Dual-photon absorptiometry (DPA) allows separation of body mass into bone mineral, fat, and fat-free soft tissue. This report evaluates the potential of DPA to isolate appendages of human subjects and to quantify extremity skeletal muscle mass (limb fat-free soft tissue). The method was evaluated in 34 healthy adults who underwent DPA study, anthropometry of the limbs, and estimation of whole-body skeletal muscle by models based on total body potassium (TBK) and nitrogen (TBN) and on fat-free body mass (FFM). DPA appendicular skeletal muscle (22.0 +/- 3.1 kg, mean +/- SD) represented 38.7% of FFM, with similar proportions in males and females. There were strong correlations (all p less than 0.001) between limb muscle mass estimated by DPA and anthropometric limb muscle areas (r = 0.82-0.92), TBK (r = 0.94), and total-body muscle mass based on TBK-FFM (r = 0.82) and TBK-TBN (r = 0.82) models. Appendicular skeletal muscle mass estimated by DPA is thus a potentially practical and accurate method of quantifying human skeletal muscle mass in vivo.

The Carney triad (CT) is gastrointestinal stromal tumor (GIST), paraganglioma, and pulmonary chondroma. The GISTs of CT show different clinical, molecular, and morphologic features to usual adult GISTs but are similar to the majority of pediatric GISTs. We postulated that these GISTs would show negative staining for succinate dehydrogenase B (SDHB). We performed SDHB immunohistochemistry on GISTs arising in 5 individuals with CT, 1 child, 7 individuals with GIST in young adulthood including 2 with germline KIT mutations, 3 individuals with neurofibromatosis 1, one 63-year-old female with multifocal gastric epithelioid GIST with lymph node metastases, and 104 consecutive unselected individuals with apparently sporadic GIST. The GISTs and paragangliomas arising in CT, the pediatric GIST, and the multifocal gastric GIST from the 63-year-old showed negative SDHB staining. GISTs from the 7 young adults and 3 with neurofibromatosis were SDHB positive. Of the unselected GISTs, 101 (97%) were positive. One of the negative GISTs arose in a 48-year-old female with previous recurrent multifocal gastric GISTs and the other 2 arose in females also in their 40s with gastric GISTs with epithelioid morphology. We conclude that negative staining for SDHB is characteristic of the GISTs of CT and the subgroup of pediatric GISTs which it resembles. Furthermore, when negative staining occurs in apparently sporadic GISTs in adults, the GISTs show morphologic and clinical features similar to pediatric and CT type GISTs. GISTs may therefore be divided into type 1 (SDHB positive) and type 2 (SDHB negative) subtypes.

Autoimmunity has long been linked to myocarditis and its sequela, dilated cardiomyopathy, the leading causes of heart failure in young patients. However, the underlying mechanisms are poorly defined, with most clinical investigations focused on humoral autoimmunity as the target for intervention. Here, we show that the α-isoform of myosin heavy chain (α-MyHC, which is encoded by the gene Myh6) is the pathogenic autoantigen for CD4 + T cells in a spontaneous mouse model of myocarditis. Further, we found that Myh6 transcripts were absent in mouse medullary thymic epithelial cells (mTECs) and peripheral lymphoid stromal cells, which have been implicated in mediating central and peripheral T cell tolerance, respectively. Transgenic expression of α-MyHC in thymic epithelium conferred tolerance to cardiac myosin and prevented myocarditis, demonstrating that α-MyHC is a primary autoantigen in this disease process. Remarkably, we found that humans also lacked α-MyHC in mTECs and had high frequencies of α-MyHC-specific T cells in peripheral blood, with markedly augmented T cell responses to α-MyHC in patients with myocarditis. Since α-MyHC constitutes a small fraction of MyHC in human heart, these findings challenge the longstanding notion that autoimmune targeting of MyHC is due to its cardiac abundance and instead suggest that it is targeted as a result of impaired T cell tolerance mechanisms. These results thus support a role for T cell-specific therapies for myocarditis.

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