Lipoproteins and diabetic microvascular complications

Risk factors for the microvascular complications (nephropathy and retinopathy) of Type 1 and Type 2 diabetes mellitus and the associated accelerated atherosclerosis include: age, diabetes duration, genetic factors, hyperglycaemia, hypertension, smoking, inflammation, glycation and oxidative stress and dyslipoproteinaemia. Hypertriglyceridaemia, low HDL and small dense LDL are common features of Type 2 diabetes and Type 1 diabetes with poor glycaemic control or renal complications. With the expansion of knowledge and of clinical and research laboratory tools, a broader definition of 'lipid' abnormalities in diabetes is appropriate. Dyslipoproteinaemia encompasses alterations in lipid levels, lipoprotein subclass distribution, composition (including modifications such as non-enzymatic glycation and oxidative damage), lipoprotein-related enzymes, and receptor interactions and subsequent cell signaling. Alterations occur in all lipoprotein classes; chylomicrons, VLDL, LDL, HDL, and Lp(a). There is also emerging evidence implicating lipoprotein related genotypes in the development of diabetic nephropathy and retinopathy. Lipoprotein related mechanisms associated with damage to the cardiovascular system may also be relevant to damage to the renal and ocular microvasculature. Adverse tissue effects are mediated by both alterations in lipoprotein function and adverse cellular responses. Recognition and treatment of lipoprotein-related risk factors, supported by an increasing array of assays and therapeutic agents, may facilitate early recognition and treatment of high complication risk diabetic patients. Further clinical and basic research, including intervention trials, is warranted to guide clinical practice. Optimal lipoprotein management, as part of a multi-faceted approach to diabetes care, may reduce the excessive personal and economic burden of microvascular complications and the related accelerated atherosclerosis.

Immune complexes containing modified lipoproteins are related to the progression of internal carotid intima-media thickness in patients with type 1 diabetes

Modified lipoproteins induce autoimmune responses including the synthesis of autoantibodies with pro-inflammatory characteristics. Circulating modified lipoprotein autoantibodies combine with circulating antigens and form immune complexes (IC). We now report the results of a study investigating the role of circulating IC containing modified lipoproteins in the progression of carotid intima-media thickness (IMT) in patients enrolled in the Epidemiology of Diabetes Interventions and Complications (EDIC) Trial, a follow-up study of the Diabetes Control and Complications Trial (DCCT). This cohort includes 1229 patients with type 1 diabetes in whom B-mode ultrasonography of internal and common carotid arteries was performed in 1994-1996 and in 1998-2000. Conventional CHD risk factors, antibodies against modified forms of LDL and modified lipoprotein IC were determined in 1050 of these patients from blood collected in 1996-1998. Cholesterol and apolipoprotein B content of IC (surrogate markers of modified ApoB-rich lipoproteins) were significantly higher in patients who showed progression of the internal carotid IMT than in those showing no progression, regression or mild progression. Multivariate linear and logistic regression modeling using conventional and non-conventional risk factors showed that the cholesterol content of IC was a significant positive predictor of internal carotid IMT progression. In conclusion these data demonstrate that increased levels of modified ApoB-rich IC are associated with increased progression of internal carotid IMT in the DCCT/EDIC cohort of type 1 diabetes.

Focal irradiation of perforating eye injuries

Plaques constructed with 125I were used to irradiate the sites of perforating ocular injuries in rabbits. An approximate dose of 16Gy given over a period of 6 days was shown to significantly reduce intraocular cellular proliferation when irradiation was commenced within 24 hours after injury. If irradiation was delayed until day 5, this reduction in cellular proliferation and intraocular membrane formation did not occur. Smaller radiation doses of approximately 6Gy given within 24 hours post-injury and administered over 6 days also reduced the extent of cellular proliferation but was not as effective as the 16Gy dose.

α-adrenoceptors do not mediate reflex mydriasis in rabbits

The aim of this study was to identify receptors that mediate reflex mydriasis in pentobarbital-anesthetized rabbits, in which the cervical sympathetic nerve was sectioned unilaterally. Voltage-response curves of pupillary dilation were generated bilaterally by stimulation of the sciatic nerve. Evoked mydriatic responses were mediated mainly by efferent parasympathetic innervation, and, to a lesser extent, by sympathetic innervation. The a-adrenergic antagonist, phenoxybenzamine (0.3 mg/kg, intravenously (i.v.)), antagonized mydriasis of the neurally intact eye, but not that on the sympathectomized side. The a- adrenergic antagonist, RS 79948 (0.3 mg/kg, i.v.), potentiated mydriasis of the normal eye, but was without either a potentiating or inhibitory effect on the mydriasis of the sympathectomized eye. In addition, the dopamine-receptor antagonist, haloperidol (1 mg/kg, i.v.), inhibited evoked mydriasis of the sympathectomized eye. These results suggest that, unlike some other species (cats and rats), a-adrenoceptors do not mediate reflex mydriasis elicited by sciatic-nerve stimulation in the rabbit, and support the previous finding in humans that dopamine receptors may mediate this response.

