Unique Capabilities of Genome Sequencing for Rare Disease Diagnosis

Background

Causal variants underlying rare disorders may remain elusive even after expansive gene panels or exome sequencing (ES). Clinicians and researchers may then turn to genome sequencing (GS), though the added value of this technique and its optimal use remain poorly defined. We therefore investigated the advantages of GS within a phenotypically diverse cohort.

Methods

GS was performed for 744 individuals with rare disease who were genetically undiagnosed. Analysis included review of single nucleotide, indel, structural, and mitochondrial variants.

Results

We successfully solved 218/744 (29.3%) cases using GS, with most solves involving established disease genes (157/218, 72.0%). Of all solved cases, 148 (67.9%) had previously had non-diagnostic ES. We systematically evaluated the 218 causal variants for features requiring GS to identify and 61/218 (28.0%) met these criteria, representing 8.2% of the entire cohort. These included small structural variants (13), copy neutral inversions and complex rearrangements (8), tandem repeat expansions (6), deep intronic variants (15), and coding variants that may be more easily found using GS related to uniformity of coverage (19).

Conclusion

We describe the diagnostic yield of GS in a large and diverse cohort, illustrating several types of pathogenic variation eluding ES or other techniques. Our results reveal a higher diagnostic yield of GS, supporting the utility of a genome-first approach, with consideration of GS as a secondary or tertiary test when higher-resolution structural variant analysis is needed or there is a strong clinical suspicion for a condition and prior targeted genetic testing has been negative.

Segmental Uniparental Isodisomy Causing an "Inside-to-Outside" Limb-Girdle Muscular Dystrophy Due to a Homozygous Mutation in POGLUT1

This case report describes segmental uniparental isodisomy causing an “inside-to-outside” limb-girdle muscular dystrophy due to a homozygous mutation in POGLUT1.

RFC1 in an Australasian neurological disease cohort: extending the genetic heterogeneity and implications for diagnostics

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome is a progressive, generally late-onset, neurological disorder associated with biallelic pentanucleotide expansions in Intron 2 of the RFC1 gene. The locus exhibits substantial genetic variability, with multiple pathogenic and benign pentanucleotide repeat alleles previously identified. To determine the contribution of pathogenic RFC1 expansions to neurological disease within an Australasian cohort and further investigate the heterogeneity exhibited at the locus, a combination of flanking and repeat-primed PCR was used to screen a cohort of 242 Australasian patients with neurological disease. Patients whose data indicated large gaps within expanded alleles following repeat-primed PCR, underwent targeted long-read sequencing to identify novel repeat motifs at the locus. To increase diagnostic yield, additional probes at the RFC1 repeat region were incorporated into the PathWest diagnostic laboratory targeted neurological disease gene panel to enable first-pass screening of the locus for all samples tested on the panel. Within the Australasian cohort, we detected known pathogenic biallelic expansions in 15.3% (n = 37) of patients. Thirty indicated biallelic AAGGG expansions, two had biallelic 'Māori alleles' [(AAAGG)exp(AAGGG)exp], two samples were compound heterozygous for the Māori allele and an AAGGG expansion, two samples had biallelic ACAGG expansions and one sample was compound heterozygous for the ACAGG and AAGGG expansions. Forty-five samples tested indicated the presence of biallelic expansions not known to be pathogenic. A large proportion (84%) showed complex interrupted patterns following repeat-primed PCR, suggesting that these expansions are likely to be comprised of more than one repeat motif, including previously unknown repeats. Using targeted long-read sequencing, we identified three novel repeat motifs in expanded alleles. Here, we also show that short-read sequencing can be used to reliably screen for the presence or absence of biallelic RFC1 expansions in all samples tested using the PathWest targeted neurological disease gene panel. Our results show that RFC1 pathogenic expansions make a substantial contribution to neurological disease in the Australasian population and further extend the heterogeneity of the locus. To accommodate the increased complexity, we outline a multi-step workflow utilizing both targeted short- and long-read sequencing to achieve a definitive genotype and provide accurate diagnoses for patients.

