PLAG1 alterations in lipoblastoma: involvement in varied mesenchymal cell types and evidence for alternative oncogenic mechanisms.

Lipoblastomas are rare soft tissue tumors that occur primarily in young children. They typically contain variably differentiated adipocytes, primitive mesenchymal cells, myxoid matrix, and fibrous trabeculae. Abnormalities in chromosome 8, leading to rearrangements of the PLAG1 gene, were demonstrated recently in four lipoblastomas. In the present report, we determine the frequency of PLAG1 alterations in 16 lipoblastomas from children aged 13 years or younger, and we also evaluate the stages of lipoblastoma differentiation at which PLAG1 genomic alterations are found. Eleven lipoblastomas (69%), including those with either classic or lipoma-like histology, had rearrangements of the 8q12 PLAG1 region. Another three lipoblastomas had polysomy for chromosome 8 in the absence of PLAG1 rearrangement. Only two cases (13%) lacked a chromosome 8 abnormality. Notably, the lipoblastomas with chromosome 8 polysomy had up to five copies of chromosome 8 as an isolated cytogenetic finding in an otherwise diploid cell. We also demonstrate that PLAG1 alterations are found in a spectrum of mesenchymal cell types in lipoblastomas, including lipoblasts, mature adipocytes, primitive mesenchymal cells, and fibroblast-like cells. This finding is consistent with neoplastic origin in a primitive mesenchymal precursor and with variable differentiation to a mature adipocyte end-point. Hence, our studies provide biological validation for the clinical observation that lipoblastomas can evolve into mature, lipoma-like, lesions. They also suggest that PLAG1 dosage alterations caused by polysomy 8 might represent an alternative oncogenic mechanism in lipoblastoma.     

Myogenic regulatory protein expression in adult soft tissue sarcomas. A sensitive and specific marker of skeletal muscle differentiation.

Myogenic regulatory protein (MyoD1) is a DNA-binding nuclear protein that initiates myogenesis in mesenchymal stem cells. Its expression has proved a very sensitive marker of myogenic differentiation in malignant tumors of childhood. In this study, the reliability of MyoD1 expression as marker of skeletal muscle differentiation has been tested in 38 cases of adult-type sarcomas. Frozen sections were stained with the monoclonal antibody 5.8A (MAb 5.8A) developed against recombinant wild-type murine MyoD1. Routinely processed formalin-fixed sections from the same cases were reviewed and immunostained with a panel of antibodies commonly used to detect muscle differentiation: myoglobin, fast-myosin, desmin, muscle-specific actin, alpha-smooth muscle actin. Four cases positively stained with MAb 5.8A: three were high-grade spindle cell sarcomas, the fourth case was an alveolar soft-part sarcoma. Only tumors in which skeletal muscle histogenesis was inferred on the basis of histology and/or immunohistochemistry with conventional muscle markers stained positively for MAb 5.8A. All the other tumors tested, including leiomyosarcomas (11), liposarcomas, (10, including four dedifferentiated liposarcomas), fibrosarcomas (three), malignant fibrous histiocytomas (two), synovial sarcomas (two), and malignant peripheral nerve sheath tumors (two), were negative for MAb 5.8A. The high level of sensitivity and specificity of MyoD1 expression indicates the value of this marker in the diagnosis of soft tissue sarcomas.     

Myxoid mesenchymal tumors of uterus: endometrial stromal and smooth muscle tumors, myxoid variant

Four myxoid variant of uterine mesenchymal tumors are reported. One was a low grade stromal sarcoma with infiltrative margins and the others were well circumscribed tumors corresponding to an endometrial stromal nodule and two leiomyomas. They were hypocellular neoplasms composed of stellated cells with an abundant Alcian Blue positive myxoid matrix. The myxoid nature of the neoplasms obscured their cellular nature and made the distinction between smooth muscle and endometrial stromal tumors difficult. Endometrial stromal tumors, showed very focal areas of small basophilic cells, characteristic of endometrial stroma. The diagnosis was based on the presence of a spiral arteriolar network, a CD10 positivity as well as the absence of h-caldesmon and desmin expression. The two myxoid leiomyomas showed more spindle cells and a desmin expression while h-caldesmon was negative and CD10 focally positive in both cases. Myxoid variant of endometrial stromal tumors does not necessarily exhibit the typical morphology of endometrial stroma. They may demonstrate morphological features of smooth muscle tumors in the uterus. Also, myxoid changes in uterin smooth muscle tumors may modify the classical immunoreactivity of smooth muscle markers in these tumors and make it difficult to distinguish between benign and malignant neoplasms. An immunohistochemical panel of antibodies including CD10, h-caldesmon and desmin may help in establishing the correct diagnosis.     

