What Is Nijmegen Breakage Syndrome?

Nijmegen breakage syndrome (NBS) is a rare inherited condition in which the body’s ability to repair damaged DNA is severely impaired, leading to a cascade of problems that include a small head at birth, weakened immunity, and an exceptionally high risk of cancer in childhood and young adulthood. It is caused by mutations in a gene called NBN, and because of a single founder mutation, the condition is overwhelmingly concentrated in people of Slavic descent. Fewer than a few hundred cases have been formally documented worldwide, but the carrier rate in some Eastern European populations is surprisingly high, giving NBS an outsized significance in genetics and cancer biology.

What Goes Wrong at the Molecular Level

Every cell in your body sustains DNA damage constantly, from normal metabolism, sunlight, and environmental exposures. Among the most dangerous types of damage is a double-strand break, where both rails of the DNA ladder snap. Healthy cells have a rapid-response system for these breaks. A key part of that system is a three-protein sensor complex called MRN, made up of the proteins MRE11, RAD50, and NBS1 (also known as nibrin). When a double-strand break occurs, the MRN complex is the first to arrive at the scene. It recognizes the damage, holds the broken ends in place, and activates a signaling cascade that pauses cell division and summons repair machinery.¹

In NBS, the NBN gene that encodes nibrin is mutated, so the protein is absent or non-functional.² Without nibrin, the MRN complex cannot assemble properly, and the entire damage-response chain falls apart. The cell cannot reliably detect breaks, cannot properly signal them, and cannot efficiently route them to the correct repair pathway.³ Cell lines from NBS patients are hypersensitive to ionizing radiation and show abnormal localization of the other two MRN proteins, MRE11 and RAD50, which normally cluster at damage sites but fail to do so without nibrin.⁴ The practical result is that chromosomes accumulate breaks, rearrangements, and other structural errors at a rate far higher than normal, which is why the condition was originally named for the “breakage” visible under a microscope.⁵

Recognizing the Condition

The most visible hallmark of NBS is microcephaly, a head that is noticeably smaller than expected. It is present at birth and becomes progressively more pronounced as the child grows. Despite the small head size, most children with NBS reach developmental milestones on time in the early years, which can initially be misleading. Intellectual disability, when it develops, tends to be in the mild-to-moderate range.⁶

Facial features become more distinctive with age. The forehead slopes backward, and the lower jaw is relatively small, while the middle part of the face appears prominent. These features are consistent enough across patients that experienced clinicians can suspect NBS on appearance alone. Growth is mildly delayed, and most patients are shorter than their peers, though not dramatically so.

Female patients face an additional problem. The same DNA-repair failure that causes chromosomal instability also damages the ovaries. This leads to a condition called premature ovarian insufficiency, meaning the ovaries stop functioning normally well before the expected age. Affected girls may fail to enter puberty on their own or may experience early menopause. Hormonal evaluation often reveals elevated levels of the hormones that stimulate the ovaries, a sign the ovaries are not responding.⁷

Immunodeficiency and Infections

The DNA-repair pathway that nibrin supports is not only needed after radiation damage. It also plays a role in the normal rearrangement of immune-system genes during the development of infection-fighting cells. Without functional nibrin, both the antibody-producing arm and the cell-mediated arm of the immune system develop poorly. The result is combined immunodeficiency, meaning patients lack adequate levels of protective antibodies and also have reduced T-cell function.

In practice, this translates to frequent and sometimes chronic respiratory tract infections, including sinusitis, bronchitis, and pneumonia. These recurrent infections can cause lasting lung damage over time and are a major factor in long-term prognosis.⁸ Many patients require regular immunoglobulin replacement therapy, where pooled antibodies from donor blood are infused to compensate for what the patient’s own immune system cannot produce. Prophylactic antibiotics are also commonly prescribed.

The immunodeficiency creates a further complication for vaccination. Live vaccines, which contain weakened versions of a virus or bacterium, can theoretically cause disease in someone whose immune system cannot mount a proper response. Clinicians managing NBS patients typically avoid live vaccines and rely on inactivated alternatives where available, though even those may not generate a strong protective response.

Cancer Risk

The most serious consequence of NBS is an extraordinarily high cancer risk. By age 20, more than 40 percent of patients develop a malignancy, and the cancers that arise are overwhelmingly lymphoid, meaning they originate in the cells of the immune system.⁹ Non-Hodgkin lymphomas are the most common, but Hodgkin lymphomas, leukemias, and other blood cancers also occur at elevated rates.¹⁰

This cancer susceptibility is a direct consequence of the chromosomal instability that defines the condition. When double-strand breaks go unrepaired or are repaired incorrectly, sections of chromosomes can be lost, duplicated, or rearranged. Cytogenetic analysis of tumor cells from NBS patients frequently reveals complex karyotypes with multiple structural abnormalities, reflecting this ongoing genomic chaos.¹¹ Even cells grown in the laboratory from NBS patients accumulate chromosomal errors at a rapid pace.¹²

Some patients develop a second cancer after treatment for their first, adding another layer of difficulty to an already challenging clinical picture.

