Mary E. Brunkow

The immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) is caused by mutations of FOXP3

IPEX is a fatal disorder characterized by immune dysregulation, polyendocrinopathy, enteropathy and X-linked inheritance (MIM 304930). We present genetic evidence that different mutations of the human gene FOXP3, the ortholog of the gene mutated in scurfy mice (Foxp3), causes IPEX syndrome. Recent linkage analysis studies mapped the gene mutated in IPEX to an interval of 17–20-cM at Xp11.23–Xq13.3 (refs. 1,2).

Disruption of a new forkhead/winged-helix protein, scurfin, results in the fatal lymphoproliferative disorder of the scurfy mouse

Scurfy (sf) is an X-linked recessive mouse mutant resulting in lethality in hemizygous males 16–25 days after birth, and is characterized by overproliferation of CD4+CD8– T lymphocytes, extensive multiorgan infiltration and elevation of numerous cytokines. Similar to animals that lack expression of either Ctla-4 (refs. 5,6) or Tgf-β (refs. 7,8), the pathology observed in sf mice seems to result from an inability to properly regulate CD4+CD8– T-cell activity. Here we identify the gene defective in sf mice by combining high-resolution genetic and physical mapping with large-scale sequence analysis. The protein encoded by this gene (designated Foxp3) is a new member of the forkhead/winged-helix family of transcriptional regulators and is highly conserved in humans. In sf mice, a frameshift mutation results in a product lacking the forkhead domain. Genetic complementation demonstrates that the protein product of Foxp3, scurfin, is essential for normal immune homeostasis.

X-linked neonatal diabetes mellitus, enteropathy and endocrinopathy syndrome is the human equivalent of mouse scurfy.

To determine whether human X-linked neonatal diabetes mellitus, enteropathy and endocrinopathy syndrome (IPEX; MIM 304930) is the genetic equivalent of the scurfy (sf) mouse, we sequenced the human ortholog (FOXP3) of the gene mutated in scurfy mice (Foxp3), in IPEX patients. We found four non-polymorphic mutations. Each mutation affects the forkhead/winged-helix domain of the scurfin protein, indicating that the mutations may disrupt critical DNA interactions.

Osteocyte control of bone formation via sclerostin, a novel BMP antagonist

There is an unmet medical need for anabolic treatments to restore lost bone. Human genetic bone disorders provide insight into bone regulatory processes. Sclerosteosis is a disease typified by high bone mass due to the loss of SOST expression. Sclerostin, the SOST gene protein product, competed with the type I and type II bone morphogenetic protein (BMP) receptors for binding to BMPs, decreased BMP signaling and suppressed mineralization of osteoblastic cells. SOST expression was detected in cultured osteoblasts and in mineralizing areas of the skeleton, but not in osteoclasts. Strong expression in osteocytes suggested that sclerostin expressed by these central regulatory cells mediates bone homeostasis. Transgenic mice overexpressing SOST exhibited low bone mass and decreased bone strength as the result of a significant reduction in osteoblast activity and subsequently, bone formation. Modulation of this osteocyte‐derived negative signal is therapeutically relevant for disorders associated with bone loss.

The Amount of Scurfin Protein Determines Peripheral T Cell Number and Responsiveness

In the absence of the recently identified putative transcription factor scurfin, mice develop a lymphoproliferative disorder resulting in death by 3 wk of age from a pathology that resembles TGF-β or CTLA-4 knockout mice. In this report, we characterize mice that overexpress the scurfin protein and demonstrate that these animals have a dramatically depressed immune system. Mice transgenic for the Foxp3 gene (which encodes the scurfin protein) have fewer T cells than their littermate controls, and those T cells that remain have poor proliferative and cytolytic responses and make little IL-2 after stimulation through the TCR. Although thymic development appears normal in these mice, peripheral lymphoid organs, particularly lymph nodes, are relatively acellular. In a separate transgenic line, forced expression of the gene specifically in the thymus can alter thymic development; however, this does not appear to affect peripheral T cells and is unable to prevent disease in mice lacking a functional Foxp3 gene, indicating that the scurfin protein acts on peripheral T cells. The data indicate a critical role for the Foxp3 gene product in the function of the immune system, with both the number and functionality of peripheral T cells under the aegis of the scurfin protein.

