Infantile myofibromatosis (IMF) is a rare tumor found in 1 in 150,000 to 1 in 400,000 live births. It is nonetheless the most common tumor derived from fibrous connective tissue that occurs primarily in infants and young children. IMF tumors are benign in the sense that they do not metastasize to distant tissues although when occurring in the viscera, i.e. internal organs, carry guarded to poor prognoses and can be life-threatening, particularly in newborns and young infants. The condition was first described by Arthur Purdy Stout as congenital generalized fibromatosis – in which he coined the word fibromatosis – in 1954. IMF tumors occur in three clinical patterns: 1) solitary IMF tumors (also called myofibromas) which often regress spontaneously and rarely cause serious issues; 2) multiple tumors (no viscera involvement) which consists of numerous (i.e. dozens to >100) IMF lesions most of which are located in the skin and subcutaneous tissues but not visceral organs, may regress spontaneously, and rarely cause serious issues; and 3) multiple tumors (with viscera involvement) (also called generalized myofibromatosis) which rarely regress spontaneously and consist of numerous IMF lesions in non-visceral tissues plus one or more visceral tumors that may be life-threatening. A minority of infantile myofibromatosis tumors present in individuals with a strong family history of the disease. These familial cases are associated with mutations in either the PDGFRB or NOTCH3 gene. However, most IMF cases have no family history of the disease but nonetheless have PDGFRB gene mutations in their tumor cells; these mutations are similar to those occurring in the familial PDGFRB gene mutations. Regardless of these genetic variations, all IMF tumors consist of bland-appearing, benign (i.e. non-malignant) myofibroblasts, i.e. cells that blend a variable set of features seen in fibroblasts, the most common cell type in connective tissue, with a variable set of features seen in smooth muscle cells. Treatment of IMF tumors depends upon the tumor numbers, locations, and genetic abnormalities found in each individual and often include more than one therapeutic regimen. Individuals with solitary tumors are usually treated by observation with the expectation that many of these tumors will regress spontaneiously. Single tumors located in vital areas (e.g. intracranial tumors) and tumors that do not regress over suitable observation periods are often treated by surgical removal. Multiple tumors (with viscera involvement) and surgically inaccessible life-threatening IMF tumors have been treated with one or a combination of chemotherapy drugs, radiation therapy or, in tumors with certain PDGFRB gene mutations, drugs directed specifically against this mutated gene's protein product.
Presentation IMF tumors are usually painless, well-encapsulated, rubbery to hard, and freely movable-to-fixed masses. They may be evident at birth in up to 60% of cases but generally go undetected until they are diagnosed in the first year of life, uncommonly in older infants and young (<10 years/old) children, or rarely in older children and adults (one individual was diagnosed with IMF at 85 years of age). The tumors may be flesh-colored papules or nodules; plaques (i.e. flat-topped papules that are equal to or greater than 10 mm in largest diameter); pedunculated, calcified masses; or ulcerated, necrotic masses. Most of these tumors are <1 cm in diameter but can be much larger, e.g. 5.2 cm in diameter. At least 9 IMF cases described in the English literature were diagnosed in fetuses based on the findings of fetal spectrogram and/or magnetic resonance imaging. IMF tumors in the viscera or deep tissues may present with serious or life-threatening signs and symptoms of organ and/or deep tissue damage due to these tumors space-filling and pressuring effects. Some individuals with IMF present with a family history of IMF tumors. To date (2021), 20 families have been reported to carry various germline (i.e. inherited) mutations in one of their two PDGFRB genes. All of these mutations were gain of function mutations (i.e. the mutant gene's product is overactive) and inherited in an autosomal dominant manner with incomplete penetrance. Among 36 identified familial carriers of these mutant genes, 4 had no history of IMF, 3 had developed a solitary IMF, 29 had developed multiple tumors (no viscera involvement) or multiple tumors (with viscera involvement) IMF, and 2 had died. Most of these individuals were diagnosed with IMF in their infancy or early childhood. Subsequent studies have found that up to 70% of individuals with IMF and no family history of the disease also carry mutations in one of their two PDGFRB genes in the cells of their tumor tissues. These mutations are somatic mutations (i.e. non-inherited mutations developing only after conception) that are identical or similar to those seen in familial cases. The familial and non-familial mutated PDGFRB genes produce highly overactive platelet-derived growth factor receptor beta (i.e. PDGFβ receptor) proteins that contribute to the development and/or progression of IMF. A mutation in one of the two NOTCH3 genes has been reported to occur in a single family with autosomal dominant inheritance of IMF: 9 of this family's members had IMF plus the mutation while 7 of its members did not have IMF or the mutation. Since the mutated NOTCH3 gene in this family produces an overactive neurogenic locus notch homolog protein 3 product that in cell culture studies increases the expression of the PDGFRB gene's product, the PDGFβ receptor, this NOTCH3 mutation may promote the development and/or progression of IMF in a manner similar to PDGFRB mutant genes, i.e. by increasing the number and therefore overall activity of the PDGFβ receptor. Further studies are needed to define NOTCH3 gene mutation's frequency of occurrence in, and contribution to the development of, IMF.
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