Amanda J. Churchill

Expression of pro- and anti-angiogenic isoforms of VEGF is differentially regulated by splicing and growth factors

Vascular endothelial growth factor A (VEGFA; hereafter referred to as VEGF) is a key regulator of physiological and pathological angiogenesis. Two families of VEGF isoforms are generated by alternate splice-site selection in the terminal exon. Proximal splice-site selection (PSS) in exon 8 results in pro-angiogenic VEGFxxx isoforms (xxx is the number of amino acids), whereas distal splice-site selection (DSS) results in anti-angiogenic VEGFxxxb isoforms. To investigate control of PSS and DSS, we investigated the regulation of isoform expression by extracellular growth factor administration and intracellular splicing factors. In primary epithelial cells VEGFxxxb formed the majority of VEGF isoforms (74%). IGF1, and TNFα treatment favoured PSS (increasing VEGFxxx) whereas TGFβ1 favoured DSS, increasing VEGFxxxb levels. TGFβ1 induced DSS selection was prevented by inhibition of p38 MAPK and the Clk/sty (CDC-like kinase, CLK1) splicing factor kinase family, but not ERK1/2. Clk phosphorylates SR protein splicing factors ASF/SF2, SRp40 and SRp55. To determine whether SR splicing factors alter VEGF splicing, they were overexpressed in epithelial cells, and VEGF isoform production assessed. ASF/SF2, and SRp40 both favoured PSS, whereas SRp55 upregulated VEGFxxxb (DSS) isoforms relative to VEGFxxx. SRp55 knockdown reduced expression of VEGF165b. Moreover, SRp55 bound to a 35 nucleotide region of the 3′UTR immediately downstream of the stop codon in exon 8b. These results identify regulation of splicing by growth and splice factors as a key event in determining the relative pro-versus anti-angiogenic expression of VEGF isoforms, and suggest that p38 MAPK-Clk/sty kinases are responsible for the TGFβ1-induced DSS selection, and identify SRp55 as a key regulatory splice factor.

Diabetic retinopathy is associated with a switch in splicing from anti- to pro-angiogenic isoforms of vascular endothelial growth factor

Aims/hypothesisProliferative diabetic retinopathy results from excess blood vessel growth into the vitreous fluid of the eye. Retinal angiogenesis is regulated by expression of vascular endothelial growth factor (VEGF), and many studies have shown that VEGF is critically involved in proliferative diabetic retinopathy. VEGF is alternatively spliced to form the angiogenic (VEGFxxx) and potentially anti-angiogenic (VEGFxxxb) family of isoforms. The VEGFxxxb family is found in normal tissues, but down-regulated in renal and prostate cancer. Previous studies on endogenous expression of VEGF in the eye have not distinguished between the two families of isoforms.MethodsWe measured VEGFxxxb isoform expression in normal human eye tissue (lens, sclera, retina and iris) and vitreous fluid using enzyme-linked immunosorbent assay and Western blotting with a VEGFxxxb-specific antibody.ResultsVEGFxxxb protein was expressed in lens, sclera, retina, iris and vitreous fluid. Multiple isoforms were seen, including VEGF165b, VEGF121b, VEGF145b, VEGF183b and VEGF189b. In non-diabetic patients, 64±7% of the VEGF in the vitreous was VEGFxxxb (n=18), whereas in diabetic patients only 12.5±3.6% of total VEGF was VEGFxxxb.Conclusions/interpretationSince VEGFxxxb inhibits VEGFxxx-induced angiogenesis in a one-to-one stoichiometric manner, these results show that in the eye of diabetic patients VEGF splicing was switched from an anti-angiogenic to a pro-angiogenic environment. This occurred through changes to the ratio of VEGFxxx : VEGFxxxb. Alterations to splicing, and through that to the balance of VEGF isoforms, could therefore be a potential therapeutic strategy for diabetic retinopathy.

