Pilomatricoma, also known as pilomatrixoma, is a type of noncancerous (benign) skin tumor associated with hair follicles. Hair follicles are specialized structures in the skin where hair growth occurs. Pilomatricomas occur most often on the head or neck, although they can also be found on the arms, torso, or legs. A pilomatricoma feels like a small, hard lump under the skin. This type of tumor grows relatively slowly and usually does not cause pain or other symptoms. Most affected individuals have a single tumor, although rarely multiple pilomatricomas can occur. If a pilomatricoma is removed surgically, it tends not to grow back (recur).
Most pilomatricomas occur in people under the age of 20. However, these tumors can also appear later in life. Almost all pilomatricomas are benign, but a very small percentage are cancerous (malignant). Unlike the benign form, the malignant version of this tumor (known as a pilomatrix carcinoma) occurs most often in middle age or late in life.
Pilomatricoma usually occurs without other signs or symptoms (isolated), but this type of tumor has also rarely been reported with inherited conditions. Disorders that can be associated with pilomatricoma include Gardner syndrome, which is characterized by multiple growths (polyps) and cancers of the colon and rectum; myotonic dystrophy, which is a form of muscular dystrophy; and Rubinstein-Taybi syndrome, which is a condition that affects many parts of the body and is associated with an increased risk of both benign and malignant tumors.
Pilomatricoma is an uncommon tumor. The exact prevalence is unknown, but pilomatricoma probably accounts for less than 1 percent of all benign skin tumors.
Mutations in the CTNNB1 gene are found in almost all cases of isolated pilomatricoma. These mutations are somatic, which means they are acquired during a person's lifetime and are present only in tumor cells. Somatic mutations are not inherited.
The CTNNB1 gene provides instructions for making a protein called beta-catenin. This protein plays an important role in sticking cells together (cell adhesion) and in communication between cells. It is also involved in cell signaling as part of the Wnt signaling pathway. This pathway promotes the growth and division (proliferation) of cells and helps determine the specialized functions a cell will have (differentiation). Wnt signaling is involved in many aspects of development before birth, as well as the maintenance and repair of adult tissues.
Among its many activities, beta-catenin appears to be necessary for the normal function of hair follicles. This protein is active in cells that make up a part of the hair follicle known as the matrix. These cells divide and mature to form the different components of the hair follicle and the hair shaft. As matrix cells divide, the hair shaft is pushed upward and extends beyond the skin.
Mutations in the CTNNB1 gene lead to a version of beta-catenin that is always turned on (constitutively active). The overactive protein triggers matrix cells to divide too quickly and in an uncontrolled way, leading to the formation of a pilomatricoma.
Most pilomatrix carcinomas, the malignant version of pilomatricoma, also have somatic mutations in the CTNNB1 gene. It is unclear why some pilomatricomas are cancerous but most others are not.
Most people with isolated pilomatricoma do not have any other affected family members. However, rare families with multiple affected members have been reported. In these cases, the inheritance pattern of the condition (if any) is unknown.
Other Names for This Condition
- Benign pilomatricoma
- Benign pilomatrixoma
- Calcifying epithelioma of Malherbe
- Malherbe calcifying epithelioma
Additional Information & Resources
Genetic Testing Information
Genetic and Rare Diseases Information Center
Research Studies from ClinicalTrials.gov
Catalog of Genes and Diseases from OMIM
Scientific Articles on PubMed
- Chan EF, Gat U, McNiff JM, Fuchs E. A common human skin tumour is caused by activating mutations in beta-catenin. Nat Genet. 1999 Apr;21(4):410-3. doi: 10.1038/7747. Citation on PubMed
- Julian CG, Bowers PW. A clinical review of 209 pilomatricomas. J Am Acad Dermatol. 1998 Aug;39(2 Pt 1):191-5. doi: 10.1016/s0190-9622(98)70073-8. Citation on PubMed
- Lan MY, Lan MC, Ho CY, Li WY, Lin CZ. Pilomatricoma of the head and neck: a retrospective review of 179 cases. Arch Otolaryngol Head Neck Surg. 2003 Dec;129(12):1327-30. doi: 10.1001/archotol.129.12.1327. Citation on PubMed
- Lazar AJ, Calonje E, Grayson W, Dei Tos AP, Mihm MC Jr, Redston M, McKee PH. Pilomatrix carcinomas contain mutations in CTNNB1, the gene encoding beta-catenin. J Cutan Pathol. 2005 Feb;32(2):148-57. doi: 10.1111/j.0303-6987.2005.00267.x. Citation on PubMed
- Moreno-Bueno G, Gamallo C, Perez-Gallego L, Contreras F, Palacios J. beta-catenin expression in pilomatrixomas. Relationship with beta-catenin gene mutations and comparison with beta-catenin expression in normal hair follicles. Br J Dermatol. 2001 Oct;145(4):576-81. doi: 10.1046/j.1365-2133.2001.04455.x. Citation on PubMed