Expanding the phenotype of DNMT3A as a cause a congenital myopathy with rhabdomyolysis

Pathogenic variants in DNMT3A are most commonly associated with Tatton-Brown-Rahman Syndrome (TBRS), but includes other phenotypes such as Heyn-Sproul-Jackson syndrome and acute myeloid leukemia (AML). We describe a patient presenting to the neuromuscular clinic with a de novo missense variant in DNMT3A where the striking clinical feature is that of a congenital myopathy with associated episodes of rhabdomyolysis, severe myalgias and chest pain along with phenotypic features associated with TBRS. Muscle biopsy showed minor myopathic features and cardiac investigations revealed mildly impaired bi-ventricular systolic function. We confirmed the DNA methylation profile matched haplo-insufficient TBRS cases, consistent with a loss of methyltransferase activity. Our report emphasizes the phenotypic overlap of patients with syndromic disorders presenting to neuromuscular clinics and limitations of gene panels in establishing a molecular diagnosis.

Patient Care Standards for Primary Mitochondrial Disease in Australia. An Australian adaptation of the Mitochondrial Medicine Society recommendations

This document provides consensus‐based recommendations for general physicians and primary care physicians who diagnose and manage patients with mitochondrial diseases (MD). It builds on previous international guidelines, with particular emphasis on clinical management in the Australian setting. This statement was prepared by a working group of medical practitioners, nurses and allied health professionals with clinical expertise and experience in managing Australian patients with MD. As new treatments and management plans emerge, these consensus‐based recommendations will continue to evolve, but current standards of care are summarised in this document.

Progressive cranial neuropathy and uterine involvement in myeloid sarcoma

A rare extramedullary manifestation of haematological malignancy, myeloid sarcoma is most commonly seen in patients with acute myeloid leukaemia. We report on an adult patient who presented with an atypical phenotype of progressive cranial neuropathy without blood or bone marrow involvement, and in whom obtaining material for pathological diagnosis was made challenging by unusual findings of absent fluorodeoxyglucose-positron emission tomography avidity and involvement of sites not readily accessible to biopsy (orbital apex and cauda equina). The eventual diagnosis was obtained through biopsy of the uterine cervix before being verified on repeat lymph node and cerebrospinal fluid sampling prior to initiation of chemotherapy.

Pathogenic Abnormal Splicing Due to Intronic Deletions that Induce Biophysical Space Constraint for Spliceosome Assembly

A precise genetic diagnosis is the single most important step for families with genetic disorders to enable personalized and preventative medicine. In addition to genetic variants in coding regions (exons) that can change a protein sequence, abnormal pre-mRNA splicing can be devastating for the encoded protein, inducing a frameshift or in-frame deletion/insertion of multiple residues. Non-coding variants that disrupt splicing are extremely challenging to identify. Stemming from an initial clinical discovery in two index Australian families, we define 25 families with genetic disorders caused by a class of pathogenic non-coding splice variant due to intronic deletions. These pathogenic intronic deletions spare all consensus splice motifs, though they critically shorten the minimal distance between the 5' splice-site (5'SS) and branchpoint. The mechanistic basis for abnormal splicing is due to biophysical constraint precluding U1/U2 spliceosome assembly, which stalls in A-complexes (that bridge the 5'SS and branchpoint). Substitution of deleted nucleotides with non-specific sequences restores spliceosome assembly and normal splicing, arguing against loss of an intronic element as the primary causal basis. Incremental lengthening of 5'SS-branchpoint length in our index EMD case subject defines 45-47 nt as the critical elongation enabling (inefficient) spliceosome assembly for EMD intron 5. The 5'SS-branchpoint space constraint mechanism, not currently factored by genomic informatics pipelines, is relevant to diagnosis and precision medicine across the breadth of Mendelian disorders and cancer genomics.

Congenital Titinopathy: Comprehensive characterization and pathogenic insights

OBJECTIVE

Comprehensive clinical characterization of congenital titinopathy to facilitate diagnosis and management of this important emerging disorder.

METHODS

Using massively parallel sequencing we identified 30 patients from 27 families with 2 pathogenic nonsense, frameshift and/or splice site TTN mutations in trans. We then undertook a detailed analysis of the clinical, histopathological and imaging features of these patients.

