Birt-Hogg-Dubé syndrome (BHD) is a rare inherited condition caused by mutations in a single gene called FLCN, which codes for a protein named folliculin. It affects three main organ systems: the skin, the lungs, and the kidneys. People with BHD develop small benign skin bumps (usually on the face and neck), lung cysts that can rupture and cause a collapsed lung, and an elevated lifetime risk of kidney cancer. Because these features seem unrelated at first glance, BHD often goes undiagnosed for years, and understanding how its parts fit together matters for anyone carrying the mutation or caring for someone who does.
What Causes Birt-Hogg-Dubé Syndrome
BHD traces back to mutations in one gene: FLCN, located on chromosome 17. This is the only gene known to cause the syndrome.1PubMed Central. The folliculin mutation database: an online database of mutations associated with Birt-Hogg-Dubé syndrome The condition follows an autosomal dominant inheritance pattern, meaning you only need one faulty copy of the gene (inherited from one parent) to be affected.2The Lancet Oncology. Birt-Hogg-Dubé syndrome If a parent carries the mutation, each child has a 50 percent chance of inheriting it.
Most FLCN mutations result in a shortened, non-functional version of the folliculin protein. Deletions account for roughly 45 percent of known mutations, with substitutions making up about 32 percent and duplications around 15 percent.1PubMed Central. The folliculin mutation database: an online database of mutations associated with Birt-Hogg-Dubé syndrome New mutations continue to be discovered in families worldwide, including novel variants identified in Korean and other populations that expand the known mutation spectrum.3PubMed Central. A novel FLCN gene mutation causing Birt-Hogg-Dubé syndrome in a Korean family4PubMed Central. A Case of Birt-Hogg-Dubé (BHD) Syndrome Harboring a Novel Folliculin (FLCN) Gene Mutation
Folliculin does not do just one thing in the cell. It partners with other proteins (FNIP1 and FNIP2) and interacts with a key energy-sensing enzyme called AMPK, as well as the mTOR pathway, which controls cell growth and metabolism.5PubMed Central. FLCN: The causative gene for Birt-Hogg-Dubé syndrome When folliculin is missing or broken, these growth-regulating pathways go haywire, which helps explain how a single gene can produce such different problems in the skin, lungs, and kidneys. Research in animal models has also linked folliculin loss to changes in immune signaling, suggesting the protein’s reach extends even further than originally thought.6PubMed Central. The Transcription Factors TFEB and TFE3 Link the FLCN-AMPK Signaling Axis to Innate Immune Response and Pathogen Resistance
Skin Symptoms
The hallmark skin feature of BHD is the fibrofolliculoma, a small, dome-shaped, skin-colored or whitish bump that arises from the hair follicle. When the syndrome was first described in 1977 by three Canadian physicians, it was defined by three types of skin growths: fibrofolliculomas, trichodiscomas, and acrochordons (skin tags).7PubMed Central. Birt-Hogg-Dubé syndrome: from gene discovery to molecularly targeted therapies Research since then has suggested that all three are probably variations of the same underlying lesion rather than truly separate tumors.8PubMed. Birt-Hogg-Dubé syndrome: a review of the literature and the differential diagnosis of firm facial papules
These bumps typically start appearing between a person’s twenties and forties, and they tend to show up on the face, neck, and upper trunk. Over time, more lesions appear, and existing ones grow larger.9Aesthetic Surgery Journal Open Forum. Birt–Hogg–Dubé Syndrome: A Rare Genodermatosis Presenting as Skin Papillomas The bumps are benign and painless, but because they cluster on the face, they can be cosmetically distressing. They are sometimes mistaken for acne, warts, or other common skin conditions, which contributes to delayed diagnosis.
Treatment for the skin lesions is purely cosmetic. Options include electrocautery, shave removal, dermabrasion, excision, topical rapamycin, and ablative laser therapy (such as CO₂ laser).10PubMed Central. Fibrofolliculomas in Birt-Hogg-Dubé syndrome treated with nonfractionated ablative CO2 laser None of these are permanent cures; new bumps tend to develop over time because the underlying genetic cause remains. But for people bothered by the appearance, periodic treatment can help.
