build a molecular anchor that holds dozens of protective proteins onto the cell surface. Without that anchor, the offspring of the mutated stem cell lack key complement-regulating proteins, leaving them vulnerable to destruction by the body’s own immune system. The size and behavior of this clone determine whether a person has mild laboratory findings or full-blown paroxysmal nocturnal hemoglobinuria (PNH), a disease marked by chronic red blood cell destruction, dangerous blood clots, and bone marrow failure. What makes the PNH clone so unusual among acquired blood disorders is the paradox at its center: the mutation itself is not rare, but the conditions that allow it to flourish are.

The Mutation Behind the Clone

Every PNH clone traces back to a somatic mutation in PIGA, a gene located on the X chromosome. PIGA encodes an enzyme required in the first step of building glycosylphosphatidylinositol (GPI) anchors, the lipid tethers that hold certain proteins on the outer surface of cells. When PIGA is knocked out, the cell and all its descendants lose every GPI-anchored protein at once. Researchers identified PIGA as the culprit in the early 1990s, and the gene’s X-chromosome location explains why a single hit is enough: because cells use only one active copy of the X chromosome, one mutation silences the only working copy of the gene.¹

Among the dozens of surface proteins lost, two matter most for disease: CD55 and CD59, both of which normally shield red blood cells from complement, a cascade of immune proteins that punches holes in cell membranes. Without CD55 and CD59, PNH red blood cells are defenseless against complement-mediated destruction.² Laboratory experiments confirm the connection directly: when those two proteins are stripped from normal red blood cells using enzymes, the cells become just as vulnerable to complement attack as PNH cells.³

For years PNH was considered a straightforward single-gene disease: one stem cell picks up a PIGA mutation, and the clone that grows from it produces all the trouble. Newer genomic work has complicated that picture, revealing that PNH stem cells often carry additional mutations beyond PIGA, which may influence how aggressively the clone expands.⁴

Why the Clone Grows Instead of Staying Small

Here is the part that puzzled researchers for decades. PIGA mutations turn up in tiny numbers of blood cells in virtually all healthy people, yet those “microclones” never expand into anything clinically meaningful.⁵ The mutation alone is not enough. Something else has to change in the bone marrow environment to give the mutant stem cell a survival advantage over its normal neighbors.

The leading explanation is autoimmune attack. In this model, cytotoxic T cells target normal stem cells but spare GPI-negative ones, essentially creating a hostile environment in the marrow where only the mutant clone thrives. Several lines of evidence support this. PNH overlaps heavily with aplastic anemia, a disease driven by immune destruction of marrow cells. When PIGA is intentionally knocked out in mice, the resulting GPI-negative blood cells gradually disappear rather than expand, because the mice lack the autoimmune pressure that would select for them. And in patients who have received bone marrow transplants from genetically identical twins, PNH went into remission only when the transplant included immune-suppressing conditioning.⁶ The picture fits a Darwinian model inside the body: a random mutation that is normally harmless becomes advantageous when the environment shifts in a specific way.

The mechanisms behind clonal selection likely involve immune evasion and improved cell survival in a marrow under immune attack.⁷ This also explains why PNH clones are found so often in patients with aplastic anemia: the autoimmune marrow environment is already there, providing the selective pressure the clone needs to grow.

What the Clone Does to the Body

The clinical consequences of a PNH clone flow from one central event: complement chews up unprotected red blood cells inside the bloodstream. This intravascular hemolysis releases free hemoglobin, which scavenges nitric oxide, a molecule the body depends on to keep blood vessels relaxed and functioning properly.⁸ When nitric oxide drops, the downstream effects are wide-ranging.

Smooth muscle spasms cause abdominal pain, difficulty swallowing, and erectile dysfunction. Research in animal models has shown that the esophageal problems PNH patients report are directly tied to the loss of nitric oxide from excess free hemoglobin in the blood.⁹ Blood clots are the most dangerous complication, and the leading cause of death in untreated PNH. The clotting risk comes from a convergence of complement activation, platelet and white blood cell activation, endothelial damage, and a self-reinforcing loop between the complement system and the clotting cascade.¹⁰ Fatigue, dark urine (especially in the morning, when urine has concentrated overnight), shortness of breath, and anemia round out the classic symptom picture.¹¹