Polymicrogyria and absence of pineal gland due to PAX6 mutation

Identification of genes involved in human cerebral development is important for our understanding of disorders with potential neurodevelopmental causes such as epilepsy and learning disability. Murine models suggest that PAX6 plays a key role in human brain development. With magnetic resonance imaging in 24 humans heterozygous for defined PAX6 mutations, we demonstrated widespread structural abnormalities including absence of the pineal gland and unilateral polymicrogyria. Ann Neurol 2003;53:658–663

Deletion of the OPA1 gene in a dominant optic atrophy family: evidence that haploinsufficiency is the cause of disease

Dominant optic atrophy (DOA) is the most common form of autosomally inherited optic neuropathy.1 The disease typically presents in childhood with slow bilateral loss of visual acuity, visual field defects, abnormal colour discrimination, and pallor of the optic discs. The majority of DOA families published to date have shown linkage to a major locus on chromosome 3q28 ( OPA1 ). The OPA1 gene was recently identified and found to encode a ubiquitously expressed, dynamin related GTPase.2,3 In order to determine the mutation spectrum of OPA1 in DOA, we and others have screened the coding exons and their flanking splice sites in large patient cohorts.4–7 Over 60 different mutations have been reported, most of which are specific to individual families. It has been speculated that haploinsufficiency is the cause of disease,3–6 but to date there has been no evidence to prove that this mechanism, rather than aberrant function of mutated proteins, is responsible for the disease. Using a combined approach of single stranded conformational polymorphism and heteroduplex analysis, we detected mutations in 57% (20/35) of our affected DOA patients.5 Additional DNA samples from an Australian family, in which no OPA1 mutation was identified in the above mutation screen, have recently been obtained enabling further investigation of the cause of disease in this family. All available family members underwent clinical evaluation to determine their disease status. This included best corrected visual acuity (BCVA), assessment of colour vision with Isihara plates and/or City University testing, automated perimetry (Humphrey Field Analyzer, San Leandro, CA), and dilated fundus examination with stereo disc photography. The ocular phenotype observed was typical of DOA with visual acuity reduction ranging from mild to moderate, mild to moderate colour vision disturbance, and visual field analysis ranging from normal to mild paracentral …

VEGF-A165b is cytoprotective and antiangiogenic in the retina

PURPOSE A number of key ocular diseases, including diabetic retinopathy and age-related macular degeneration, are characterized by localized areas of epithelial or endothelial damage, which can ultimately result in the growth of fragile new blood vessels, vitreous hemorrhage, and retinal detachment. VEGF-A(165), the principal neovascular agent in ocular angiogenic conditions, is formed by proximal splice site selection in its terminal exon 8. Alternative splicing of this exon results in an antiangiogenic isoform, VEGF-A(165)b, which is downregulated in diabetic retinopathy. Here the authors investigate the antiangiogenic activity of VEGF(165)b and its effect on retinal epithelial and endothelial cell survival. METHODS VEGF-A(165)b was injected intraocularly in a mouse model of retinal neovascularization (oxygen-induced retinopathy [OIR]). Cytotoxicity and cell migration assays were used to determine the effect of VEGF-A(165)b. RESULTS VEGF-A(165)b dose dependently inhibited angiogenesis (IC(50), 12.6 pg/eye) and retinal endothelial migration induced by 1 nM VEGF-A(165) across monolayers in culture (IC(50), 1 nM). However, it also acts as a survival factor for endothelial cells and retinal epithelial cells through VEGFR2 and can stimulate downstream signaling. Furthermore, VEGF-A(165)b injection, while inhibiting neovascular proliferation in the eye, reduced the ischemic insult in OIR (IC(50), 2.6 pg/eye). Unlike bevacizumab, pegaptanib did not interact directly with VEGF-A(165)b. CONCLUSIONS The survival effects of VEGF-A(165)b signaling can protect the retina from ischemic damage. These results suggest that VEGF-A(165)b may be a useful therapeutic agent in ischemia-induced angiogenesis and a cytoprotective agent for retinal pigment epithelial cells.