RESULTS

All patients had prenatal or early onset hypotonia and/or congenital contractures. None had ophthalmoplegia. Scoliosis and respiratory insufficiency typically developed early and progressed rapidly, whereas limb weakness was often slowly progressive, and usually did not prevent independent walking. Cardiac involvement was present in 46% of patients. Relatives of 2 patients had dilated cardiomyopathy. Creatine kinase levels were normal to moderately elevated. Increased fiber size variation, internalized nuclei and cores were common histopathological abnormalities. Cap-like regions, whorled or ring fibers, and mitochondrial accumulations were also observed. Muscle magnetic resonance imaging showed gluteal, hamstring and calf muscle involvement. Western blot analysis showed a near-normal sized titin protein in all samples. The presence of 2 mutations predicted to impact both N2BA and N2B cardiac isoforms appeared to be associated with greatest risk of cardiac involvement. One-third of patients had 1 mutation predicted to impact exons present in fetal skeletal muscle, but not included within the mature skeletal muscle isoform transcript. This strongly suggests developmental isoforms are involved in the pathogenesis of this congenital/early onset disorder.

INTERPRETATION

This detailed clinical reference dataset will greatly facilitate diagnostic confirmation and management of patients, and has provided important insights into disease pathogenesis. Ann Neurol 2018;83:1105-1124.

Genetic mimics of cerebral palsy

The term "cerebral palsy mimic" is used to describe a number of neurogenetic disorders that may present with motor symptoms in early childhood, resulting in a misdiagnosis of cerebral palsy. Cerebral palsy describes a heterogeneous group of neurodevelopmental disorders characterized by onset in infancy or early childhood of motor symptoms (including hypotonia, spasticity, dystonia, and chorea), often accompanied by developmental delay. The primary etiology of a cerebral palsy syndrome should always be identified if possible. This is particularly important in the case of genetic or metabolic disorders that have specific disease-modifying treatment. In this article, we discuss clinical features that should alert the clinician to the possibility of a cerebral palsy mimic, provide a practical framework for selecting and interpreting neuroimaging, biochemical, and genetic investigations, and highlight selected conditions that may present with predominant spasticity, dystonia/chorea, and ataxia. Making a precise diagnosis of a genetic disorder has important implications for treatment, and for advising the family regarding prognosis and genetic counseling. © 2019 International Parkinson and Movement Disorder Society.

Targeted Re-Sequencing Emulsion PCR Panel for Myopathies: Results in 94 Cases

BACKGROUND

Molecular diagnostics in the genetic myopathies often requires testing of the largest and most complex transcript units in the human genome (DMD, TTN, NEB). Iteratively targeting single genes for sequencing has traditionally entailed high costs and long turnaround times. Exome sequencing has begun to supplant single targeted genes, but there are concerns regarding coverage and needed depth of the very large and complex genes that frequently cause myopathies.

OBJECTIVE

To evaluate efficiency of next-generation sequencing technologies to provide molecular diagnostics for patients with previously undiagnosed myopathies.

METHODS

We tested a targeted re-sequencing approach, using a 45 gene emulsion PCR myopathy panel, with subsequent sequencing on the Illumina platform in 94 undiagnosed patients. We compared the targeted re-sequencing approach to exome sequencing for 10 of these patients studied.

RESULTS

We detected likely pathogenic mutations in 33 out of 94 patients with a molecular diagnostic rate of approximately 35%. The remaining patients showed variants of unknown significance (35/94 patients) or no mutations detected in the 45 genes tested (26/94 patients). Mutation detection rates for targeted re-sequencing vs. whole exome were similar in both methods; however exome sequencing showed better distribution of reads and fewer exon dropouts.

CONCLUSIONS

Given that costs of highly parallel re-sequencing and whole exome sequencing are similar, and that exome sequencing now takes considerably less laboratory processing time than targeted re-sequencing, we recommend exome sequencing as the standard approach for molecular diagnostics of myopathies.

Improving genetic diagnosis in Mendelian disease with transcriptome sequencing

Exome and whole-genome sequencing are becoming increasingly routine approaches in Mendelian disease diagnosis. Despite their success, the current diagnostic rate for genomic analyses across a variety of rare diseases is approximately 25 to 50%. We explore the utility of transcriptome sequencing [RNA sequencing (RNA-seq)] as a complementary diagnostic tool in a cohort of 50 patients with genetically undiagnosed rare muscle disorders. We describe an integrated approach to analyze patient muscle RNA-seq, leveraging an analysis framework focused on the detection of transcript-level changes that are unique to the patient compared to more than 180 control skeletal muscle samples. We demonstrate the power of RNA-seq to validate candidate splice-disrupting mutations and to identify splice-altering variants in both exonic and deep intronic regions, yielding an overall diagnosis rate of 35%. We also report the discovery of a highly recurrent de novo intronic mutation in COL6A1 that results in a dominantly acting splice-gain event, disrupting the critical glycine repeat motif of the triple helical domain. We identify this pathogenic variant in a total of 27 genetically unsolved patients in an external collagen VI-like dystrophy cohort, thus explaining approximately 25% of patients clinically suggestive of having collagen VI dystrophy in whom prior genetic analysis is negative. Overall, this study represents a large systematic application of transcriptome sequencing to rare disease diagnosis and highlights its utility for the detection and interpretation of variants missed by current standard diagnostic approaches.