How PNH Clones Are Measured

The gold standard for detecting a PNH clone is flow cytometry, a technique that identifies cells based on the proteins they carry on their surface. Modern assays look at red blood cells, neutrophils, and monocytes for the absence of GPI-anchored markers. Standardized protocols can reliably detect PNH clones as small as about 0.01% of a cell population.¹²

A reagent called FLAER (fluorescent aerolysin) has become particularly important. Aerolysin is a bacterial toxin that binds specifically to GPI anchors, so a fluorescent version of it lights up normal white blood cells while leaving PNH cells dark. FLAER-based assays on white blood cells are more sensitive than older CD59-based assays on red blood cells. In one comparison over three years, the FLAER approach detected PNH clones in about 12% of tested samples, while the red blood cell method caught only about 6%, always with smaller clone sizes.¹³ The reason for the gap is straightforward: transfusions dilute the PNH red blood cell pool, and complement destroys PNH red cells faster than normal ones, so red cell testing consistently underestimates the true clone size. White blood cells are not affected by either factor, making them a more accurate mirror of what is actually happening in the marrow.

Clone size is reported as a percentage of the cell population tested. A “small” clone might be under 1%, often found incidentally in patients with aplastic anemia or myelodysplastic syndromes. A “large” clone, sometimes over 50%, is more typical in classical PNH with active hemolysis and thrombotic risk. The clinical severity generally tracks with clone size, although the relationship is not perfectly linear; some patients with moderately sized clones have significant symptoms while others do not.

Clone Size Can Change Over Time

PNH clones are not static. A study tracking clone sizes in aplastic anemia patients over time found that roughly a quarter stayed stable, about half grew, and another quarter shrank.¹⁴ This variability is why regular monitoring matters. A patient whose clone is small today may need treatment in a few years, while another patient’s clone may quietly fade. The trajectory depends in part on what is happening in the bone marrow: if the autoimmune pressure that favored the clone eases (sometimes after immunosuppressive therapy for aplastic anemia), the clone can lose its survival advantage and shrink.

Spontaneous remission is documented but rare. For most patients with established PNH and large clones, the disease is chronic and lifelong without intervention.

PNH Clones in Aplastic Anemia and Myelodysplastic Syndromes

One of the most clinically relevant facts about PNH clones is that they show up frequently in bone marrow failure disorders, even when the patient does not have classic PNH. In aplastic anemia, PNH clones are found in roughly 60% of cases. In myelodysplastic syndromes (MDS), the rate is lower, around 20%, and the clones tend to be smaller.¹⁵ When PNH clones do appear in MDS, they tend to cluster in lower-risk subtypes with less than 5% blasts and relatively modest clone sizes, often well below 1%.¹⁶

Finding a PNH clone in an aplastic anemia patient is considered a positive prognostic sign by many hematologists. It suggests an immune-mediated mechanism is driving the marrow failure, and such patients tend to respond better to immunosuppressive therapy. The presence of large PNH clones (over 10%) is more common in aplastic anemia than in MDS, and these patients tend to have deeper drops in blood counts along with higher markers of red cell destruction.¹⁷

Treating the Consequences Without Eliminating the Clone

Complement inhibitors are the backbone of PNH treatment, but it is worth understanding what they do and do not accomplish. Drugs like eculizumab block the complement cascade at a specific point (the C5 protein), which prevents the final “attack” step that punches holes in red blood cells. This dramatically reduces intravascular hemolysis and slashes the clotting risk.¹⁸ But these drugs do not touch the PNH clone itself. The mutant stem cells keep dividing; the GPI-negative blood cells keep being produced. Treatment manages the downstream damage, not the source.

This distinction matters because blocking complement at C5 creates a new problem: earlier complement fragments, particularly C3, still land on PNH red blood cells. Under normal circumstances, C3-coated cells would go on to be destroyed by the terminal complement pathway. When that pathway is blocked, the C3 fragments accumulate, and macrophages in the spleen and liver recognize and eat these tagged cells. The result is a low-level extravascular hemolysis that was previously masked by the more dramatic intravascular destruction.¹⁹ About a quarter of patients on eculizumab remain dependent on transfusions because of this residual hemolysis.²⁰

Newer drugs aim to address this gap. Pegcetacoplan, for example, inhibits complement further upstream at the C3 level, blocking both intravascular and extravascular destruction. Ravulizumab works at the same C5 target as eculizumab but lasts longer, allowing infusions every eight weeks instead of every two. A cost-effectiveness analysis from the Netherlands found that switching from eculizumab to ravulizumab saved roughly €267,000 per patient while also improving quality-adjusted life years, driven almost entirely by the reduced frequency of infusions.²¹