Why Standard Cancer Treatment Is Dangerous

Treating cancer in a patient with NBS is one of the hardest problems in pediatric oncology. The same DNA-repair defect that causes cancer also makes the patient dangerously sensitive to the tools doctors normally use to fight it. Radiation therapy works by inflicting double-strand breaks on tumor DNA, and chemotherapy drugs like alkylating agents do similar damage. In a patient whose cells cannot repair those breaks, the collateral damage to healthy tissue can be catastrophic.

A case report described fatal toxicity in a child with unrecognized NBS who received standard-dose radiation and chemotherapy for a brain tumor, highlighting the importance of screening for DNA-repair disorders before initiating aggressive treatment in children with certain physical features.¹³ Based on a systematic review of outcomes, clinicians now recommend reducing chemotherapy doses by up to half for NBS patients with severe immunodeficiency, particularly for drugs like anthracyclines, methotrexate, alkylating agents, and certain plant-derived agents. Radiation therapy and the drug bleomycin should be avoided entirely when possible.¹⁴

These dose reductions are a difficult balancing act. Use too little chemotherapy and the cancer may not respond. Use too much and the patient may die from treatment toxicity, bone marrow failure, or overwhelming infection. There is no one-size-fits-all protocol, and treatment decisions rely heavily on the individual patient’s immune status and the type of malignancy involved.

Stem Cell Transplantation

Hematopoietic stem cell transplant, essentially replacing the patient’s blood and immune system with a donor’s, is the only approach that can address both the immunodeficiency and an existing malignancy at the same time. But transplant for NBS carries its own significant risks, again because of the underlying sensitivity to the conditioning regimens used to prepare the body for the new cells.

Data from a multi-center analysis of transplant outcomes in patients with DNA double-strand break repair disorders, including NBS, showed an overall survival rate of about 69 percent. The type of conditioning regimen made a dramatic difference. Patients who received reduced-intensity conditioning, which uses lower doses of chemotherapy and radiation to prepare the body, had a survival rate of roughly 79 percent. Those who received full-strength myeloablative conditioning had a survival rate of only 41 percent, with most deaths occurring early in the transplant course, suggesting poor tolerance of the regimen itself.¹⁵

A smaller case series of six NBS patients who underwent transplant reported that five of the six survived, and all surviving patients showed restored T-cell immunity after the procedure. The single death occurred in the patient who received myeloablative conditioning. Graft-versus-host disease, when the donor cells attack the recipient’s body, was mild in those who experienced it.¹⁶ These findings strongly support using reduced-intensity conditioning protocols whenever transplant is indicated.

How NBS Differs from Ataxia-Telangiectasia

NBS is sometimes confused with ataxia-telangiectasia (A-T), and for good reason. Both conditions involve defective DNA double-strand break repair, and at the cellular level they are almost indistinguishable: cells from both conditions show chromosomal instability, radiation sensitivity, and the same defects in cell-cycle checkpoints triggered by radiation.¹⁷ Both conditions also share immunodeficiency and a strong predisposition to cancer.

The key clinical difference is neurological. A-T causes progressive cerebellar degeneration, leading to increasingly severe problems with balance, coordination, and movement. NBS does not. An NBS patient will have microcephaly and may have mild intellectual disability, but they do not develop the characteristic wobbling gait and movement difficulties of A-T. The connection between the two conditions runs deep, though: nibrin (the protein missing in NBS) normally works by activating ATM, the very protein that is defective in A-T.¹⁸ They are defects at different steps of the same pathway, which explains why the downstream consequences overlap so heavily.

The Slavic Founder Mutation and Carrier Frequency

The vast majority of NBS cases worldwide trace back to a single mutation: a five-base-pair deletion in the NBN gene, designated 657del5. This mutation arose in a common ancestor of modern Slavic populations and has persisted at a surprisingly high frequency. A study across the Czech Republic, Poland, and Ukraine found a carrier rate of about 1 in 177, far higher than would be expected for a mutation that causes such a serious disease.¹⁹

Why hasn’t natural selection eliminated a mutation that causes profound disease when inherited from both parents? One intriguing piece of evidence suggests that carriers, people with one working copy and one mutated copy, may have a subtle reproductive advantage. A study comparing carrier and non-carrier women from the same communities found that carriers had significantly more children on average: about 3.0 offspring per carrier versus 2.4 per non-carrier. This difference held across age groups and was statistically robust.²⁰ The mechanism behind this fertility boost is unknown, but the finding raises the possibility that the 657del5 mutation persists in Slavic populations partly because carriers, who are healthy themselves, tend to have larger families.