Lung Cysts and Collapsed Lungs
Lung involvement is one of the most common and potentially dangerous aspects of BHD. The vast majority of people with the syndrome develop multiple thin-walled air-filled cysts in their lungs, even when they have no respiratory symptoms. These cysts tend to concentrate in the lower portions of the lungs and sit near the lung’s outer edges and blood vessels.11PubMed. Thoracic CT findings in Birt-Hogg-Dube syndrome12PubMed Central. Pulmonary manifestations of Birt-Hogg-Dubé syndrome They range from a few millimeters to nearly 8 centimeters across, and the largest ones tend to sit in the lower lobes. About 87 percent of patients in one imaging study had cysts in both lungs.11PubMed. Thoracic CT findings in Birt-Hogg-Dube syndrome
The real danger is spontaneous pneumothorax, a collapsed lung that happens without any obvious injury. One large study of confirmed BHD patients found that 76 percent had experienced at least one spontaneous pneumothorax during their lifetime, and 82 percent of those had more than one episode. The average patient with a first pneumothorax went on to have about 3.6 total episodes. Recurrences could happen on the same side or the opposite side.13PubMed Central. Spontaneous Pneumothoraces in Patients with Birt-Hogg-Dubé Syndrome The first collapse tends to happen around the mid-thirties, though it has been reported as early as age 14.13PubMed Central. Spontaneous Pneumothoraces in Patients with Birt-Hogg-Dubé Syndrome
Another study found a somewhat lower pneumothorax rate of about 24 percent among confirmed mutation carriers, but that study also confirmed a clear link between cyst size and number and the likelihood of lung collapse.14American Journal of Respiratory and Critical Care Medicine. Lung Cysts, Spontaneous Pneumothorax, and Genetic Associations in 89 Families with Birt-Hogg-Dubé Syndrome The variation in reported rates likely reflects differences in how patients were recruited: studies based at pneumothorax clinics naturally capture people who have already had a collapse, while broader family screening studies catch more carriers who have not.
Air travel is a specific concern. In the larger pneumothorax study, 11 collapse episodes among eight patients occurred either during flights or within 24 hours of landing, yielding a rate of about 0.12 percent per flight.13PubMed Central. Spontaneous Pneumothoraces in Patients with Birt-Hogg-Dubé Syndrome That sounds small, but for someone who flies frequently and already has large lung cysts, it adds up. Clinicians generally recommend that BHD patients be aware of the risk and discuss it with their care team before frequent flying.
Kidney Cancer Risk
The most medically serious aspect of BHD is an increased risk of kidney tumors. These tumors tend to appear at younger ages than typical kidney cancers and are more likely to be multiple or affect both kidneys.15PubMed. The ABCs of BHD: An In-Depth Review of Birt-Hogg-Dubé Syndrome The types of kidney cancer seen in BHD are distinctive: they span several subtypes, including chromophobe renal cell carcinoma, hybrid tumors (chromophobe-oncocytoma), oncocytomas, and less commonly clear cell or papillary renal cell carcinoma. This mixed tumor profile is unusual and can itself be a diagnostic clue.
In one single-center study of BHD patients with kidney tumors, the median tumor size at detection was 2 centimeters, and the maximum growth rate was about 0.23 centimeters per year. Only one patient in that group developed metastatic disease after initial surgery.16PubMed Central. Diagnosis of renal tumors in Birt-Hogg-Dube syndrome: Clinical presentation and risk factors in a single-center retrospective cohort A multicentre European study likewise found that while some patients did develop metastases, the behavior of the cancers tended to be more indolent than in sporadic (non-inherited) kidney cancers, and no one in their cohort died from it.17PubMed Central. Renal cell tumour characteristics in patients with the Birt-Hogg-Dubé cancer susceptibility syndrome: a retrospective, multicentre study
That said, the risk of metastasis is not zero. An analysis of 115 confirmed mutation carriers from 35 families found that 5 of 14 patients who developed renal cancer went on to develop metastatic disease, an unexpectedly high fraction that underscored the importance of early detection through surveillance.18British Journal of Cancer. Renal cancer and pneumothorax risk in Birt–Hogg–Dubé syndrome; an analysis of 115 FLCN mutation carriers from 35 BHD families The discrepancy between studies probably reflects differences in screening practices and how early tumors were caught. The takeaway is clear: when caught early, BHD kidney cancers are usually very treatable, but surveillance cannot be skipped.