Diagnosis and etiology of congenital muscular dystrophy: We are halfway there

OBJECTIVE

To evaluate the diagnostic outcomes in a large cohort of congenital muscular dystrophy (CMD) patients using traditional and next generation sequencing (NGS) technologies.

METHODS

A total of 123 CMD patients were investigated using the traditional approaches of histology, immunohistochemical analysis of muscle biopsy, and candidate gene sequencing. Undiagnosed patients available for further testing were investigated using NGS.

RESULTS

Muscle biopsy and immunohistochemical analysis found deficiencies of laminin α2, α-dystroglycan, or collagen VI in 50% of patients. Candidate gene sequencing and chromosomal microarray established a genetic diagnosis in 32% (39 of 123). Of 85 patients presenting in the past 20 years, 28 of 51 who lacked a confirmed genetic diagnosis (55%) consented to NGS studies, leading to confirmed diagnoses in a further 11 patients. Using the combination of approaches, a confirmed genetic diagnosis was achieved in 51% (43 of 85). The diagnoses within the cohort were heterogeneous. Forty-five of 59 probands with confirmed or probable diagnoses had variants in genes known to cause CMD (76%), and 11 of 59 (19%) had variants in genes associated with congenital myopathies, reflecting overlapping features of these conditions. One patient had a congenital myasthenic syndrome, and 2 had microdeletions. Within the cohort, 5 patients had variants in novel (PIGY and GMPPB) or recently published genes (GFPT1 and MICU1), and 7 had variants in TTN or RYR1, large genes that are technically difficult to Sanger sequence.

INTERPRETATION

These data support NGS as a first-line tool for genetic evaluation of patients with a clinical phenotype suggestive of CMD, with muscle biopsy reserved as a second-tier investigation. Ann Neurol 2016;80:101-111.

Use of Whole-Exome Sequencing for Diagnosis of Limb-Girdle Muscular Dystrophy: Outcomes and Lessons Learned

IMPORTANCE

To our knowledge, the efficacy of transferring next-generation sequencing from a research setting to neuromuscular clinics has never been evaluated.

OBJECTIVE

To translate whole-exome sequencing (WES) to clinical practice for the genetic diagnosis of a large cohort of patients with limb-girdle muscular dystrophy (LGMD) for whom protein-based analyses and targeted Sanger sequencing failed to identify the genetic cause of their disorder.

DESIGN, SETTING, AND PARTICIPANTS

We performed WES on 60 families with LGMDs (100 exomes). Data analysis was performed between January 6 and December 19, 2014, using the xBrowse bioinformatics interface (Broad Institute). Patients with LGMD were ascertained retrospectively through the Institute for Neuroscience and Muscle Research Biospecimen Bank between 2006 and 2014. Enrolled patients had been extensively investigated via protein studies and candidate gene sequencing and remained undiagnosed. Patients presented with more than 2 years of muscle weakness and with dystrophic or myopathic changes present in muscle biopsy specimens.

MAIN OUTCOMES AND MEASURES

The diagnostic rate of LGMD in Australia and the relative frequencies of the different LGMD subtypes. Our central goals were to improve the genetic diagnosis of LGMD, investigate whether the WES platform provides adequate coverage of known LGMD-related genes, and identify new LGMD-related genes.