Fibronexus in "Malignant Fibrous Histiocytoma" of the Bone: A Case Report of Pleomorphic Myofibrosarcoma

The fibronexus (fibronexus junction) has been thought to be a characteristic ultrastructural feature of myofibroblasts, but it is controversial whether the fibronexus is also a characteristic of various myofibroblastic tumors. We report here a case of pleomorphic bone sarcoma (pleomorphic/storiform malignant fibrous histiocytoma) with fibronexus junctions arising in the head of the left humerus of a 70-year-old woman. By light microscopy the tumor was composed of large spindle or polygonal cells occasionally arranged in fascicles. Foamy giant cells with bizarre nuclei were not uncommon. Immunohistochemically, the tumor cells were positive diffusely for vimentin and focally for muscle actin ( HHF35 ) and alpha-smooth muscle actin but were negative for desmin, high-molecular weight caldesmon, and S-100 protein. Ultrastructurally, the tumor cells had indented nuclei and spindle or polygonal cytoplasm, with little rough endoplasmic reticulum and small vesicles, and rather numerous mitochondria. The tumor cells had myofilaments with focal densities in the periphery, fibronectin fibrils adjacent to intracellular myofilaments, and by definition, therefore, fibronexus junctions. These findings suggest myofibroblastic differentiation, and a diagnosis of pleomorphic myofibrosarcoma is thought to be appropriate. We believe that the fibronexus is a characteristic and useful ultrastructural feature for differentiating myofibroblastic tumors from other pleomorphic myogenic sarcomas.     

Fibronexus in "malignant fibrous histiocytoma" of the bone: a case report of pleomorphic myofibrosarcoma

The fibronexus (fibronexus junction) has been thought to be a characteristic ultrastructural feature of myofibroblasts, but it is controversial whether the fibronexus is also a characteristic of various myofibroblastic tumors. We report here a case of pleomorphic bone sarcoma (pleomorphic/storiform malignant fibrous histiocytoma) with fibronexus junctions arising in the head of the left humerus of a 70-year-old woman. By light microscopy the tumor was composed of large spindle or polygonal cells occasionally arranged in fascicles. Foamy giant cells with bizarre nuclei were not uncommon. Immunohistochemically, the tumor cells were positive diffusely for vimentin and focally for muscle actin ( HHF35 ) and alpha-smooth muscle actin but were negative for desmin, high-molecular weight caldesmon, and S-100 protein. Ultrastructurally, the tumor cells had indented nuclei and spindle or polygonal cytoplasm, with little rough endoplasmic reticulum and small vesicles, and rather numerous mitochondria. The tumor cells had myofilaments with focal densities in the periphery, fibronectin fibrils adjacent to intracellular myofilaments, and by definition, therefore, fibronexus junctions. These findings suggest myofibroblastic differentiation, and a diagnosis of pleomorphic myofibrosarcoma is thought to be appropriate. We believe that the fibronexus is a characteristic and useful ultrastructural feature for differentiating myofibroblastic tumors from other pleomorphic myogenic sarcomas.     

Intramuskuläre Fasciitis nodularis

Nodular fasciitis is a frequently occurring pseudosarcomatous fibrous proliferation which usually affects the subcutaneous tissue. In the present study, we investigated tissue specimens from 26 patients with intramuscular nodular fasciitis. The median age of patients was 31 years, with the tumors occurring mainly in the extremities (11 patients) and the trunk (11 patients). These had an average size of 2.6 cm. Males predominated (male:female 9:4). Histological examination revealed a proliferation of myofibroblasts (positive for smooth muscle-actin) embedded in a myxoid or fibrous stroma. In 25/26 cases, myogenic giant cells were observed (positive for desmin, negative for smooth-muscle actin and CD 68). Less frequently (11/26 patients), osteoclastic giant cells were found (positive for CD 68, negative for desmin and smooth-muscle actin). In intramuscular nodular fasciitis, myogenic giant cells are encountered more often than previously described. They are relevant for the differential diagnosis of sarcoma and fibromatoses, since they can be misinterpreted as neoplastic giant cells. However, the uniformity of the myofibroblastic proliferate and the myogenic phenotype of the giant cells without proliferative activity (no mitoses, negativity for Ki-67) lead to a correct diagnosis.     

