Glomerular Lesions in Proteinuric Miniature Schnauzer Dogs.

Miniature Schnauzer dogs are predisposed to idiopathic hypertriglyerceridemia, which increases risk for diseases such as pancreatitis and gallbladder mucocele. Recently, elevated triglyceride concentrations have been associated with proteinuria in this breed, although it is difficult to determine which abnormality is primary. Retrospective review of renal tissue from 27 proteinuric Miniature Schnauzers revealed that 20 dogs had ultrastructural evidence of osmophilic globules consistent with lipid in glomerular tufts. Seven of these dogs had lipid thromboemboli in glomerular capillary loops that distorted their shape and compressed circulating erythrocytes. Triglyceride concentrations were reported in 6 of these 7 dogs, and all were hypertriglyceridemic. In addition, glomerular lipidosis (defined as accumulation of foam cells within peripheral capillary loops) was identified in a single dog. The remaining 12 dogs had smaller amounts of lipid that could only be identified ultrastructurally. Neither signalment data nor clinicopathologic parameters (serum albumin, serum creatinine, urine protein-to-creatinine ratio, and blood pressure) differed among the various types of lipid lesions. During the time course of this study, all dogs diagnosed with glomerular lipid thromboemboli were Miniature Schnauzers, underscoring the importance of recognizing these clear spaces within capillary loops as lipid.

X-Linked Hereditary Nephropathy in Navasota Dogs: Clinical Pathology, Morphology, and Gene Expression During Disease Progression.

X-linked hereditary nephropathy (XLHN) in Navasota dogs is a spontaneously occurring disease caused by a mutation resulting in defective production of type IV collagen and juvenile-onset renal failure. The study was aimed at examining the evolution of renal damage and the expression of selected molecules potentially involved in the pathogenesis of XLHN. Clinical data and renal samples were obtained in 10 XLHN male dogs and 5 controls at 4 (T0), 6 (T1), and 9 (T2) months of age. Glomerular and tubulointerstitial lesions were scored by light microscopy, and the expression of 21 molecules was investigated by quantitative real-time polymerase chain reaction with selected proteins evaluated by immunohistochemistry. No significant histologic lesions or clinicopathologic abnormalities were identified in controls at any time-point. XLHN dogs had progressive proteinuria starting at T0. At T1, XLHN dogs had a mesangioproliferative glomerulopathy with glomerular loss, tubular necrosis, and interstitial fibrosis. At T2, glomerular and tubulointerstitial lesions were more severe, particularly glomerular loss, interstitial fibrosis, and inflammation. At T0, transforming growth factor ß, connective tissue growth factor, and platelet-derived growth factor α mRNA were overexpressed in XLHN dogs compared with controls. Clusterin and TIMP1 transcripts were upregulated in later stages of the disease. Transforming growth factor ß, connective tissue growth factor, and platelet-derived growth factor α should be considered as key players in the initial events of XHLN. Clusterin and TIMP1 appear to be more associated with the progression rather than initiation of tubulointerstitial damage in chronic renal disease.

Correlation of Urine and Serum Biomarkers with Renal Damage and Survival in Dogs with Naturally Occurring Proteinuric Chronic Kidney Disease.

BACKGROUND: Urine protein loss is common in dogs with chronic kidney disease (CKD). HYPOTHESIS/OBJECTIVES: To evaluate new biomarkers of glomerular and tubulointerstitial (TI) damage compared with histology and as survival indicators in dogs with naturally occurring, proteinuric CKD. ANIMALS: One hunderd and eighty dogs with naturally occurring kidney disease. METHODS: Retrospective study using urine, serum, and renal biopsies from dogs with kidney disease, 91% of which had proteinuric CKD. Biomarkers were evaluated and correlated with pathologic renal damage, and significant associations, sensitivities, and specificities of biomarkers for renal disease type were determined. RESULTS: Fractional excretions of immunogloblin M (IgM_FE) and immunoglobulin G (IgG_FE) correlated most strongly with glomerular damage based on light microscopy (r = 0.58 and 0.56, respectively; P < .01). Serum creatinine (SCr) correlated most strongly with TI damage (r = 0.70, P < .01). Urine IgM/creatinine and urine NAG/creatinine had the highest sensitivity (75%) and specificity (78%) for detection of immune complex-mediated glomerulonephritis. Although individually most biomarkers were significantly associated with decreased survival time (P < .05), in a multivariate analysis, SCr, IgM_FE, and glomerular damage based on transmission electron microscopy (TEM) were the only biomarkers significantly associated with survival time (SCr: P = .001; IgM_FE: P = .008; TEM: P = .017). CONCLUSIONS AND CLINICAL IMPORTANCE: Novel urine biomarkers and FEs are useful for detection of glomerular and TI damage in dogs with proteinuric CKD and might predict specific disease types and survival.

