Birt–Hogg–Dubé syndrome (BHD), also Hornstein–Birt–Hogg–Dubé syndrome, Hornstein–Knickenberg syndrome, and fibrofolliculomas with trichodiscomas and acrochordons is a human, adult onset, autosomal dominant genetic disorder caused by a mutation in the folliculin (FLCN) gene. It can cause susceptibility to kidney cancer, renal and pulmonary cysts, and noncancerous tumors of the hair follicles, called fibrofolliculomas. The symptoms seen in each family are unique, and can include any combination of the three symptoms. Fibrofolliculomas are the most common manifestation, found on the face and upper trunk in over 80% of people with BHD over the age of 40. Pulmonary cysts are equally common (84%) and 24% of people with BHD eventually experience a collapsed lung (spontaneous pneumothorax). Kidney tumors, both cancerous and benign, occur in 14–34% of people with BHD; the associated kidney cancers are often rare hybrid tumors.
Any of these conditions that occurs in a family can indicate a diagnosis of Birt–Hogg–Dubé syndrome, though it is only confirmed by a genetic test for a mutation in the FLCN gene, which codes for the protein folliculin. Though its function is not fully understood, it appears to be a tumor suppressor gene that restricts cell growth and division. Versions of FLCN have been found in other animals, including fruit flies, German Shepherds, rats, and mice. The disease was discovered in 1977, but the connection with FLCN was not elucidated until 2002, after kidney cancer, collapsed lungs, and pulmonary cysts were all definitively connected to BHD.
Birt–Hogg–Dubé syndrome can manifest similarly to other diseases, which must be ruled out when making a diagnosis. These include tuberous sclerosis, which causes skin lesions similar to fibrofolliculomas, and Von Hippel–Lindau disease, which causes hereditary kidney cancers. Once diagnosed, people with BHD are treated preventatively, with monitoring of kidneys and lungs using medical imaging. Fibrofolliculomas can be removed surgically and pneumothorax and kidney cancer are treated according to the normal standard of care. Dermatologic examinations, neck ultrasounds and colonoscopies should be considered as well.
Signs and symptoms Skin
Birt–Hogg–Dubé syndrome affects the skin and increases the risk of tumors in the kidneys and lungs. The condition is characterized by multiple noncancerous, dome-shaped tumors of the hair follicles (fibrofolliculomas), particularly on the face, neck, and more rarely, the upper chest. The fibrofolliculomas are generally described as having an opaque white color or a yellowish tone and have a waxy, smooth texture. The tumors are always found on and around the nose and on and behind the outer ear. Typically, they first appear in a person's 20s or 30s, and are found in more than 80% of people with the syndrome above the age of 40. The tumors become larger and more numerous over time. Tumors differ between individuals; they may appear merged in plaques, look similar to a comedo with a plug of keratin, or include epidermoid cysts. A large number of tumors on the face can be associated with hyperseborrhea (abnormally elevated sebum production). The presence of fibrofolliculomas on a person's face can cause significant psychological distress.
Other tumors can include trichodiscomas (tumors of the hair disc, which may be identical to fibrofolliculomas), angiofibromas, and perifollicular fibromas. However, angiofibromas are more common in tuberous sclerosis. Along with the tumors, other skin conditions are seen in people with Birt–Hogg–Dubé syndrome. About 40% of people or families with the disease have papules in their mouths, which can be located on the cheeks (buccal mucosa), tongue, gums, or lips. Either white or mucosa-colored, they are discrete, small, and soft, and consist of fibrous tissue covered in thickened epithelium. Collagenomas of the skin are also found in some families. Many people with BHD have skin lesions that appear to be acrochordons (skin tags), but may instead be fibrofolliculomas. These lesions are usually found in the armpit, on the eyelids, and in folds of skin. Not all individuals develop the facial tumors; some families with the mutation that causes BHD develop only kidney tumors or spontaneous pneumothorax.
Kidneys
People over 20 years of age with BHD have an increased risk of developing slow-growing kidney tumors (chromophobe renal carcinoma and renal oncocytoma), kidney cysts, and possibly tumors in other organs and tissues. These tumors often occur in both kidneys and in multiple locations in each kidney. The average number of kidney tumors found in a person with BHD is 5.3, though up to 28 tumors have been found. Hybrid oncocytoma/chromophobe carcinoma, found in 50% of cases, is the most commonly found cancer, followed by chromophobe renal carcinoma, clear cell renal carcinoma, renal oncocytoma, and papillary renal cell carcinoma. People over 40 years old and men are more likely to develop kidney tumors, which are diagnosed at a median age of 48. Kidney cancer associated with BHD have been diagnosed in people at ages as young as 20.