Calponin and h-caldesmon in soft tissue tumors: consistent h-caldesmon immunoreactivity in gastrointestinal stromal tumors indicates traits of smooth muscle differentiation.

Currently, the immunohistochemical evaluation of smooth muscle differentiation is usually based on desmin, which also reacts with skeletal muscle and is not present in all smooth muscle tumors, and alpha-smooth muscle actin, which reacts with myoepithelial cells. Neither marker typically reacts with gastrointestinal stromal tumors (GISTs), previously classified as smooth muscle tumors or presently often classified as smooth muscle/stromal tumors. Two cytoskeleton-associated actin-binding proteins, calponin (CALP) and h-caldesmon (HCD), are putative smooth muscle markers that also react with myoepithelia. These markers are of particular interest in the immunohistochemical analysis of tumors; neither of them has been extensively documented in soft tissue tumors. In this study, we evaluated selected normal and reactive tissues and more than 250 mesenchymal tumors for CALP and HCD. Both markers were expressed in parenchymal and vascular smooth muscle cells in various organs and in myoepithelial cells. CALP also reacted with myofibroblasts of desmoplastic stroma. All of our 25 benign smooth muscle tumors from various locations were positive for CALP and HCD, as were most of the retroperitoneal and uterine leiomyosarcomas. HCD was more specific, because CALP also reacted with myofibroblastic lesions. The common reactivity of malignant fibrous histiocytomas with CALP and HCD suggests a combination of myofibroblastic and smooth muscle differentiation in these tumors. The GISTs (c-kit positive, usually actin negative) showed nearly consistent HCD reactivity, suggesting traits of smooth muscle differentiation. GISTs were usually CALP negative and showed a CALP expression pattern similar to that of alpha-smooth muscle actin. Although nonmuscle, nonmyofibroblastic tumors were negative for CALP and HCD, synovial sarcomas showed streaks of CALP-positive cells of unknown significance. CALP and HCD should be explored as markers to identify myofibroblastic and smooth muscle cell differentiation in mesenchymal tumors.     

PRRX‐NCOA1/2 rearrangement characterizes a distinctive fibroblastic neoplasm

Fibroblastic/myofibroblastic neoplasms represent a broad, and occasionally diagnostically challenging, category of soft tissue neoplasms. A subset of these tumors defy conventional classification. However, with the advent of next‐generation sequencing, the identification of disease‐defining molecular alterations is gradually improving their subclassification. Following identification of two index cases of a distinctive fibroblastic neoplasm with a fusion gene involving PRRX1 and NCOA1, we performed a retrospective review to further characterize this entity. We identified two additional cases, including one with a fusion between PRRX1 and NCOA2. The average patient age was 38 years, and three patients were female. Two tumors occurred on the neck, and the others involved the groin and thigh. Tumors were centered in the subcutis and ranged from 2.3 to 14.0 cm (average 5.8 cm). Morphologically, they were predominantly hypocellular, with focal hypercellularity. They were composed of monomorphic spindle‐stellate cells with a vague fascicular pattern. The nuclei were bland with only rare mitotic activity, and occasional multinucleation. The intervening stroma was typically abundant and ranged from myxoid to collagenous, with frequent rope‐like collagen bundles. Three of the cases had a prominent vasculature ranging from numerous small curvilinear vessels to ectatic and branching staghorn‐like vessels. Immunohistochemistry was negative for desmin, smooth muscle actin, S100, CD34, keratin, and epithelial membrane antigen. Each of the patients was treated by simple excision and none of the tumors were associated with local recurrence or metastasis. Based on their unique morphological and molecular attributes, we believe this represents a novel fibroblastic tumor for which we have tentatively proposed the name “PRRX‐NCOAx‐rearranged fibroblastic tumor.”     