Recombinant Human VEGF165b Inhibits Experimental Choroidal Neovascularization

PURPOSE Vascular endothelial growth factor (VEGF-A) is the principal stimulator of angiogenesis in wet age-related macular degeneration (AMD). However, VEGF-A is generated by alternate splicing into two families, the proangiogenic VEGF-A(xxx) family and the antiangiogenic VEGF-A(xxx)b family. It is the proangiogenic family that is responsible for the blood vessel growth seen in AMD. METHODS To determine the role of antiangiogenic isoforms of VEGF-A as inhibitors of choroidal neovascularization, the authors used a model of laser-induced choroidal neovascularization in the mouse eye and investigated VEGF-A(165)b effects on endothelial cells and VEGFRs in vitro. RESULTS VEGF-A(165)b inhibited VEGF-A(165)-mediated endothelial cell migration with a dose effect similar to that of ranibizumab and bevacizumab and 200-fold more potent than that of pegaptanib. VEGF-A(165)b bound both VEGFR1 and VEGFR2 with affinity similar to that of VEGF-A(165). After laser injury, mice were injected either intraocularly or subcutaneously with recombinant human VEGF-A(165)b. Intraocular injection of rhVEGF-A(165)b gave a pronounced dose-dependent inhibition of fluorescein leakage, with an IC(50) of 16 pg/eye, neovascularization (IC(50), 0.8 pg/eye), and lesion as assessed by histologic staining (IC(50), 8 pg/eye). Subcutaneous administration of 100 microg twice a week also inhibited fluorescein leakage and neovascularization and reduced lesion size. CONCLUSIONS These results show that VEGF-A(165)b is a potent antiangiogenic agent in a mouse model of age-related macular degeneration and suggest that increasing the ratio of antiangiogenic-to-proangiogenic isoforms may be therapeutically effective in this condition.

VEGF 121 b, a new member of the VEGF xxx b family of VEGF-A splice isoforms, inhibits neovascularisation and tumour growth in vivo

Background:The key mediator of new vessel formation in cancer and other diseases is VEGF-A. VEGF-A exists as alternatively spliced isoforms - the pro-angiogenic VEGFxxx family generated by exon 8 proximal splicing, and a sister family, termed VEGFxxxb, exemplified by VEGF165b, generated by distal splicing of exon 8. However, it is unknown whether this anti-angiogenic property of VEGF165b is a general property of the VEGFxxxb family of isoforms.Methods:The mRNA and protein expression of VEGF121b was studied in human tissue. The effect of VEGF121b was analysed by saturation binding to VEGF receptors, endothelial migration, apoptosis, xenograft tumour growth, pre-retinal neovascularisation and imaging of biodistribution in tumour-bearing mice with radioactive VEGF121b.Results:The existence of VEGF121b was confirmed in normal human tissues. VEGF121b binds both VEGF receptors with similar affinity as other VEGF isoforms, but inhibits endothelial cell migration and is cytoprotective to endothelial cells through VEGFR-2 activation. Administration of VEGF121b normalised retinal vasculature by reducing both angiogenesis and ischaemia. VEGF121b reduced the growth of xenografted human colon tumours in association with reduced microvascular density, and an intravenous bolus of VEGF121b is taken up into colon tumour xenografts.Conclusion:Here we identify a second member of the family, VEGF121b, with similar properties to those of VEGF165b, and underline the importance of the six amino acids of exon 8b in the anti-angiogenic activity of the VEGFxxxb isoforms.

Cytokine polymorphism in noninfectious uveitis.