How BHD Is Diagnosed
Diagnosing BHD can be tricky because its three main features — skin bumps, lung cysts, and kidney tumors — appear at different ages, and many patients initially present with only one of them. A young person might show up at the emergency room with an unexplained collapsed lung; a dermatologist might notice a pattern of facial papules; or a kidney tumor might be found incidentally on imaging. The challenge is connecting these dots and thinking of BHD in the first place.
European clinical practice guidelines recommend that clinicians consider BHD in several scenarios: recurrent spontaneous pneumothorax (especially with a family history of it), characteristic lower-zone lung cysts on chest CT, bilateral or multifocal kidney tumors in a younger person, and the distinctive skin lesions.19European Journal of Human Genetics. ERN GENTURIS clinical practice guidelines for the diagnosis, surveillance and management of people with Birt-Hogg-Dubé syndrome The definitive diagnosis usually comes from genetic testing that identifies a pathogenic FLCN mutation. However, a small number of patients meet all the clinical criteria for BHD without a detectable mutation using standard testing methods, possibly because the mutation lies in a non-coding region or is a large structural rearrangement that standard sequencing misses.20PubMed Central. Folliculin gene-negative Birt-Hogg-Dube syndrome: a case report
Because diagnosis often starts from a single feature, getting the full picture typically requires a skin exam, a chest CT scan, and kidney imaging (usually an abdominal MRI). Family members of a confirmed patient should be offered genetic testing, since half of first-degree relatives carry the mutation on average, and many may have undetected lung cysts or kidney tumors.
How Lung Cysts in BHD Differ from Other Conditions
Several conditions produce multiple lung cysts, and telling them apart matters because the cancer screening implications are very different. Lymphangioleiomyomatosis (LAM), for instance, is another cystic lung disease that primarily affects women of childbearing age and has its own cancer associations. On CT scans, BHD cysts have a recognizable pattern: they concentrate in the lower lungs, tend to be fewer in number, are often irregular or elliptical in shape, and frequently sit against the pleural surface (the lung’s outer lining) or along the blood vessels and dividing walls of the lungs.21PubMed Central. Birt-Hogg-Dubé syndrome: characteristic CT findings differentiating it from other diffuse cystic lung diseases
Compared with LAM, BHD patients tend to be older at diagnosis and are more often male. Their cysts are more irregular, more likely to have internal walls (septation), tend to be larger at their maximum size, and cluster in the lower and peripheral lung zones rather than being spread diffusely throughout both lungs.22PubMed. Differentiation Between Lymphangioleiomyomatosis and Birt-Hogg-Dubé Syndrome: Analysis of Pulmonary Cysts on CT Images A Chinese study found that about 58 percent of BHD patients had fewer than 50 lung cysts, whereas all the patients with LAM and other non-BHD cystic diseases had more than 50. In BHD, the biggest cyst was typically in the lower lobes, while in LAM the distribution of the largest cysts was more scattered.23PubMed Central. Characterization of CT scans of patients with Birt-Hogg-Dubé syndrome compared with those of Chinese patients with non-BHD diffuse cyst lung diseases These imaging patterns can sometimes point a radiologist toward BHD even before genetic testing is done.
Managing Kidney Tumors
The cornerstone of kidney cancer management in BHD is lifelong surveillance imaging, typically with MRI, starting once the diagnosis is confirmed. The goal is to catch tumors early while they are small and treatable with kidney-sparing surgery. Most guidelines recommend imaging at least every one to two years.