Studies of alpha-adrenoceptor antagonists on sympathetic mydriasis in rabbits

This study was undertaken to identify the alpha-adrenergic receptor type responsible for sympathetically evoked mydriasis in pentobarbital-anesthetized rabbits. Frequency-response curves of pupillary dilation were generated by stimulation of the preganglionic cervical sympathetic nerve (1-64 Hz). Evoked mydriatic responses were inhibited by systemic administration of nonselective alpha-adrenergic antagonists, phentolamine (0.3-10 mg/kg) and phenoxybenzamine (0.03-0.3 mg/kg), as well as the selective alpha(1)-adrenergic antagonist, prazosin (0.1-1 mg/kg). The alpha(2)-adrenergic antagonist, RS 79948 (0.3 mg/kg, i.v.) was without inhibitory effect, but potentiated the mydriatic response. In addition, the selective alpha(1A)-adrenoceptor antagonist, 5-methylurapidil (0.1-1 mg/kg, i.v.), antagonized the elicited mydriasis in a dose-dependent fashion. Unlike previous observations that prazosin does not block the adrenoceptor in rabbit iris dilator muscle, our results suggest that prazosin is effective in inhibiting neuronally elicited mydriasis in this species, and that alpha(1A)-adrenoceptors appear to mediate the response.

Clinical and experimental links between diabetes and glaucoma

Glaucoma is a leading cause of blindness. It is a multifactorial condition, the risk factors for which are increasingly well defined from large-scale epidemiological studies. One risk factor that remains controversial is the presence of diabetes. It has been proposed that diabetic eyes are at greater risk of injury from external stressors, such as elevated intraocular pressure. Alternatively, diabetes may cause ganglion cell loss, which becomes additive to a glaucomatous ganglion cell injury. Several clinical trials have considered whether a link exists between diabetes and glaucoma. In this review, we outline these studies and consider the causes for their lack of concordant findings. We also review the biochemical and cellular similarities between the two conditions. Moreover, we review the available literature that attempts to answer the question of whether the presence of diabetes increases the risk of developing glaucoma. At present, laboratory studies provide robust evidence for an association between diabetes and glaucoma.

Glial and neuronal dysfunction in streptozotocin-induced diabetic rats

Neuronal dysfunction has been noted very soon after the induction of diabetes by streptozotocin injection in rats. It is not clear from anatomical evidence whether glial cell dysfunction accompanies the well-documented neuronal deficit. Here, we isolate the Müller cell driven slow-P3 component of the full-field electroretinogram and show that it is attenuated at 4 weeks following the onset of streptozotocin-hyperglycaemia. We also found a concurrent reduction in the sensitivity of the phototransduction cascade, as well as in the components of the electroretinogram known to indicate retinal ganglion cell and amacrine cell integrity. Our data support the idea that neuronal and Müller cell dysfunction occurs at the same time in streptozotocin-induced hyperglycaemia.

Treatment of choroidal neovascularisation secondary to membranoproliferative glomerulonephritis type II with intravitreal ranibizumab

Membranoproliferative glomerulonephritis type II (MPGN II) is characterised by electron-dense deposits of complement components in the glomerular basement membrane and retinal pigment epithelium. Approximately, 10% of affected individuals develop serious ocular complications similar to age-related macular degeneration such as choroidal neovascularisation (CNV), which has been managed with photocoagulation or photodynamic therapy; however, these treatments can impact visual acuity. We report the case of a 42-year-old woman with MPGN II presenting with decreased visual acuity and paracentral scotoma in her left eye due to an extrafoveal choroidal neovascular membrane (growth of new vessels under the retina). The patient was successfully treated with intravitreal ranibizumab (Lucentis) with restoration of visual function. This case highlights the successful management of CNV secondary to MPGN II with the antivascular endothelial growth factor agent ranibizumab and emphasises the importance of early referral of patients with MPGN II who are reporting of visual 'distortion'.

Enrichment of pathogenic alleles in the brittle cornea gene, ZNF469, in keratoconus

Keratoconus, a common inherited ocular disorder resulting in progressive corneal thinning, is the leading indication for corneal transplantation in the developed world. Genome-wide association studies have identified common SNPs 100 kb upstream of ZNF469 strongly associated with corneal thickness. Homozygous mutations in ZNF469 and PR domain-containing protein 5 (PRDM5) genes result in brittle cornea syndrome (BCS) Types 1 and 2, respectively. BCS is an autosomal recessive generalized connective tissue disorder associated with extreme corneal thinning and a high risk of corneal rupture. Some individuals with heterozygous PRDM5 mutations demonstrate a carrier ocular phenotype, which includes a mildly reduced corneal thickness, keratoconus and blue sclera. We hypothesized that heterozygous variants in PRDM5 and ZNF469 predispose to the development of isolated keratoconus. We found a significant enrichment of potentially pathologic heterozygous alleles in ZNF469 associated with the development of keratoconus (P = 0.00102) resulting in a relative risk of 12.0. This enrichment of rare potentially pathogenic alleles in ZNF469 in 12.5% of keratoconus patients represents a significant mutational load and highlights ZNF469 as the most significant genetic factor responsible for keratoconus identified to date.