The Only Way to Eliminate the Clone Entirely

Allogeneic stem cell transplant remains the sole cure for PNH, meaning the only treatment that replaces the mutant stem cells with healthy donor cells. A multicenter analysis found that after transplant, the PNH clone was reduced to 0% in about half of classical PNH patients and to less than 1% in nearly all the rest, with three-year overall survival around 89%.²²

But transplant carries serious risks, including graft-versus-host disease, infections during immune suppression, and treatment-related death. Given that complement inhibitors now control symptoms effectively for most patients, transplant is generally reserved for people who also have underlying bone marrow failure or life-threatening clotting disease that has not responded to other treatment.²³ The calculus shifted after complement inhibitors became available: hemolytic anemia alone is no longer considered sufficient justification for the risks of transplant.

PNH Clones During Pregnancy

Pregnancy adds a layer of complexity because complement activity naturally increases, and the physiological changes of pregnancy independently raise clotting risk. For women with PNH clones, this creates a dangerous overlap. Eculizumab has been used during pregnancy, and the data, while limited to a few dozen pregnancies, are broadly reassuring. In one series, about 84% of pregnancies treated with eculizumab resulted in live births, with no maternal deaths and no cases of maternal thrombosis.²⁴

The management is not simple, though. Over half of pregnant women needed their eculizumab dose increased to keep hemolysis under control, reflecting the rising complement burden of pregnancy.²⁵ Nearly all received anticoagulation with low-molecular-weight heparin. Premature birth rates were higher than in the general population, with about a quarter to a third of babies arriving early. Hemorrhagic events occurred in roughly one in ten pregnancies. These numbers reflect active, aggressive management, so the outcomes, while encouraging, should not be read as evidence that PNH pregnancies are low-risk.

Living With a PNH Clone on Treatment

Even with modern complement inhibitors, the disease takes a measurable toll on daily life. Patients on treatment still report worse quality of life across physical, social, and cognitive domains compared to the general population.²⁶ Fatigue is the single biggest driver of impaired function and correlates with nearly every measured aspect of quality of life.²⁷ Part of the fatigue is residual anemia from incomplete hemolysis control. Part of it may come from the chronic inflammatory state that PNH sustains even when the most visible symptoms are managed.

The treatment burden itself is substantial. Eculizumab requires intravenous infusions every two weeks, typically at an infusion center. The longer-acting ravulizumab reduced this to every eight weeks, a meaningful improvement for people who had been structuring their lives around biweekly appointments. But complement inhibitors are among the most expensive drugs in the world, with annual costs that can exceed several hundred thousand dollars per patient, making access a persistent challenge in healthcare systems worldwide.

PNH in Children

PNH is overwhelmingly a disease of adults, but it does occur in children, usually presenting as bone marrow failure rather than the classic hemolytic picture seen in adults. In a pediatric cohort, the majority of patients initially presented with signs of marrow failure, and about half had features of myelodysplasia without progressing to leukemia.²⁸ Thrombosis was still a major problem, occurring in half the patients and directly causing two deaths. Several children in that series underwent stem cell transplant, with three out of five surviving disease-free. The rarity of pediatric PNH means there are no large controlled trials, and treatment decisions often rely on extrapolating from adult data along with close monitoring of the clone over time.

Microclones in Healthy People

One of the more surprising findings in PNH biology is that tiny populations of PIGA-mutant cells exist in the blood of nearly all healthy individuals. These microclones are detectable only with high-sensitivity assays and typically make up a vanishingly small fraction of circulating cells. For a long time, their existence suggested that PIGA mutations are an unavoidable accident of normal blood cell production, occurring at a steady rate in everyone’s bone marrow. More recent work, however, indicates that most of these mutations in healthy people arise in short-lived progenitor cells rather than true long-lived stem cells, which is why they never grow into clinically meaningful populations.²⁹ The distinction matters: a mutation in a stem cell can sustain a clone indefinitely, while a mutation in a downstream progenitor produces a burst of abnormal cells that eventually washes out as the progenitor exhausts its lifespan. This is why having detectable PIGA-mutant cells in your blood is not, in itself, a reason to worry.