PURPOSE Noninfectious uveitis is a sight-threatening immune-mediated intraocular inflammatory disorder. The inheritance of uveitis in multiplex families and its association with known monogenic and polygenic immunologic disorders suggests that common genetic variants underlie susceptibility to uveitis as well as to other immunologic disorders. TNFA and IL10 are strong candidate genes, given the influence of these cytokines on inflammation, immune tolerance, and apoptosis. METHODS The role of 12 polymorphisms spanning the TNFA and IL10 genomic regions was investigated in 192 uveitis patients and 92 population control subjects from four regional centers in the United Kingdom and Republic of Ireland. RESULTS The results demonstrate that uveitis is associated with three haplotype-tagging SNPs (htSNPs) in the IL10 gene: htSNP2 (rs6703630), htSNP5 (rs2222202), and htSNP6 (rs3024490). IL10htSNP2AG/htSNP5TC was the most significantly associated haplotype (P = 0.00085), whereas the LTA+252AA/TNFhtSNP2GG haplotype was protective (P = 0.00031). Furthermore, subgroup analysis showed that the frequency of the TNFd4 allele was higher in patients with nonremitting ocular disease and/or those requiring higher levels of maintenance immunosuppression. Although these associations lost significance after Bonferroni correction, they infer a relationship that may be validated by a larger study. CONCLUSIONS Since these variants are implicated in the susceptibility and severity of several immunologic disorders, the results support the hypothesis that common genetic determinants influence shared mechanisms of autoimmunity.

Quantitative MR image analysis in subjects with defects in the PAX6 gene.

Cerebral development is governed by genetic and environmental factors. Animal models provide much of our knowledge about genetic influences in the brain but it is not clear how similar or relevant these are to the human brain. The investigation of human subjects with mutations in genes known to be expressed in the developing brain is an alternative approach to improving our understanding of the role of genetic factors in brain development. We investigated 24 subjects with known mutations in the PAX6 gene (including representatives of all the known mutations of the gene) which are associated with characteristic ocular abnormalities in humans and animals. We have quantified MRI data using several techniques to assess qualities of cerebral structure which are difficult to interpret visually. Abnormalities were identified using voxel-based morphometry, statistical morphometrics, and measurement of corpus callosum cross-sectional area when comparing data from subjects with PAX6 abnormality and 72 age-and sex-matched control subjects. These abnormalities include grey matter changes in the cerebellum and occipital poles and white matter loss in the corpus callosum, alteration of sulcal orientation in the occipital lobe, and alteration to overall neuronal connectivity. These abnormalities complement and exceed the changes seen in the mouse models, and those seen on visual inspection alone of the human MRI data. Structural differences were also identified between the two largest genotype mutation subgroups.

Autosomal dominant retinitis pigmentosa with intrafamilial variability and incomplete penetrance in two families carrying mutations in PRPF8.

PURPOSE The aim of this study was to report detailed genotype/phenotype correlation in two British autosomal dominant retinitis pigmentosa (adRP) families with recently described mutations in PRPF8. METHODS Ten affected members from the two families (excluded for PRPF31 mutations) were assessed clinically. Seven subjects had fundus photography; some had electrophysiology, autofluorescence imaging, and visual field testing. Linkage analysis was performed from genomic DNA in one family. RNA was extracted from lymphocytes of the proband from both families, reverse transcribed into cDNA and subsequently screened for mutations in PRPF8. Segregation of mutations in each family was tested by direct genomic sequencing of the specific exons carrying the mutation. RESULTS All affected members complained of nyctalopia with variable age of onset. In the first family, there was marked variation in the clinical phenotype among affected individuals ranging from severe rod-cone dystrophy to a 67-year-old patient with a normal retinal appearance and mild rod dysfunction on scotopic electroretinography (ERG). The second family demonstrated similar variability and a history of a nonpenetrant individual. Linkage analysis in the first family showed strong evidence for linkage to markers on chromosome 17p implicating PRPF8 as a candidate gene. A c.6353 C>T change causing a nonconservative missense mutation p.S2118F was found in exon 38 of PRPF8 by direct sequencing of the cDNA. The mutation c.6930G>C (p.R2310S) was found in the second family. CONCLUSIONS This is the first report of marked intrafamilial variability associated with mutations in the PRPF8 gene, including incomplete penetrance. PRPF8 mutations should be suspected in patients with adRP and variable expressivity.