World Small Animal Veterinary Association Renal Pathology Initiative: Classification of Glomerular Diseases in Dogs.

Evaluation of canine renal biopsy tissue has generally relied on light microscopic (LM) evaluation of hematoxylin and eosin-stained sections ranging in thickness from 3 to 5 µm. Advanced modalities, such as transmission electron microscopy (TEM) and immunofluorescence (IF), have been used sporadically or retrospectively. Diagnostic algorithms of glomerular diseases have been extrapolated from the World Health Organization classification scheme for human glomerular disease. With the recent establishment of 2 veterinary nephropathology services that evaluate 3-µm sections with a panel of histochemical stains and routinely perform TEM and IF, a standardized objective species-specific approach for the diagnosis of canine glomerular disease was needed. Eight veterinary pathologists evaluated 114 parameters (lesions) in renal biopsy specimens from 89 dogs. Hierarchical cluster analysis of the data revealed 2 large categories of glomerular disease based on the presence or absence of immune complex deposition: The immune complex-mediated glomerulonephritis (ICGN) category included cases with histologic lesions of membranoproliferative or membranous patterns. The second category included control dogs and dogs with non-ICGN (glomerular amyloidosis or focal segmental glomerulosclerosis). Cluster analysis performed on only the LM parameters led to misdiagnosis of 22 of the 89 cases-that is, ICGN cases moved to the non-ICGN branch of the dendrogram or vice versa, thereby emphasizing the importance of advanced diagnostic modalities in the evaluation of canine glomerular disease. Salient LM, TEM, and IF features for each pattern of disease were identified, and a preliminary investigation of related clinicopathologic data was performed.

Symmetric Dimethylarginine Assay Validation, Stability, and Evaluation as a Marker for the Early Detection of Chronic Kidney Disease in Dogs.

BACKGROUND: Symmetric dimethylarginine (SDMA) is a small molecule formed by methylation of arginine, and released into blood during protein degradation. SDMA is primarily eliminated by renal excretion and is a promising endogenous marker of glomerular filtration rate (GFR). OBJECTIVES: To validate an assay for SDMA measurement, determine stability of SDMA in blood, and compare SDMA with serum creatinine concentration (sCr) and GFR for early detection of decreasing kidney function in dogs with chronic kidney disease (CKD). ANIMALS: Eight male dogs affected with X-linked hereditary nephropathy and 4 unaffected male littermates. METHODS: Prospective study validating SDMA measurement using liquid chromatography-mass spectrometry, assessing stability of SDMA in serum and plasma, and serially determining sCr, SDMA, and GFR (using iohexol clearance) in dogs during progression from preclinical disease to end-stage renal failure. Correlations were determined using linear regression. Timepoints at which sCr, SDMA, and GFR identified decreased renal function were compared using defined cutoffs, trending in an individual dog, and comparison with unaffected littermates. RESULTS: Symmetric dimethylarginine was highly stable in serum and plasma, and the assay demonstrated excellent analytical performance. In unaffected dogs, SDMA remained unchanged whereas in affected dogs, SDMA increased during disease progression, correlating strongly with an increase in sCr (r = 0.95) and decrease in GFR (r = -0.95). Although trending improved sCr's sensitivity, SDMA identified, on average, <20% decrease in GFR, which was earlier than sCr using any comparison method. CONCLUSIONS AND CLINICAL IMPORTANCE: Symmetric dimethylarginine is useful for both early identification and monitoring of decreased renal function in dogs with CKD.

The incidence rate of colectomy for medically refractory ulcerative colitis has declined in parallel with increasing anti-TNF use: a time-trend study.