RESULTS

With WES, we identified likely pathogenic mutations in known myopathy genes for 27 of 60 families. Twelve families had mutations in known LGMD-related genes. However, 15 families had variants in disease-related genes not typically associated with LGMD, highlighting the clinical overlap between LGMD and other myopathies. Common causes of phenotypic overlap were due to mutations in congenital muscular dystrophy-related genes (4 families) and collagen myopathy-related genes (4 families). Less common myopathies included metabolic myopathy (2 families), congenital myasthenic syndrome (DOK7), congenital myopathy (ACTA1), tubular aggregate myopathy (STIM1), myofibrillar myopathy (FLNC), and mutation of CHD7, usually associated with the CHARGE syndrome. Inclusion of family members increased the diagnostic efficacy of WES, with a diagnostic rate of 60% for "trios" (an affected proband with both parents) vs 40% for single probands. A follow-up screening of patients whose conditions were undiagnosed on a targeted neuromuscular disease-related gene panel did not improve our diagnostic yield.

CONCLUSIONS AND RELEVANCE

With WES, we achieved a diagnostic success rate of 45.0% in our difficult-to-diagnose cohort of patients with LGMD. We expand the clinical phenotypes associated with known myopathy genes, and we stress the importance of accurate clinical examination and histopathological results for interpretation of WES, with many diagnoses requiring follow-up review and ancillary investigations of biopsy specimens or serum samples.

Mutations in HSPB8 causing a new phenotype of distal myopathy and motor neuropathy

OBJECTIVE

To report novel disease and pathology due to HSPB8 mutations in 2 families with autosomal dominant distal neuromuscular disease showing both myofibrillar and rimmed vacuolar myopathy together with neurogenic changes.

METHODS

We performed whole-exome sequencing (WES) in tandem with linkage analysis and candidate gene approach as well as targeted next-generation sequencing (tNGS) to identify causative mutations in 2 families with dominant rimmed vacuolar myopathy and a motor neuropathy. Pathogenic variants and familial segregation were confirmed using Sanger sequencing.

RESULTS

WES and tNGS identified a heterozygous change in HSPB8 in both families: c.421A > G p.K141E in family 1 and c.151insC p.P173SfsX43 in family 2. Affected patients had a distal myopathy that showed myofibrillar aggregates and rimmed vacuoles combined with a clear neurogenic component both on biopsy and neurophysiologic studies. MRI of lower limb muscles demonstrated diffuse tissue changes early in the disease stage progressing later to fatty replacement typical of a myopathy.

CONCLUSION

We expand the understanding of disease mechanisms, tissue involvement, and phenotypic outcome of HSPB8 mutations. HSPB8 is part of the chaperone-assisted selective autophagy (CASA) complex previously only associated with Charcot-Marie-Tooth type 2L (OMIM 60673) and distal hereditary motor neuronopathy type IIa. However, we now demonstrate that patients can develop a myopathy with histologic features of myofibrillar myopathy with aggregates and rimmed vacuoles, similar to the pathology in myopathies due to gene defects in other compounds of the CASA complex such as BAG3 and DNAJB6 after developing the early neurogenic effects.

Expanding the phenotype of GMPPB mutations

Dystroglycanopathies are a heterogeneous group of diseases with a broad phenotypic spectrum ranging from severe disorders with congenital muscle weakness, eye and brain structural abnormalities and intellectual delay to adult-onset limb-girdle muscular dystrophies without mental retardation. Most frequently the disease onset is congenital or during childhood. The exception is FKRP mutations, in which adult onset is a common presentation. Here we report eight patients from five non-consanguineous families where next generation sequencing identified mutations in the GMPPB gene. Six patients presented as an adult or adolescent-onset limb-girdle muscular dystrophy, one presented with isolated episodes of rhabdomyolysis, and one as a congenital muscular dystrophy. This report expands the phenotypic spectrum of GMPPB mutations to include limb-girdle muscular dystrophies with adult onset with or without intellectual disability, or isolated rhabdomyolysis.

Muscle disorders: the latest investigations

Patients with muscle disorders can present a diagnostic challenge to physicians because of the different ways they can present and the large number of different underlying causes. Recognition of the 'myopathic phenotype' coupled with investigations usually including electrodiagnostic and histological investigations have been essential for diagnosing the underlying cause of a myopathy. Despite these standard investigations, some patients can remain undiagnosed. New tests including more specific antibody tests for immune-mediated myopathies and the introduction of next-generation sequencing promise to revolutionise diagnostic approaches for immune and inherited myopathies, but clinical expertise remains essential to choose the most appropriate tests and interpret the results. The aim of this review is to provide an overview of the different presentations to the neuromuscular clinic and the latest investigations that can be helpful in the diagnosis of muscle disorders.