Is anti-h-caldesmon useful for distinguishing smooth muscle and myofibroblastic tumors? An immunohistochemical study

Misinterpretation of positive staining of antibodies to desmin, smooth muscle actin, and muscle actin as representing smooth muscle differentiation in the context of a spindle cell tumor is not uncommon. Anti-h-caldesmon is a promising novel immunohistochemical reagent for more specific smooth muscle differentiation. We studied 72 tumors (11 leiomyosarcomas, 26 malignant fibrous histiocytomas [MFHs], 11 fibromatoses, 11 cellular cutaneous fibrous histiocytomas [CCFHs], 5 malignant peripheral nerve sheath tumors, 4 synovial sarcomas, and 4 cases of nodular fasciitis), the reactive myofibroblastic response in 5 cases of acute cholecystitis, and the desmoplastic response surrounding 5 invasive breast carcinomas. Tissues were examined for expression of h-caldesmon, desmin, smooth muscle actin, and muscle actin. Diffuse staining for h-caldesmon was present only within the leiomyosarcomas. Focal staining for h-caldesmon involving less than 1% of lesional cells was present in 3 of 26 MFHs and 1 of 11 CCFHs. There was overlap in staining for the other "myoid" markers in all of the lesions that contained myofibroblasts. Anti-h-caldesmon seems to be a reliable marker of smooth muscle differentiation, and its inclusion in a panel of myoid immunohistochemical reagents should allow distinction of smooth muscle and myofibroblastic tumors.     

Inflammatory pseudotumor (inflammatory myofibroblastic tumor) of the mitral valve of the heart

We report a rare case of inflammatory pseudotumor/inflammatory myofibroblastic tumor (IPT/IMT) of the heart, involving the mitral valve. A 58‐year‐old woman presenting with dyspnea was immediately admitted to the hospital, and found to have congestive heart failure due to the invagination of a tumor‐like mass of the mitral valve. This mass arose from and involved almost the entire posterior leaflet of the mitral valve and occupied almost the entire mitral valve orifice. The tumor was a yellowish‐white well‐circumscribed mass with a smooth surface. The excised mass was 3.0 × 2.3 × 1.8 cm, and consisted of abundant Sudan III‐positive foam cells, histiocytes, lymphocytes, plasma cells, and loosely arrayed spindle cells, in vascular‐rich fibrous tissue. Immunohistochemistry showed that the spindle cells were positive for vimentin and α‐smooth muscle actin, and negative for desmin, CD34, and human muscle actin (HHF‐35), suggesting they were myofibroblastic cells. The plasma cells and lymphocytes showed no monoclonality. There were few mitotic cells, and, except for the lymphocytes, few Ki‐67‐positive cells. The findings suggested IPT/IMT. The 39 cardiac IPT/IMT cases appearing in the English‐language literature are discussed.     

Myogenic Stromal Tumor of the Male Breast (So-Called Myofibroblastoma)

A case of a distinctive stromal tumor of the male breast, recently designated as myofibroblastoma, is reported. A fascicular clustering of spindle cells with intervening hyaline bands was the dominant pattern, but areas of myxoid transformation were present. Ultrastructure revealed a large predominance of fibroblastlike cells, often containing dispersed or aggregated intermediate and/or thin filaments. These cells depicted numerous thin processes encircling collagenous bundles. Primitive mesenchymal and typical smooth muscle cells were present, but myofibroblasts were not identified. Vimentin was diffusely expressed, whereas desmin and muscle-specific actin were detected in, respectively, 50% to 60% and 15% to 25% of the cell population. The designation of myogenic stromal tumor is, therefore, considered more appropriate.     

Gastrointestinal spindle cell tumours of the dog: histological and immunohistochemical study