BACKGROUND: Medical therapy is standard treatment for ulcerative colitis with colectomy reserved for medically refractory disease or malignancy. The introductions of ciclosporin in 1994 and anti-TNF therapy in 2005 have extended medical management options. AIM: To determine whether the colectomy incidence rate for medically refractory ulcerative colitis has changed since the introduction of anti-TNF therapy. METHODS: Adult patients with a diagnosis of ulcerative colitis and who subsequently underwent an urgent or elective colectomy for medically refractory disease in Edmonton, Canada between 1 January 1998 and 31 December 2011 were identified. Log-linear regression was used to estimate the annual percent change in the total colectomy incidence rate (urgent and elective combined) and the urgent and elective incidence rates individually, before and after 2005, the year infliximab was approved for use in ulcerative colitis. Temporal trends of drug utilisation in this study population were also described. RESULTS: During 1998-2011, 481 patients with ulcerative colitis underwent a colectomy for medically refractory disease. There was negligible change in the total colectomy incidence rate from 1998 to 2005, with an annual percent change of 4.4% (95% confidence interval (CI): -1.12% to 10.16%). From 2005-2011, following the approval and increasing use of anti-TNF therapy, the total colectomy incidence rate decreased by 16.1% (95% CI: -21.32% to -10.54%) every year to 0.9 per 100 ulcerative colitis patients in 2011. CONCLUSION: The total incidence rate of colectomy for medically refractory ulcerative colitis has declined substantially since 2005, paralleling the increased use of anti-TNF therapy in this patient population.

Epithelial mesenchymal transition in the progression of renal disease in dogs.

Chronic kidney disease (CKD) in dogs is the final common pathway resulting from persistent renal injury and is characterized by progressive tubulointerstitial damage (TID). Pathogenesis of CKD is divided into an initial inflammatory phase with a predominantly mononuclear infiltrate followed by a fibrotic phase with increased numbers of fibroblasts and extracellular matrix deposition that causes a progressive reduction of functional parenchyma. Proteinuria is a common manifestation of renal diseases in dogs, and its role in the pathogenesis of CKD is still uncertain. Nevertheless, the degree of proteinuria in dogs correlates with TID progression. Increased protein filtration may have direct effects on tubular epithelial cells (TECs) that induce them to express the major histocompatibility complex type II, and thereby contribute to lymphocyte recruitment. Thus, an active pro-inflammatory role is proposed for TECs in TID progression. Moreover TECs are believed to actively participate in the mechanisms of renal fibrosis. Epithelial-Mesenchymal-Transition (EMT) of TECs in canine TID has been studied in the last decade. Down-regulation of adhesion molecules and loss of epithelial markers in TECs directly correlate with the severity of TID and with de novo expression of mesenchymal markers. Tubular basement membrane (TBM) disruption is an early EMT event. Increased activity of Matrix Metalloproteinase-2 and its co-localization with TBM splitting suggests an active role for the enzyme in inducing EMT. Processes occurring in canine CKD share many similarities with its human counterpart, making the dog a good model in which to examine the mechanisms of TID progression.

The trans-sphincteric posterior sagittal repair of recto-urinary and recto-vaginal fistulae using Surgisis™ mesh and fibrin sealant.

Recto-urinary, recto-vaginal and ileo-anal pouch-associated fistulae are rare yet a significant clinical problem due to their profound impact on patients' quality of life and are a challenge to repair. In this report, we describe repair of these complex fistulae using a modified trans-sphincteric posterior sagittal approach with Surgisis™ mesh and fibrin sealant and review our repair outcomes.

De novo expression of human leukocyte antigen-DR (HLA-DR) and loss of beta-catenin expression in tubular epithelial cells: a possible event in epithelial-mesenchymal transition in canine renal diseases.

Tubulointerstitial fibrosis (TIF) plays a central role in the progression to end-stage renal disease. Tubular epithelial cells (TECs) undergo epithelial-mesenchymal transition (EMT) and may contribute to the progression of TIF. Using immunohistochemistry, the primary aim of this study was to assess the expression of ß-catenin, human leukocyte antigen-DR (HLA-DR) and vimentin in renal biopsies from dogs with spontaneous kidney diseases of varying severities. Morphological diagnosis, severity of inflammation, TIF, HLA-DR expression and clinicopathological variables were compared in dogs with renal injury to identify any potential relationship between the different factors; ß-catenin down-regulation was used as a marker of EMT. Fibrosis, HLA-DR expression, serum creatinine concentration (SCr), and urine protein-to-creatinine ratio (UPC) were all increased and ß-catenin expression decreased in dogs with primary glomerular disease compared with dogs with acute tubular necrosis. HLA-DR expression by TECs was positively correlated to fibrosis, inflammation, UPC, and SCr. ß-catenin expression was negatively correlated to fibrosis, inflammation and HLA-DR expression. The progression of renal failure correlated closely with tubulointerstitial damage. De novo HLA-DR expression associated with ß-catenin down-regulation by TECs may represent a possible step in the progression of TIF and EMT.