The Prevalence of Age-Related Macular Degeneration in Alzheimer's Disease

BACKGROUND:
Age-related macular degeneration (AMD) and Alzheimer's disease (AD) share several features, including the presence of extracellular abnormal deposits associated with neuronal degeneration, drusen, and plaques, respectively. Investigation of any association of AMD and specifically AD is worthwhile but has rarely been done.
OBJECTIVES:
The aim of this study was to determine the prevalence of AMD in subjects with AD in comparison with an age-matched cognitively normal cohort.
METHODS:
Cases were defined as those diagnosed with AD using standardized criteria as part of their clinical care, while controls were cognitively intact individuals aged 65 years or more. Dilated retinal photographs were taken, and a range of potentially confounding factors measured including APOE genotype. AMD features were recorded and AMD grades given.
RESULTS:
Data was collected on 322 controls and 258 cases. While AMD was associated with AD, and the proportion of cases of advanced AMD in AD cases was twice that of controls, when corrected the association was lost. AD was associated with age, the presence of an APOE allele, and smoking, while being 'generally unwell recently' was associated with a reduced risk of AD.
CONCLUSION:
AD and AMD are both associated with age, but our study does not find evidence they are associated with each other. However the retina offers an opportunity to non-invasively image neuronal tissue, and more sophisticated imaging techniques may shed light on ocular biomarkers of AD.

Mutational Analysis of the Rhodopsin Gene in Sector Retinitis Pigmentosa

Background: To determine the role of rhodopsin (RHO) gene mutations in patients with sector retinitis pigmentosa (RP) from Northern Ireland.

Design: A case series of sector RP in a tertiary ocular genetics clinic.

Participants: Four patients with sector RP were recruited from the Royal Victoria Hospital (Belfast, Northern Ireland) and Altnagelvin Hospital (Londonderry, Northern Ireland) following informed consent.

Methods: The diagnosis of sector RP was based on clinical examination, International Society for Clinical Electrophysiology of Vision (ISCEV) standard electrophysiology, and visual field analysis. DNA was extracted from peripheral blood leucocytes and the coding regions and adjacent flanking intronic sequences of the RHO gene were polymerase chain reaction (PCR) amplified and cycle sequenced.

Main Outcome Measure: Rhodopsin mutational status.

Results: A heterozygous missense mutation in RHO (c.173C > T) resulting in a non-conservative substitution of threonine to methionine (p. Thr58Met) was identified in one patient and was absent from 360 control individuals. This non-conservative substitution (p.Thr58Met) replaces a highly evolutionary conserved polar hydrophilic threonine residue with a non-polar hydrophobic methionine residue at position 58 near the cytoplasmic border of helix A of RHO.

Conclusions: The study identified a RHO gene mutation (p.Thr58Met) not previously reported in RP in a patient with sector RP. These findings outline the phenotypic variability associated with RHO mutations. It has been proposed that the regional effects of RHO mutations are likely to result from interplay between mutant alleles and other genetic, epigenetic and environmental factors.

Multimodal functional and anatomical imaging identifies preclinical microvascular abnormalities in type 1 diabetes mellitus

Structural and functional change in the microcirculation in type 1 diabetes mellitus predicts future end-organ damage and macrovascular events. We explored the utility of novel signal processing techniques to detect and track change in ocular hemodynamics in patients with this disease. 24 patients with uncomplicated type 1 diabetes mellitus, and 18 age-and-sex matched control subjects were studied. Doppler ultrasound was used to interrogate the carotid and ophthalmic arteries and digital photography to image the retinal vasculature. Frequency analysis algorithms were applied to quantify velocity waveform structure and retinal photographic data at baseline and following inhalation of 100% oxygen. Frequency data was compared between groups. No significant differences were found in the resistive index between groups at baseline or following inhaled oxygen. Frequency analysis of the Doppler flow velocity waveforms identified significant differences in bands 3-7 between patients and controls in data captured from the ophthalmic artery (p<0.01 for each band). In response to inhaled oxygen, changes in the frequency band amplitudes were significantly greater in control subjects compared with patients (p<0.05). Only control subjects demonstrated a positive correlation (R=0.61) between change in retinal vessel diameter and frequency band amplitudes derived from ophthalmic artery waveform data. The use of multimodal signal processing techniques applied to Doppler flow velocity waveforms and retinal photographic data identified preclinical change in the ocular microcirculation in patients with uncomplicated diabetes mellitus. An impaired autoregulatory response of the retinal microvasculature may contribute to the future development of retinopathy in such patients.