Clinical spectrum of MTOR-related hypomelanosis of Ito with neurodevelopmental abnormalities.

PURPOSE: Hypomelanosis of Ito (HI) is a skin marker of somatic mosaicism. Mosaic MTOR pathogenic variants have been reported in HI with brain overgrowth. We sought to delineate further the pigmentary skin phenotype and clinical spectrum of neurodevelopmental manifestations of MTOR-related HI. METHODS: From two cohorts totaling 71 patients with pigmentary mosaicism, we identified 14 patients with Blaschko-linear and one with flag-like pigmentation abnormalities, psychomotor impairment or seizures, and a postzygotic MTOR variant in skin. Patient records, including brain magnetic resonance image (MRI) were reviewed. Immunostaining (n = 3) for melanocyte markers and ultrastructural studies (n = 2) were performed on skin biopsies. RESULTS: MTOR variants were present in skin, but absent from blood in half of cases. In a patient (p.[Glu2419Lys] variant), phosphorylation of p70S6K was constitutively increased. In hypopigmented skin of two patients, we found a decrease in stage 4 melanosomes in melanocytes and keratinocytes. Most patients (80%) had macrocephaly or (hemi)megalencephaly on MRI. CONCLUSION: MTOR-related HI is a recognizable neurocutaneous phenotype of patterned dyspigmentation, epilepsy, intellectual deficiency, and brain overgrowth, and a distinct subtype of hypomelanosis related to somatic mosaicism. Hypopigmentation may be due to a defect in melanogenesis, through mTORC1 activation, similar to hypochromic patches in tuberous sclerosis complex.

NODAL/TGFß signalling mediates the self-sustained stemness induced by PIK3CAH1047R homozygosity in pluripotent stem cells.

Activating PIK3CA mutations are known 'drivers' of human cancer and developmental overgrowth syndromes. We recently demonstrated that the 'hotspot' PIK3CAH1047R variant exerts unexpected allele dose-dependent effects on stemness in human pluripotent stem cells (hPSCs). In this study, we combine high-depth transcriptomics, total proteomics and reverse-phase protein arrays to reveal potentially disease-related alterations in heterozygous cells, and to assess the contribution of activated TGFß signalling to the stemness phenotype of homozygous PIK3CAH1047R cells. We demonstrate signalling rewiring as a function of oncogenic PI3K signalling strength, and provide experimental evidence that self-sustained stemness is causally related to enhanced autocrine NODAL/TGFß signalling. A significant transcriptomic signature of TGFß pathway activation in heterozygous PIK3CAH1047R was observed but was modest and was not associated with the stemness phenotype seen in homozygous mutants. Notably, the stemness gene expression in homozygous PIK3CAH1047R hPSCs was reversed by pharmacological inhibition of NODAL/TGFß signalling, but not by pharmacological PI3Kα pathway inhibition. Altogether, this provides the first in-depth analysis of PI3K signalling in hPSCs and directly links strong PI3K activation to developmental NODAL/TGFß signalling. This work illustrates the importance of allele dosage and expression when artificial systems are used to model human genetic disease caused by activating PIK3CA mutations. This article has an associated First Person interview with the first author of the paper.

Oncogenic PIK3CA promotes cellular stemness in an allele dose-dependent manner.

The PIK3CA gene, which encodes the p110α catalytic subunit of PI3 kinase (PI3K), is mutationally activated in cancer and in overgrowth disorders known as PIK3CA-related overgrowth spectrum (PROS). To determine the consequences of genetic PIK3CA activation in a developmental context of relevance to both PROS and cancer, we engineered isogenic human induced pluripotent stem cells (iPSCs) with heterozygous or homozygous knockin of PIK3CAH1047R While heterozygous iPSCs remained largely similar to wild-type cells, homozygosity for PIK3CAH1047R caused widespread, cancer-like transcriptional remodeling, partial loss of epithelial morphology, up-regulation of stemness markers, and impaired differentiation to all three germ layers in vitro and in vivo. Genetic analysis of PIK3CA-associated cancers revealed that 64% had multiple oncogenic PIK3CA copies (39%) or additional PI3K signaling pathway-activating "hits" (25%). This contrasts with the prevailing view that PIK3CA mutations occur heterozygously in cancer. Our findings suggest that a PI3K activity threshold determines pathological consequences of oncogenic PIK3CA activation and provide insight into the specific role of this pathway in human pluripotent stem cells.

Receptor Quaternary Organization Explains G Protein-Coupled Receptor Family Structure.

The organization of Rhodopsin-family G protein-coupled receptors (GPCRs) at the cell surface is controversial. Support both for and against the existence of dimers has been obtained in studies of mostly individual receptors. Here, we use a large-scale comparative study to examine the stoichiometric signatures of 60 receptors expressed by a single human cell line. Using bioluminescence resonance energy transfer- and single-molecule microscopy-based assays, we found that a relatively small fraction of Rhodopsin-family GPCRs behaved as dimers and that these receptors otherwise appear to be monomeric. Overall, the analysis predicted that fewer than 20% of ∼700 Rhodopsin-family receptors form dimers. The clustered distribution of the dimers in our sample and a striking correlation between receptor organization and GPCR family size that we also uncover each suggest that receptor stoichiometry might have profoundly influenced GPCR expansion and diversification.

Nephroblastomatosis or Wilms tumor in a fourth patient with a somatic PIK3CA mutation.