Pathologic evaluation of canine renal biopsies: methods for identifying features that differentiate immune-mediated glomerulonephritides from other categories of glomerular diseases.

BACKGROUND: Human renal biopsies are routinely evaluated with light microscopy (LM) using a panel of histologic stains, transmission electron microscopy (TEM), and immunofluorescence (IF) microscopy to obtain a diagnosis. In contrast, the pathologic evaluation of glomerular disease in veterinary medicine has relied mostly on LM and was of limited utility. To address this problem, recently established veterinary renal diagnostic centers have adopted methods used in human nephropathology for evaluation of renal biopsies. Three broad categories of disease, which have the greatest implications for clinical management of proteinuric dogs, have been established and include amyloidosis, immune complex-mediated glomerulonephritis (ICGN), and non-ICGN. OBJECTIVE: To demonstrate histopathologic, ultrastructural, and IF findings in renal biopsy specimens that experienced veterinary nephropathologists utilize to make accurate and clinically useful diagnoses in dogs with proteinuric glomerular disease and to provide guidelines for the proper evaluation of renal biopsies. METHODS: Renal biopsy specimens were routinely examined by LM, IF, and TEM. Samples were reviewed by members of the World Small Animal Veterinary Association Renal Standardization Study Group to identify lesions that were diagnostic for, or suggestive of, the presence of immune complexes (IC) or amyloidosis in all modalities. Ten guidelines for renal biopsy evaluation were formulated. RESULTS: Each method of investigation contributed important findings that were integrated to make an accurate final morphological diagnosis. The guidelines were validated by an independent group of veterinary pathologists. CONCLUSIONS AND CLINICAL IMPORTANCE: Routine evaluation of renal biopsies with LM, TEM, and IF is feasible and necessary for making accurate, morphologic diagnoses that can be used to guide clinical management of dogs with glomerular disease.

Consensus recommendations for the diagnostic investigation of dogs with suspected glomerular disease.

BACKGROUND: The International Renal Interest Society (IRIS) offers guidelines for chronic kidney disease and acute kidney injury. As dogs with glomerular disease may present differently and require different treatment than those with whole nephron or tubular disease, the IRIS Canine Glomerulonephritis (GN) Study Group was convened to formulate guidelines for these cases. The Diagnosis Subgroup was asked to make recommendations for diagnostic evaluation of such cases. OBJECTIVE: To seek consensus among renal specialists for the evaluation of dogs with proteinuria because of suspected glomerular disease. METHODS: After reviewing the literature, subgroup members discussed and wrote the draft paper and recommendations, which members of the IRIS Canine GN Study Group voted upon by electronic secret ballot, with comments noted. Consensus was declared if votes showed strong or general agreement from 85% of the respondents. RESULTS: Diagnostic tests were categorized as essential, recommended, or potentially helpful, with prioritization dependent on case characteristics, eg, for cases with uncomplicated proteinuria versus complicated with hypoalbuminemia, azotemia, or both. Consensus was reached with 86-100% agreement on all questions posed. All cases should have basic examinations including blood pressure measurement, blood, and urine testing, and a search for infectious diseases relevant to their environs. The majority ranked imaging (chest radiographs, abdominal ultrasonogram) and renal biopsy procured and interpreted by experienced personnel as essential evaluations in complicated cases, but a few respondents deemed these to be essential in uncomplicated cases as well. CONCLUSIONS AND CLINICAL IMPORTANCE: Strong consensus about recommendations for diagnostic evaluation of dogs with suspected glomerular protein loss was attained. These guidelines help clinicians characterize disease processes for more informed therapeutic decision-making.

Prevalence of immune-complex glomerulonephritides in dogs biopsied for suspected glomerular disease: 501 cases (2007-2012).