Serological markers of autoimmunity in patients infected with hepatitis C virus: impact of HIV co-infection

OBJECTIVES

We sought to evaluate the prevalence, predictors and significance of autoantibody expression in patients with chronic hepatitis C (CHC) with or without HIV co-infection.

METHODS

Retrospective review of laboratory and histologic data for all patients with CHC who had a liver biopsy available. HIV status was documented in all patients. Results analyzed in SPSS10, Chicago, IL, a p value <0.05 was considered significant.

RESULTS

170 patients with hepatitis C viremia, including 107 (63%) HIV co-infection, who had testing for anti-nuclear antibody (ANA) or anti-smooth muscle antibody (ASMA) and anti-mitochondrial antibody (AMA) were included in the study. Overall, 63% (74/117) of patients were ASMA seropositive and 6% (9/153) were positive for ANA. All 117 patients tested for AMA were negative. HIV co-infected patients were significantly more likely to be ASMA positive 71% (53/75) compared to those with hepatitis C alone (50%) [P=0.026]. There were no significant differences in age, gender, race, risk group, alanine aminotransferase (ALT) levels or grade of inflammation on histology between autoantibody positive and negative patients. ASMA positive patients had significantly higher globulin levels (P=0.036) and a trend towards more bridging fibrosis or cirrhosis. Patients with autoantibody expression rarely had histologic features of AIH.

CONCLUSION

We found a high rate of ASMA seropositivity in our cohort of patients with chronic hepatitis C, and HIV co-infection was associated with significantly higher rates of ASMA expression. Autoantibody expression was not associated with demographic or clinical characteristics and does not necessarily preclude antiviral therapy.

Optimisation of the comet genotoxicity assay in freshly isolated murine hepatocytes: detection of strong in vitro DNA damaging properties for styrene

While the comet assay is used to detect DNA damage in isolated cells following exposure to chemicals in vitro, few publications report the use of the procedure in liver cells isolated from mice. Our initial efforts to use the assay to assess DNA damage in mouse hepatocytes maintained on collagen-coated dishes were hampered by high levels of baseline damage in controls, which appeared to result from mechanical damage sustained during the dislodgement of adherent cells in the early stages of the assay protocol. Here we describe an efficient version of the comet assay in cultured mouse hepatocytes that involves careful recovery of cells using a "scraping" buffer supplemented with 10% high purity grade DMSO. Use of this buffer strongly diminished the frequency of false positives. Using the industrial reagent styrene as a positive control in the optimised procedure, non-cytotoxic concentrations of this substance (2.5-10 mM) significantly increased mean comet tail length, area, and moment. Co-incubation with the CYP inhibitor SKF-525A strongly attenuated these effects of styrene. Collectively, these findings confirm this method is highly suitable for the detection of DNA damage by bioactivation-dependent compounds in freshly isolated mouse hepatocytes.

UDP-glucuronosyltransferase-dependent bioactivation of clofibric acid to a DNA-damaging intermediate in mouse hepatocytes

Glucuronidation of a number of carboxyl-containing drugs generates reactive acyl glucuronide metabolites. These electrophilic species alkylate cell proteins and may be implicated in the pathogenesis of a number of toxic syndromes seen in patients receiving the parent aglycones. Whether acyl glucuronides also attack nuclear DNA is unknown, although the acyl glucuronide formed from clofibric acid was recently found to decrease the transfection efficiency of phage DNA and generate strand breaks in plasmid DNA in vitro. To determine if such a DNA damage occurs within a cellular environment, the comet assay (i.e. single-cell gel electrophoresis) was used to detect DNA lesions in the nuclear genome of isolated mouse hepatocytes cultured with clofibric acid. Overnight exposure to 50 microM and higher concentrations of clofibric acid produced concentration-dependent increases in the comet areas of hepatocyte nuclei, with 1 mM clofibrate producing a 3.6-fold elevation over controls. These effects closely coincided with culture medium concentrations of the glucuronide metabolite formed from clofibric acid, 1-O-beta-clofibryl glucuronide. Consistent with a role for glucuronidation in the DNA damage observed, the glucuronidation inhibitor borneol diminished glucuronide formation from 100 microM clofibrate by 98% and returned comet areas to baseline levels. Collectively, these results suggest that the acyl glucuronide formed from clofibric acid is capable of migrating from its site of formation within the endoplasmic reticulum to generate strand nicks in nuclear DNA.