A Novel Mutation in CD83 Results in the Development of a Unique Population of CD4+ T Cells

Using a mouse mutagenesis screen, we have identified CD83 as being critical for the development of CD4+ T cells and for their function postactivation. CD11c+ dendritic cells develop and function normally in mice with a mutated CD83 gene but CD4+ T cell development is substantially reduced. Additionally, we now show that those CD4+ cells that develop in a CD83 mutant animal fail to respond normally following allogeneic stimulation. This is at least in part due to an altered cytokine expression pattern characterized by an increased production of IL-4 and IL-10 and diminished IL-2 production. Thus, in addition to its role in selection of CD4+ T cells, absence of CD83 results in the generation of cells with an altered activation and cytokine profile.

A point mutation in the murine Hem1 gene reveals an essential role for Hematopoietic protein 1 in lymphopoiesis and innate immunity.

Hem1 (Hematopoietic protein 1) is a hematopoietic cell-specific member of the Hem family of cytoplasmic adaptor proteins. Orthologues of Hem1 in Dictyostelium discoideum, Drosophila melanogaster, and Caenorhabditis elegans are essential for cytoskeletal reorganization, embryonic cell migration, and morphogenesis. However, the in vivo functions of mammalian Hem1 are not known. Using a chemical mutagenesis strategy in mice to identify novel genes involved in immune cell functions, we positionally cloned a nonsense mutation in the Hem1 gene. Hem1 deficiency results in defective F-actin polymerization and actin capping in lymphocytes and neutrophils caused by loss of the Rac-controlled actin-regulatory WAVE protein complex. T cell development is disrupted in Hem1-deficient mice at the CD4−CD8− (double negative) to CD4+CD8+ (double positive) cell stages, whereas T cell activation and adhesion are impaired. Hem1-deficient neutrophils fail to migrate in response to chemotactic agents and are deficient in their ability to phagocytose bacteria. Remarkably, some Rac-dependent functions, such as Th1 differentiation and nuclear factor κB (NF-κB)–dependent transcription of proinflammatory cytokines proceed normally in Hem1-deficient mice, whereas the production of Th17 cells are enhanced. These results demonstrate that Hem1 is essential for hematopoietic cell development, function, and homeostasis by controlling a distinct pathway leading to cytoskeletal reorganization, whereas NF-κB–dependent transcription proceeds independently of Hem1 and F-actin polymerization.

Immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome and the scurfy mutant mouse

IPEX is a rare X-linked syndrome characterized by neonatal infantile insulin dependent diabetes mellitus, enteropathy, endocrinopathy, eczema and severe infections with variable immune dysfunction. Scurfy, an X-linked lethal disorder of immune regulation, is the murine equivalent of IPEX, providing a model for the human disease. The gene responsible for scurfy has recently been identified by positional cloning. The scurfy/IPEX gene is a member of a family of transcriptional regulators that have in common a forkhead winged-helix DNA binding domain. The cloning of the human ortholog of the mouse gene confirmed that FOXP3 codes for the immune specific DNA binding protein responsible for IPEX. Missense and nonsense mutations, microdeletions, elimination of the downstream stop codon and mutations of the polyadenylation signal have been identified in affected members of families with classic IPEX. It has been suggested that FOXP3 acts as a trascriptional repressor in vivo. Treatment with Cyclosporin A or FK506 improves the clinical course and HLA identical bone marrow transplantation appears to provide a permanent cure.

Erratum: Alternating Hemiplegia of Childhood: Retrospective Genetic Study and Genotype-Phenotype Correlations in 187 Subjects from the US AHCF Registry (PLoS ONE (2015) 10:8 (e0137370) (doi:10.1371/journal.pone.0137370))

Louis Viollet, Gustavo Glusman, Kelley J. Murphy, Tara M. Newcomb, Sandra P. Reyna, Matthew Sweney, Benjamin Nelson, Frederick Andermann, Eva Andermann, Gyula Acsadi, Richard L. Barbano, Candida Brown, Mary E. Brunkow, Harry T. Chugani, Sarah R. Cheyette, Abigail Collins, Suzanne D. DeBrosse, David Galas, Jennifer Friedman, Lee Hood, Chad Huff, Lynn B. Jorde, Mary D. King, Bernie LaSalle, Richard J. Leventer, Aga J. Lewelt, Mylynda B. Massart, Mario R. Mérida, II, Louis J. Ptáček, Jared C. Roach, Robert S. Rust, Francis Renault, Terry D. Sanger, Marcio A. Sotero de Menezes, Rachel Tennyson, Peter Uldall, Yue Zhang, Mary Zupanc, Winnie Xin, Kenneth Silver, Kathryn J. Swoboda