BACKGROUND: Glomerulonephropathies are common causes of kidney disease in dogs. OBJECTIVE: To determine the prevalence of immune-complex glomerulonephritis (ICGN) in North American dogs biopsied for suspected glomerular disease. ANIMALS: Renal biopsies (n = 733) submitted to the Texas Veterinary Renal Pathology Service between January 1, 2007 and December 31, 2012 were reviewed. Dogs were included if the biopsy was performed for suspected glomerular disease. METHODS: Specimens were evaluated by light microscopy (LM), immunofluorescence (IF), and transmission electron microscopy (TEM). Findings were retrospectively evaluated to categorize the diagnosis for each case. For the diagnosis of ICGN, TEM findings were considered conclusive when LM and IF were equivocal. RESULTS: Of the 501 dogs included in the study, 241 (48.1%) had ICGN; 103 (20.6%) had primary glomerulosclerosis; 76 (15.2%) had amyloidosis; 45 (9.0%) had nonimmune complex (IC) glomerulopathy; 24 (4.8%) had non-IC nephropathy; and, 12 (2.4%) had primary tubulointerstitial disease. Many (66/241; 27.4%) ICGN cases required TEM for definitive diagnosis, including 14 cases (5.8%) that were not suspected on LM. Of cases not diagnosed as ICGN, a substantial proportion (60/260; 23.1%) required TEM to rule out immune complex deposits, including 14 of 189 cases (7.4%) presumptively diagnosed as ICGN on LM. CONCLUSIONS AND CLINICAL IMPORTANCE: Approximately half of all dogs biopsied for suspected glomerular disease had conditions other than ICGN. Renal biopsy is needed to accurately categorize the underlying disease and direct appropriate treatment. Additionally, TEM and IF evaluations by experienced nephropathologists are necessary to obtain an accurate diagnosis in many cases.

Urinary biomarkers of renal disease in dogs with X-linked hereditary nephropathy.

BACKGROUND: Sensitive and specific biomarkers for early tubulointerstitial injury are lacking. HYPOTHESIS: The excretion of certain urinary proteins will correlate with the state of renal injury in dogs with chronic kidney disease. ANIMALS: Twenty-five male colony dogs affected with X-linked hereditary nephropathy (XLHN) and 19 unaffected male littermates were evaluated. METHODS: Retrospective analysis of urine samples collected every 2-4 weeks was performed. Urine proteins evaluated were retinol binding protein (uRBP/c), ß2-microglobulin (uB2M), N-acetyl-ß-D-glucosaminidase (uNAG/c), neutrophil gelatinase-associated lipocalin (uNGAL/c), and immunoglobulin G (uIgG/c). Results were correlated with serum creatinine concentration (sCr), glomerular filtration rate (GFR), urine protein : creatinine ratio, and histopathologic analysis of serial renal biopsies. Analytical validation was performed for all assays; uNAG stability was evaluated. RESULTS: All urinary biomarkers distinguished affected dogs from unaffected dogs early in their disease process, increasing during early and midstages of disease. uRBP/c correlated most strongly with conventional measures of disease severity, including increasing sCr (r = 0.89), decreasing GFR (r = -0.77), and interstitial fibrosis (r = 0.80), P < .001. However, multivariate analysis revealed age, sCr, uIgG/c, and uB2M, but not uRBP/c, as significant independent predictors of GFR (P < .05). CONCLUSIONS AND CLINICAL IMPORTANCE: All urinary biomarkers were elevated before sCr increased, but typically after proteinuria developed in dogs with progressive glomerular disease because of XLHN. uRBP/c measurement might be promising as a noninvasive tool for diagnosis and monitoring of tubular injury and dysfunction in dogs.

Characterization of the genetic basis for autosomal recessive hereditary nephropathy in the English Springer Spaniel.

BACKGROUND: Autosomal recessive hereditary nephropathy (ARHN) was diagnosed in 2 English Springer Spaniels (ESS), a breed not previously reported to be affected by hereditary nephropathy (HN). OBJECTIVE: To identify and characterize the genetic cause of ARHN in ESS. ANIMALS: Sixty-three ESS (2 with ARHN, 2 obligate carriers, and 59 others), 2 mixed-breed dogs with X-linked HN, and 2 English Cocker Spaniels (ECS) with ARHN were included. METHODS: ARHN was diagnosed based on transmission electron microscopy and immunostaining of kidney. DNA from affected dogs was screened for the mutation known to cause ARHN in ECS. Quantities of COL4A3, COL4A4, and COL4A5 mRNA transcripts in renal cortex were determined using quantitative reverse transcription-polymerase chain reaction (qRT-PCR) for ARHN-affected dogs and 7 other dogs. The coding regions of COL4A3 and COL4A4 were sequenced for the 2 ARHN-affected ESS and an unaffected dog. Exon 30 of COL4A4 was sequenced for all 63 ESS. RESULTS: qRT-PCR indicated a significant reduction in transcript levels of both COL4A3 and COL4A4 mRNA in the kidney of ARHN-affected ESS. Sequencing identified a single nucleotide substitution in COL4A4 at base 2806 resulting in a premature stop codon. Thirteen of 25 related dogs were identified as carriers. CONCLUSIONS AND CLINICAL IMPORTANCE: A mutation highly likely to cause ARHN in ESS has been identified.