In general, people with this syndrome are at roughly at seven times the risk of kidney cancer compared to the unaffected population. Estimates of the incidence among people with the disease range from 14 to 34%. Rarely, it is associated with clear cell renal cell carcinoma and papillary renal cell carcinoma. If it develops in someone with BHD, renal cell carcinoma occurs later in life and has a poor prognosis. Though the types of tumors typically associated with BHD are considered less aggressive, cases of advanced or metastatic kidney cancer have been observed in people with the syndrome. Both benign and cancerous tumors can reduce kidney function over time as they grow larger.
Lungs Along with fibrofolliculomas and kidney tumors, affected individuals frequently develop cysts (blebs or bullae) in the subpleural lung base or intraparenchymal space that may rupture and cause an abnormal collection of air in the chest cavity (pneumothorax), which could result in the collapse of a lung. The cysts do not cause other symptoms and lung function is usually normal. More than 83% of people with BHD have cysts, but the syndrome does not cause conditions like progressive chronic obstructive pulmonary disease or generalized respiratory failure, though it does cause emphysema. Spontaneous, sometimes recurrent, pneumothorax occurs far more often and at a younger age with BHD than in the unaffected population. Around 24% of people with the disease have at least one spontaneous pneumothorax, 30 times the occurrence in unaffected people. Though pneumothorax caused by BHD often occurs in middle age, at a median age of 38, 17% of affected people have a spontaneous pneumothorax before turning 40. Pneumothoraces have been seen in people as young as 7 and 16 years of age. Some families have a form of BHD that only affects the lungs.
Other organs Thyroid nodules have been associated with the Birt–Hogg–Dubé phenotype, present in 65% of individuals and 90% of families with the syndrome. However, a connection between BHD and thyroid cancer has not been substantiated. Other conditions have been reported to be associated, but may not be caused by the mutation in FLCN or may not be related at all. These include multinodular goiter, medullary thyroid carcinoma, parotid oncocytoma, colonic polyposis, connective tissue nevus, lipomas, angiolipomas, parathyroid adenomas, flecked chorioretinopathy, neurothekeoma, meningiomas, angiofibromas of the face, trichoblastomas, cutaneous focal mucinosis, cutaneous leiomyoma, breast cancer, tonsillar cancer, colorectal cancer, sarcoma of the leg, lung cancer, melanoma, dermatofibrosarcoma protuberans, basal cell carcinoma, cutaneous leiomyosarcoma, and squamous cell carcinoma.
Pathophysiology Genetics
An association with the folliculin (FLCN) gene was first reported in 2002. This 14-exon gene is located on the short arm of chromosome 17 (17p11.2) and has a cytosine-rich region in exon 11 particularly susceptible to mutation. The most common mutation in this region is the insertion or deletion of a cytosine residue, found in 53% of BHD-affected families. No significant difference has been found in the symptoms experienced by families with an insertion at that location compared to those who have a deletion, but mutations in FLCN associated with BHD syndrome are heterogeneous, and are often nonsense mutations or frameshift mutations that cause early truncation of the protein product at the carboxy terminus. Very rarely, missense mutations are observed. The mutations in the FLCN gene that cause Birt–Hogg–Dubé syndrome are germline mutations, which means that they occur in every cell of the body and can be passed down to future generations. These mutations are often passed from one generation to the next in an autosomal dominant fashion, but can occur as a new mutation in an individual with no prior family history (a de novo mutation). The children of an affected parent each has a 50% chance of having the disease. BHD has very high penetrance. A correlation between different FLCN genotypes and phenotypes has not been discovered.
Function FLCN creates a protein, folliculin, that has two isoforms. It appears to act as a tumor suppressor, and is expressed strongly in the skin, distal nephrons, and type I pneumocytes. It has also been found in the parotid gland, brain, breast, pancreas, prostate, and ovaries. Tumor suppressors normally prevent cells from growing and dividing too rapidly or in an uncontrolled way. Mutations in the FLCN gene may interfere with the ability of folliculin to restrain cell growth and division, leading to the formation of noncancerous and cancerous tumors. Recent studies suggest that folliculin accomplishes this function through its involvement with cellular metabolism, possibly through modulation of the mTOR (mammalian target of rapamycin) pathway and/or oxidative phosphorylation in mitochondria. Folliculin interacts with FNIP1 and FNIP2 (FLCN-interacting protein) to form a complex with AMP-activated protein kinase. Folliculin's participation in the mTOR pathway may explain the similarity in phenotype between BHD syndrome, Cowden syndrome, tuberous sclerosis, and Peutz–Jeghers syndrome.
Most of the cancer-causing mutations cause the protein to be truncated at the carboxy terminus. The C-terminal end of folliculin has shown to be the domain through which it interacts with FNIP1, and thereby possibly the mTOR pathway. FLCN is highly conserved in vertebrates—it is very similar between many vertebrate species. The 508th amino acid, normally lysine, is affected by a missense mutation in some people with BHD. The lysine at this position is found to be conserved between invertebrate and vertebrate orthologs of folliculin, indicating that it is important to the protein's function.