To assess the relevance of spindle cell tumours in the canine gastrointestinal (GI) tract and to classify them, a retrospective study was carried out on haematoxylin and eosin-stained sections from formalin-fixed paraffin wax-embedded samples of 105 primary GI tumours. Seventeen out of 105 (16%) GI tumours were mesenchymal, 48% were epithelial and 36% were round cell tumours. Spindle cell tumours were stained by Masson trichrome, Orcein-Van Gieson and labelled immunohistochemically (vimentin, desmin, smooth muscle actin, protein S100, glial fibrillar acid protein, CD117 and MIB-1) and the histological grade, mitotic index, nuclear size and cellular density were also assessed. The 17 gastrointestinal mesenchymal tumours were classified as 10 leiomyomas (10/10 positive for desmin and smooth muscle actin; 6/10 positive for vimentin) 2 leiomyosarcomas (2/2 positive for desmin, smooth muscle actin and vimentin) and 5 gastrointestinal stromal tumours (GISTs) (5/5 positive for CD117 and vimentin; 3/5 positive for smooth muscle actin). Canine GISTs appeared as densely packed spindle cell tumours, with a diffuse, strong, cytoplasmic immunopositivity for c-kit protein (CD117). GISTs, defined as CD117-positive spindle cell or epithelioid or pleomorphic neoplasms that presumably derive from interstitial cells of Cajal, are reported in recent medical studies as the most common mesenchymal tumours of the GI tract. Our data suggest that GISTs represent a significant portion of canine GI spindle cell tumours, which can be definitely distinguished from leiomyosarcomas only by their expression of CD117.     

Variable expression of tenascin‐C,osteopontin and fibronectin in inflammatory myofibroblastic tumour of the lung

Kaarteenaho R, Sormunen R, Pääkkö P. Variable expression of tenascin‐C, osteopontin and fibronectin in inflammatory myofibroblastic tumour of the lung. APMIS 2010; 118: 91–100. The aim of this study was to analyse the expression of tenascin‐C, osteopontin and fibronectin in inflammatory myofibroblastic tumour of the lung, which is a rare tumour of unknown aetiology. Nine patients with an inflammatory myofibroblastic tumour of lung were studied by immunohistochemistry for the presence of tenascin‐C, osteopontin, fibronectin and alpha‐smooth muscle actin, which is a common marker for myofibroblasts. The ultrastructure of myofibroblasts was confirmed by transmission electron microscopy. The expression of tenascin‐C, osteopontin, fibronectin and alpha‐smooth muscle actin was also studied by immunoelectron microscopy. All cases displayed all of the studied extracellular matrix proteins and also alpha‐smooth muscle actin‐positive spindle‐shaped fibroblastic cells that were undoubtedly myofibroblasts. The immunoelectron microscopic studies demonstrated labelling for alpha‐smooth muscle actin in intracellular filament bundles within myofibroblasts, for fibronectin in the extracellular filaments of the fibronexus and for tenascin‐C extracellularly often adjacent to myofibroblasts. Labels for osteopontin were observed within myofibroblasts and plasma cells. These results demonstrate that tenascin‐C, osteopontin and fibronectin were expressed in all three kinds of subtypes of inflammatory myofibroblastic tumours of the lung and further, variable amounts of myofibroblasts could be observed by light and transmission electron microscopy as well as by immunoelectron microscopic techniques.     

Histologically low-grade dedifferentiated liposarcoma of the retroperitoneum

A low-grade dedifferentiated liposarcoma in the retroperitoneum of a 52-year-old woman is described. The excised specimens contained six nodules of lipoma-like well-differentiated liposarcoma and a nodule of dedifferentiated liposarcoma. The latter was composed predominantly of loosely arranged, benign-appearing spindle cells and fat cells. A small number of cells with irregularly shaped nuclei were scattered. There were no mitotic figures. The fat cells showed slight variation of size and shape, and a few multivacuolated lipoblasts were found. The spindle cell areas occupied approximately 60% of the tumor. The stroma was somewhat fibrous and myxoid and no dense collagenous matrix was found. The stroma vascularity was not prominent. Immunohistochemically, the spindle cells were positive for vimentin but negative for S-100 protein, desmin, muscle actin, and alpha-smooth muscle actin. Follow up for 5 months showed no evidence of recurrence or metastasis. The tumor, in which the benign-appearing spindle cell component was predominant, was considered to be a low-grade dedifferentiated liposarcoma. Close and long-term follow up is required. In retroperitoneal lipoma-like well-differentiated liposarcomas, spindle cell components like the present tumor, which represent dedifferentiation, should not be overlooked.     