The histamine H4 receptor is functionally expressed on neurons in the mammalian CNS.

BACKGROUND AND PURPOSE: The histamine H4 receptor is the most recently identified of the G protein-coupled histamine receptor family and binds several neuroactive drugs, including amitriptyline and clozapine. So far, H4 receptors have been found only on haematopoietic cells, highlighting its importance in inflammatory conditions. Here we investigated the possibility that H4 receptors may be expressed in both the human and mouse CNS. METHODS: Immunological and pharmacological studies were performed using a novel anti-H4 receptor antibody in both human and mouse brains, and electrophysiological techniques in the mouse brain respectively. Pharmacological tools, selective for the H4 receptor and patch clamp electrophysiology, were utilized to confirm functional properties of the H4 receptor in layer IV of the mouse somatosensory cortex. RESULTS: Histamine H4 receptors were prominently expressed in distinct deep laminae, particularly layer VI, in the human cortex, and mouse thalamus, hippocampal CA4 stratum lucidum and layer IV of the cerebral cortex. In layer IV of the mouse somatosensory cortex, the H4 receptor agonist 4-methyl histamine (20 micromol x L(-1)) directly hyperpolarized neurons, an effect that was blocked by the selective H4 receptor antagonist JNJ 10191584, and promoted outwardly rectifying currents in these cells. Monosynaptic thalamocortical CNQX-sensitive excitatory postsynaptic potentials were not altered by 4-methyl histamine (20 micromol x L(-1)) suggesting that H4 receptors did not act as hetero-receptors on thalamocortical glutamatergic terminals. CONCLUSIONS AND IMPLICATIONS: This is the first demonstration that histamine H4 receptors are functionally expressed on neurons, which has major implications for the therapeutic potential of these receptors in neurology and psychiatry.

A new model for the study of high-K(+)-induced preconditioning in cultured neurones: role of N-methyl-d-aspartate and alpha7-nicotinic acetylcholine receptors.

Spreading depression (SD), whether elicited by local application of high K(+) medium to the cortical surface or by other stimuli, can increase the brain's tolerance to a subsequent, severe ischaemic insult in vivo, a phenomenon termed preconditioning. Herein, we have developed and validated a robust in vitro protocol for high-K(+)-preconditioning of cultured neurones. This new model is especially appropriate to unravel the molecular mechanisms underlying neuronal preconditioning and subsequent ischaemic tolerance. With this new, optimised preparation, preconditioning was found to be dependent upon culture day in vitro, cell density, K(+) concentration and duration of treatment. Finally, preconditioning was shown to be dependent upon N-methyl-d-aspartate (NMDA), CAM-kinase II signalling and alpha7-nicotinic (alpha7 nACh) receptor function, which is analogous to in vivo preconditioning induced by various stimuli.

X chromosome inactivation patterns in normal and X-linked hereditary nephropathy carrier dogs.

Alport syndrome (AS) and hereditary nephropathy (HN) are glomerular nephropathies caused by mutations in the genes encoding the type IV collagens. In a mixed breed of dog, termed Navasota (NAV) dogs, X-linked hereditary nephropathy (XLHN) is caused by a 10-bp deletion in exon 9 of COL4A5. Males harboring this mutation succumb to end-stage renal disease before 18 months of age. In contrast, female carriers of this disease survive much longer, most have a normal life-span, and vary in disease progression as compared with XLHN-affected males. X chromosome inactivation (XCI) patterns have been studied in human X-linked AS carriers and some have been shown to have a high degree of skewed XCI. However, similar studies have never been reported in an animal model of this disease. Therefore, patterns of XCI were examined in XLHN-carrier NAV dogs. The variation in XCI among the 26 XLHN-carrier and seven normal female NAV dogs studied was low and only three were found to preferentially inactivate one X chromosome, all of which were XLHN-carriers. The average skewedness among all dogs was 59% and 57% among the XLHN-carriers. No significant difference in XCI was found between the two groups (P = 0.477). It is clear from these data that genotype does not seem to have an effect on inactivation; the majority of these dogs have random patterns of XCI. Highly skewed X chromosome inactivation also appears to be random, given that no difference was observed between the XLHN-carriers and normal females. Because of the apparent rarity of skewed XCI, these dogs may not be a suitable model for studying a potential correlation between this phenomenon and disease progression.