Wilms tumor and nephroblastomatosis are associated with syndromic conditions including hemihyperplasia. Hemihyperplasia is genetically heterogeneous and may be the result of genomic abnormalities seen in Beckwith-Wiedemann syndrome, mosaic chromosome or genomic abnormalities, or somatic point mutations. Somatic missense mutations affecting the PI3K-AKT-MTOR pathway result in segmental overgrowth and are present in numerous benign and malignant tumors. Here, we report a fourth patient with asymmetric overgrowth due to a somatic PIK3CA mutation who had nephroblastomatosis or Wilms tumor. Similar to two of three reported patients with a somatic PIK3CA mutation and renal tumors, he shared a PIK3CA mutation affecting codon 1047, presented at birth with asymmetric overgrowth, and had fibroadipose overgrowth. Codon 1047 is most commonly affected by somatic mutations in PIK3CA-related overgrowth spectrum (PROS). While the fibroadipose overgrowth phenotype appears to be common in individuals with PIK3CA mutations at codon 1047, individuals with a clinical diagnosis of Klippel-Trenaunay syndrome or isolated lymphatic malformation also had mutations affecting this amino acid. Screening for Wilms tumor in individuals with PROS-related hemihyperplasia may be considered and, until the natural history is fully elucidated in larger cohort studies, may follow guidelines for Beckwith-Wiedemann syndrome, or isolated hemihyperplasia. It is not known if the specific PIK3CA mutation, the mosaic distribution, or the clinical presentation affect the Wilms tumor or nephroblastomatosis risk in individuals with PROS. © 2016 Wiley Periodicals, Inc.

Somatic activating mutations in Pik3ca cause sporadic venous malformations in mice and humans.

Venous malformations (VMs) are painful and deforming vascular lesions composed of dilated vascular channels, which are present from birth. Mutations in the TEK gene, encoding the tyrosine kinase receptor TIE2, are found in about half of sporadic (nonfamilial) VMs, and the causes of the remaining cases are unknown. Sclerotherapy, widely accepted as first-line treatment, is not fully efficient, and targeted therapy for this disease remains underexplored. We have generated a mouse model that faithfully mirrors human VM through mosaic expression of Pik3ca(H1047R), a constitutively active mutant of the p110α isoform of phosphatidylinositol 3-kinase (PI3K), in the embryonic mesoderm. Endothelial expression of Pik3ca(H1047R)resulted in endothelial cell (EC) hyperproliferation, reduction in pericyte coverage of blood vessels, and decreased expression of arteriovenous specification markers. PI3K pathway inhibition with rapamycin normalized EC hyperproliferation and pericyte coverage in postnatal retinas and stimulated VM regression in vivo. In line with the mouse data, we also report the presence of activating PIK3CA mutations in human VMs, mutually exclusive with TEK mutations. Our data demonstrate a causal relationship between activating Pik3ca mutations and the genesis of VMs, provide a genetic model that faithfully mirrors the normal etiology and development of this human disease, and establish the basis for the use of PI3K-targeted therapies in VMs.

Phosphoinositide 3-kinase-related overgrowth: cellular phenotype and future therapeutic options.

BACKGROUND: Somatic activating mutations in PIK3CA, which encodes the p110α catalytic subunit of phosphoinositide-3-kinase (PI3K) are frequently found in cancers and have been identified in a spectrum of mosaic overgrowth disorders ranging from isolated digit enlargement to more extensive overgrowth of the body, brain, or vasculature. We aimed to study affected dermal fibroblasts with a view to inform therapeutic studies, and to observe cancer-associated mutations in isolation. METHODS: We measured PIP3 concentrations in dermal fibroblasts with endogenous PIK3CA mutations and in wild type fibroblasts using mass spectrometry, and we measured downstream signalling events with ELISA and immunoblotting. Cellular proliferation was evaluated with 5-bromo-2'-deoxyuridine incorporation, and cell size assessed by fluorescence-activated cell sorting (FACS). Glycolysis and mitochondrial tests were performed with an extracellular flux analyser (Seahorse Bioscience, Billerica, MA, USA), and mitochondrial potential was measured by FACS-based JC1 staining. Experiments were repeated after exposure to 5 nmol everolimus for 72 h. FINDINGS: Mutant fibroblasts had two times higher basal PIP3 concentrations than wild-type fibroblasts (p=0·0017), with concomitant AKT and p70S6 activation downstream. The rate of cellular proliferation was higher in mutant cells under low serum conditions, but median cell size was not statistically different. Glycolytic capacity was similar between mutant and wild type fibroblasts, but subtle differences in mitochondrial function were detected with blunted responses to uncoupling agents and reduced membrane potentials. Treatment with everolimus reversed aberrant AKT(ser473) and p70S6 signalling, slowed cellular proliferation, and reversed mitochondrial abnormalities, but was associated, paradoxically, with increases in PIP3 concentrations. INTERPRETATION: These experiments demonstrate activation of the PI3K-AKT pathway in affected fibroblasts with increased proliferation, but no hypertrophy. Moreover, we identified changes in mitochondrial function in keeping with the known propensity of AKT to modulate elements of the Warburg effect. These results suggest that inhibitors of the mammalian target of rapamycin (mTOR) might be beneficial, but these inhibitors will require formal evaluation in clinical trials. More targeted therapy with p110α inhibitors is an enticing future option. FUNDING: Wellcome Trust, Sackler Fund, National Instititute for Health Research.

Type-3 BRET, an improved competition-based bioluminescence resonance energy transfer assay.

We show that in conventional, competition-based bioluminescence resonance energy transfer (BRET) assays of membrane protein stoichiometry, the presence of competitors can alter tagged-protein density and artifactually reduce energy transfer efficiency. A well-characterized monomeric type I membrane protein, CD86, and two G protein-coupled receptors ß2AR and mCannR2, all of which behave as dimers in these conventional assays, exhibit monomeric behavior in an improved competition-based type-3 BRET assay designed to circumvent such artifacts.