Identification of COL3A1 and RAB2A as novel translocation partner genes of PLAG1 in lipoblastoma

Lipoblastoma is a rapidly growing, benign neoplasm in children. Surgical excision is usually curative, with a recurrence rate of about 20%. Because the histology of lipoblastoma is heterogeneous and overlaps with other lipomatous tumors, some lipoblastoma cases have been difficult to diagnose. The detection of PLAG1 gene rearrangement is useful for the diagnosis of lipoblastoma. Three fusion partner genes are known in relation to PLAG1 in lipoblastoma HAS2 at 8q24.1, COL1A2 at 7q22, and RAD51L1 at 14q24. Herein, we describe another two novel fusion genes in lipoblastoma tumor specimens. We checked six tumors for the presence of two known fusion genes, HAS2‐PLAG1 and COL1A2‐PLAG1. Only HAS2‐PLAG1 was found in one of the cases. Next, we attempted to identify potential PLAG1 fusion partners using 5'RACE. Sequence analysis revealed two novel fusion genes, COL3A1‐PLAG1 in three cases and RAB2A‐PLAG1 in one case, respectively. As a result of the translocations, the constitutively active promoter of the partner gene drives the ectopic expression of PLAG1. We also evaluated whether a high level of PLAG1 expression can be used to help differentiate lipomatous tumors. PLAG1 expression was evaluated by real‐time PCR in five lipoblastoma tumor specimens. The expressions were 70–150 times higher in lipoblastomas than in human adipocytes. However, PLAG1 expression was low in one case of lipoma. These results demonstrate that PLAG1 overexpression is a potential marker of lipoblastoma. Our findings, in agreement with previous studies, show that lipoblastoma is a group of lipomatous tumors with PLAG1 rearrangement and overexpression. © 2014 Wiley Periodicals, Inc.     

Solitary myofibroma in adults: clinicopathological analysis of a series

Solitary myofibroma is a recently described, benign neoplasm of superficial soft tissue, which represents the adult counterpart of infantile myofibromatosis and is poorly recognized. Eleven new cases are presented herein. The patients were mostly adults with ages ranging from 13–64 years. They presented with a solitary, usually painless nodule of variable duration in the skin or oral cavity. Histologically, each lesion had a biphasic pattern with spindle cells forming fascicular or whorled areas and rounded, more primitive cells arranged around small vessels, forming haemangioperi-cytoma-like areas. The characteristic zonation of infantile myofibromatosis was often less marked in adult lesions and there was a haphazard arrangement of the fascicular and pericytic areas in some cases. Hyalinization, especially of the peripheral spindle-celled areas, was frequent. Immunohistochemically, the spindle cells were desmin negative but muscle actin (HHF-35 and IA4) positive. The rounded cells were both desmin and actin negative. Electronmicroscopy confirmed the myofibroblastic/fibroblastic nature of tumour cells in two cases. Although vascular invasion was seen in two cases, solitary adult-type myofibroma pursues a benign clinical course.     

Immunohistochemical differentiation of leiomyocellular tumors and tumors with myogenic differentiation

The expression of alpha smooth muscle actin, muscle specific actin, desmin, h-caldesmon, and calponin was studied immunohistochemically in the following soft tissue and bone tumours and tumour-like lesions: muscle fibromatosis, inflammatory pseudotumours, chondroblastoma, enchondroma, chondrosarcoma, fibrous dysplasia, ossifying myositis, osteoblastoma, convential osteosarcoma, leiomyoma and leiomyosarcoma. Tumours and tumour-like lesions with myofibroblastic cells, osteoblasts and chondroblasts frequently exhibited intensive immunoreactivity for the muscle markers, and therefore, some of them may occasionally be confused with leiomyoma and leiomyosarcoma. Calponin does not help to differentiate various mesenchymal tumours expressing muscle markers, because it also stains intensively myofibroblasts, osteoblasts and chondroblasts. We confirmed that h-caldesmon was expressed intensely in leiomyomas and leiomyosarcomas, and never in the other tumours examined, with the exception of three chondroblastomas. The results have shown that h-caldesmon is a rather specific and sensitive marker for smooth muscle tumours, but it can also stain some actin positive myochondroblasts. It is possible that the positivity of h-caldesmon in some chondroblastomas is due to their complete myogenic transdifferentiation, and so we use the term myochondroblasts and myochondrocytes for designation of such S-100 protein, actin, and h-caldesmon positive cells.