Cancer Risk in Carriers

Being a carrier of one copy of the 657del5 mutation does not cause NBS, but it is not entirely without consequence. Carriers face a modestly increased risk of certain cancers. A large case-control study comparing nearly 5,000 breast cancer patients with over 6,000 healthy controls found the 657del5 mutation roughly twice as often in the cancer group, translating to about a twofold increase in breast cancer risk for carriers.²¹

A twofold increase sounds alarming in isolation, but for context, it is similar in magnitude to the risk increase associated with a number of moderate-penetrance breast cancer genes. It does not approach the scale of risk conferred by mutations in BRCA1 or BRCA2, which raise lifetime breast cancer risk many times over. Still, given that 1 in 177 people in certain populations carries this mutation, the population-level contribution to breast cancer burden could be meaningful. The finding has implications for genetic screening programs in Eastern European countries, where testing for the 657del5 mutation could be incorporated into broader cancer-risk panels at relatively low cost.

Prenatal Testing and Family Planning

For couples who already know they are both carriers, prenatal diagnosis is available and reliable. Because the 657del5 mutation accounts for such a large proportion of disease-causing alleles, molecular testing is straightforward. In one documented case, after a first child was diagnosed with NBS, genetic testing confirmed both parents were carriers, and prenatal diagnosis was successfully performed in the next pregnancy through amniocentesis. That pregnancy was found to be affected; the fetus was homozygous for the 657del5 mutation and had developed severe hydrocephalus, a complication consistent with the condition.²²

Preimplantation genetic testing, where embryos created through in vitro fertilization are tested before implantation, is another option for carrier couples who wish to avoid an affected pregnancy. Genetic counseling is strongly recommended for any family with a confirmed NBS diagnosis, particularly in populations where the carrier rate is high enough that both partners could carry the mutation without knowing it.

Why NBS Is Underdiagnosed

Despite its distinctive combination of features, NBS is likely underdiagnosed. Part of the problem is rarity itself: many pediatricians will never see a case in their careers. Microcephaly has a long list of potential causes, and a child who is meeting developmental milestones may not immediately trigger suspicion for a chromosomal instability syndrome. The immunodeficiency may initially be attributed to more common causes, and by the time cancer develops, the window for early intervention has narrowed.

The case of the child who died from treatment toxicity after receiving full-dose chemoradiation for a brain tumor underscores this problem.²³ That child’s NBS was not recognized until after the fatal outcome. The authors of that report recommended that all pediatric cancer patients whose height, weight, or head circumference falls below the third percentile, along with any child showing features suggestive of a DNA-repair disorder or immunodeficiency syndrome, should be screened before starting aggressive therapy. Genetic testing for the common founder mutation is simple and inexpensive, and catching NBS before treatment begins can be the difference between an adapted protocol that is tolerable and a standard protocol that is lethal.

Living with NBS

Day-to-day management of NBS revolves around immune support, cancer surveillance, and avoiding unnecessary radiation exposure. Regular immunoglobulin infusions, prophylactic antibiotics, and aggressive treatment of infections form the core of immunological management. Cancer screening, particularly for lymphoid malignancies, should begin early and be repeated regularly, though no standardized screening protocol has been universally adopted.

Dental X-rays, CT scans, and other sources of medical radiation should be minimized or replaced with non-radiation alternatives like ultrasound or MRI. Families are also advised to limit environmental radiation exposure where practical, though the natural background radiation encountered in everyday life is not something that can meaningfully be avoided. Sun protection is sensible general advice but does not address the ionizing radiation sensitivity that defines the condition.

For female patients, early endocrinological evaluation is important to detect premature ovarian insufficiency and initiate hormone replacement therapy if needed to support puberty, bone health, and overall well-being.²⁴ Fertility is likely severely compromised or absent in affected women, and this should be discussed with families as part of long-term care planning.

Life expectancy in NBS varies widely and depends heavily on whether malignancy develops and how it is managed. With improved recognition, gentler chemotherapy protocols, and the growing experience with reduced-intensity stem cell transplantation, outcomes have improved compared with the early decades after the condition was first described. But NBS remains a severe disease with major impacts on quality of life and longevity, and the families affected by it live with a level of medical complexity that few other conditions match.