When a tumor is found, clinicians generally follow the “3-centimeter rule”: the tumor is monitored with repeat imaging until it reaches about 3 centimeters in its largest dimension, at which point nephron-sparing surgery (removing the tumor while preserving as much healthy kidney tissue as possible) is recommended.24PubMed Central. Diagnosis and management of BHD-associated kidney cancer This approach was originally developed for another inherited kidney cancer syndrome (von Hippel-Lindau disease) and has been adopted for BHD because it balances cancer control against the real risk of losing too much kidney tissue over a lifetime, given that new tumors can keep forming. With this strategy, the vast majority of patients achieve a cure while avoiding the complications of total kidney removal.
There is indirect evidence supporting this threshold’s safety in BHD specifically: BHD-associated kidney tumors tend to grow more slowly than those in VHL, and until recently, no reports had documented metastases from BHD tumors smaller than 3 centimeters.25PubMed Central. Metastatic disease after removal of a renal cell carcinoma smaller than 3 cm in a patient with Birt-Hogg-Dubé syndrome, a case report That said, the existence of at least one case report of metastatic disease from a sub-3-centimeter tumor means the rule is a general guideline, not an ironclad guarantee. Close monitoring and individualized judgment remain essential.
Managing Recurrent Pneumothorax
For people with BHD who experience their first collapsed lung, the immediate treatment is standard: chest drainage to re-expand the lung. The harder question is what to do about recurrence. Conservative management (observation, chest tubes, and chemical pleurodesis, where an irritant is instilled to make the lung stick to the chest wall) carries a recurrence rate of roughly 53 percent. Surgical treatment, typically video-assisted thoracoscopic surgery (VATS) with removal of the cyst and mechanical pleurodesis, drops the recurrence rate to about 9 percent.26European Journal of Cardio-Thoracic Surgery. Familial spontaneous pneumothorax: importance of screening for Birt–Hogg–Dubé syndrome
Given the very high rate of recurrent collapses in BHD (over 80 percent of patients who have one will have another), many specialists lean toward surgical intervention earlier in the course than they would for a typical pneumothorax patient without BHD. The decision depends on which side has collapsed, how many cysts are present, and how disruptive the recurrences are to the person’s life. The surgery does not prevent new cysts from forming elsewhere in the lung, so even after a successful procedure, ongoing monitoring is important.
Living with BHD Beyond the Medical Facts
A diagnosis of BHD does not just bring medical appointments. People with the syndrome often face a web of non-medical challenges: economic pressure from ongoing imaging and procedures, physical anxiety about when the next pneumothorax might happen, and complicated family dynamics around who has been tested, who carries the mutation, and what that means for children.27PubMed Central. Birt-Hogg-Dubé: beyond the clinical manifestations These stresses can strain relationships, particularly when family members disagree about genetic testing or when a positive result in a child changes how parents approach their own health planning.
Because BHD is so rare, many patients and families feel isolated. Few local physicians will have seen a case before, which means patients often become their own advocates, explaining the condition to each new specialist they see. Online patient communities and rare-disease organizations have become important resources for sharing information and connecting families across geographic boundaries.
Research Directions and mTOR Inhibitors
Because folliculin’s loss activates the mTOR growth pathway, researchers have been interested in whether drugs that block mTOR could help treat BHD-associated kidney tumors. In laboratory models, renal cells lacking functional folliculin formed tumors, and the mTOR inhibitor sirolimus (also known as rapamycin) suppressed their growth.28PubMed Central. Flcn-deficient renal cells are tumorigenic and sensitive to mTOR suppression mTOR inhibitors are already approved for other cancers and used in transplant medicine, so the drug infrastructure exists. But moving from animal models to proven treatments in BHD patients is a long process, and no large clinical trials have yet established mTOR inhibitors as standard therapy for BHD-related kidney cancer.
There is also interest in whether rapamycin applied to the skin could help control fibrofolliculomas, since folliculin loss in skin cells may also involve the same pathway. This remains experimental. For now, the mainstay of BHD management is surveillance and timely intervention rather than preventive drug therapy, but the genetics-to-targeted-therapy pipeline is the most promising avenue for eventually changing that calculus.