ANTXR1, a stem cell-enriched functional biomarker, connects collagen signaling to cancer stem-like cells and metastasis in breast cancer

Cancer stem-like cells are thought to contribute to tumor recurrence. The anthrax toxin receptor 1 (ANTXR1) has been identified as a functional biomarker of normal stem cells and breast cancer stem-like cells. Primary stem cell-enriched basal cells (CD49f(+)/EpCAM(-)/Lin(-)) expressed higher levels of ANTXR1 compared with mature luminal cells. CD49f(+)/EpCAM(-), CD44(+)/EpCAM(-), CD44(+)/CD24(-), or ALDEFLUOR-positive subpopulations of breast cancer cells were enriched for ANTXR1 expression. CD44(+)/CD24(-)/ANTXR1(+) cells displayed enhanced self-renewal as measured by mammosphere assay compared with CD44(+)/CD24(-)/ANTXR1(-) cells. Activation of ANTXR1 by its natural ligand C5A, a fragment of collagen VI α3, increased stem cell self-renewal in mammosphere assays and Wnt signaling including the expression of the Wnt receptor-lipoprotein receptor-related protein 6 (LRP6), phosphorylation of GSK3α/β, and elevated expression of Wnt target genes. RNAi-mediated silencing of ANTXR1 enhanced the expression of luminal-enriched genes but diminished Wnt signaling including reduced LRP6 and ZEB1 expression, self-renewal, invasion, tumorigenicity, and metastasis. ANTXR1 silencing also reduced the expression of HSPA1A, which is overexpressed in metastatic breast cancer stem cells. Analysis of public databases revealed ANTXR1 amplification in medullary breast carcinoma and overexpression in estrogen receptor-negative breast cancers with the worst outcome. Furthermore, ANTXR1 is among the 10% most overexpressed genes in breast cancer and is coexpressed with collagen VI. Thus, ANTXR1:C5A interactions bridge a network of collagen cleavage and remodeling in the tumor microenvironment, linking it to a stemness signaling network that drives metastatic progression.

The expanded collagen VI family: new chains and new questions

Collagen VI is a component of the extracellular matrix of almost all connective tissues, including cartilage, bone, tendon, muscles and cornea, where it forms abundant and structurally unique microfibrils organized into different suprastructural assemblies. The precise role of collagen VI is not clearly defined although it is most abundant in the interstitial matrix of tissues and often found in close association with basement membranes. Three genetically distinct collagen VI chains, α1(VI), α2(VI) and α3(VI), encoded by the COL6A1. COL6A2 and COL6A3 genes, were first described more than 20 years ago. Their molecular assembly and role in congenital muscular dystrophy has been broadly characterized. In 2008, three additional collagen VI genes arrayed in tandem at a single gene locus on chromosome 3q in humans, and chromosome 9 in mice, were described. Following the naming scheme for collagens the new genes were designated COL6A4. COL6A5 and COL6A6 encoding the α4(VI), α5(VI) and α6(VI) chains, respectively. This review will focus on the current state of knowledge of the three new chains.

Therapy of collagen VI-related myopathies (Bethlem and Ullrich)

The collagen VI-related myopathies comprise two major forms, Bethlem myopathy (BM) and Ullrich congenital muscular dystrophy (UCMD), which show a variable combination of muscle wasting and weakness, joint contractures, distal laxity, and respiratory compromise. Specific diagnosis requires molecular genetic testing showing mutation in one of the three genes involved. This review summarizes current treatments, in particular indication for physiotherapy, orthopedic treatment for correction of foot deformity, scoliosis, and flexion contractures of elbows, and treatment of respiratory failure. The turning point in basic research on collagen VI myopathies was the discovery of an unexpected mitochondrial dysfunction as a pathogenetic mechanism underlying the myopathic syndrome seen in Col6a1 null mice. Treatment of Col6a1(-/-) mice with cyclosporin A (CsA) rescued the mitochondrial dysfunction and decreased apoptosis. Similar mitochondrial defects were revealed in cultures of UCMD patients. The results of an open pilot trial with CsA in five patients with collagen VI-related myopathies are summarized and discussed. With the availability of new potential effective treatments, several challenges must be addressed in conducting trials in orphan diseases and in neuromuscular disorders in particular. Outcome measures are discussed in the context of the expected effect of the cure. Randomized clinical trials often are not feasible for rare diseases, and sometimes would be ethically inappropriate. The need to develop alternative outcome measures or biomarkers using platforms such as genomics and proteomics is stressed in this context.

Mitochondrial dysfunction in the pathogenesis of Ullrich congenital muscular dystrophy and prospective therapy with cyclosporins

Ullrich congenital muscular dystrophy is a severe genetically and clinically heterogeneous muscle disorder linked to collagen VI deficiency. The pathogenesis of the disease is unknown. To assess the potential role of mitochondrial dysfunction in the onset of muscle fiber death in this form of dystrophy, we studied biopsies and myoblast cultures obtained from patients with different genetic defects of collagen VI and variable clinical presentations of the disease. We identified a latent mitochondrial dysfunction in myoblasts from patients with Ullrich congenital muscular dystrophy that matched an increased occurrence of spontaneous apoptosis. Unlike those in myoblasts from healthy donors, mitochondria in cells from patients depolarized upon addition of oligomycin and displayed ultrastructural alterations that were worsened by treatment with oligomycin. The increased apoptosis, the ultrastructural defects, and the anomalous response to oligomycin could be normalized by Ca(2+) chelators, by plating cells on collagen VI, and by treatment with cyclosporin A or with the specific cyclophilin inhibitor methylAla(3)ethylVal(4)-cyclosporin, which does not affect calcineurin activity. Here we demonstrate that mitochondrial dysfunction plays an important role in muscle cell wasting in Ullrich congenital muscular dystrophy. This study represents an essential step toward a pharmacological therapy of Ullrich congenital muscular dystrophy with cyclosporin A and methylAla(3)ethylVal(4) cyclosporin.