Evidence that dysregulated DNA mismatch repair characterizes human nonmelanoma skin cancer

BACKGROUND

In addition to an established role in the repair of postreplicative DNA errors, DNA mismatch repair (MMR) proteins also contribute to cellular responses to exogenous DNA damage. Previously, we have shown that Msh2-null mice display increased sensitivity to ultraviolet (UV) B-induced tumorigenesis, but squamous cell carcinomas (SCC) generated are microsatellite stable, suggesting a role for MMR other than postreplicative repair in UV-induced cutaneous tumour formation.

OBJECTIVES

We questioned whether there was evidence of MMR dysfunction in human SCC, thus validating the mouse models of MMR-dependent UVB-induced skin cancer.

METHODS

Using tissue microarrays we examined both nuclear and cytoplasmic levels of MMR proteins MSH2, MSH6, MSH3, MLH1 and PMS2 in more than 200 cases of cutaneous SCC and basal cell carcinoma (BCC).

RESULTS

We found that subsets of these 10 MMR protein measures were increased in nonmelanoma skin cancer (NMSC) compared with normal epidermal samples; this was particularly true of SCC. In fact, based on post hoc tests and MMR protein distribution patterns, BCC was distinct from SCC. With the exception of nuclear MSH2, the BCC had lower levels of identified MMR protein measures than SCC. We believe this to be important because not only is SCC more aggressive than BCC, but evidence suggests that these two NMSC subtypes arise through different molecular pathways.

CONCLUSIONS

In combination with previously established roles for MMR proteins in response to UVB-induced DNA damage, our data point towards an expanded perspective of the importance of MMR proteins in the suppression of UVB-induced tumorigenesis and, potentially, tumour behaviour.

Constitutive deficiency in DNA mismatch repair

Mutations in the DNA mismatch repair (MMR) genes are associated with the inheritance of hereditary non-polyposis colorectal cancer, also known as Lynch syndrome, a cancer syndrome with an average age at onset of 44. Individuals presenting with colorectal cancer are diagnosed with Lynch I, whereas individuals who present with extra-colonic tumors (such as endometrial, stomach, etc.) are identified as patients with Lynch syndrome II. Recently, 30 families have been reported with inheritance of biallelic mutations in the MMR genes. Here we summarize the phenotype of individuals with inheritance of homozygous or compound heterozygous mutations in the MMR genes that result in a complete lack of protein or greatly compromised protein function. In contrast to individuals with Lynch syndrome I and II, individuals with no MMR function present with childhood onset of hematological and brain malignancies, whereas residual MMR function can also result in gastrointestinal cancers and an age of onset in the second to fourth decade. Individuals with biallelic MMR mutations often present with café-au-lait spots, regardless of the level of MMR function remaining. Thus, the inheritance of two MMR gene mutations is a separate entity from Lynch I or II or the subtypes Turcot and Muir-Torre.

Constitutive deficiency in DNA mismatch repair: is it time for Lynch III?

Hereditary non-polyposis colorectal cancer (HNPCC), also known as Lynch syndrome types I and II, and the related subtypes Turcot and Muir-Torre syndrome, have all been associated with inheritance of germ line mutations in the DNA mismatch repair (MMR) genes. Fifty individuals have recently been identified with an early onset of a different spectrum of cancers associated with inheritance of two MMR mutations--resulting either in a constitutive loss of MMR function, or greatly impaired MMR function. In contrast to Lynch I and II individuals, individuals with inheritance of homozygous or compound heterozygous mutations in the MMR genes that result in a complete lack of protein, present with hematological and brain malignancies in the first decade of life. Biallelic mutations with compromised but residual protein function present with a broader spectrum of cancers (brain, hematological or gastrointestinal) in the second to fourth decades of life. We propose that inheritance of two MMR mutations in an individual and the unique tumor spectrum that occurs with an early onset should be defined separately from Lynch syndrome I and II, or the subtypes Turcot and Muir-Torre. We suggest Lynch III as an appropriate name for identifying individuals with constitutively compromised MMR associated with biallelic mutations.

Combined keratectomy, strontium-90 irradiation and permanent bulbar conjunctival grafts for corneolimbal squamous cell carcinomas in horses (1990-2002): 38 horses

OBJECTIVE

The purpose of this study was to evaluate the effectiveness of postoperative beta-irradiation with strontium-90 as an adjunctive treatment to superficial keratectomy and permanent bulbar conjunctival graft for removal of equine corneolimbal squamous cell carcinoma (SCC), in decreasing recurrence rate.

STUDY

The retrospective case study included 38 horses diagnosed and treated for SCC of the eye that involved the limbus and/or cornea. The patients were treated between 1990 and 2002, with strontium-90 irradiation immediately after corneal and conjunctival graft surgery. Recurrence was defined as the postoperative and postirradiation regrowth of SCC in the same site and globe that was previously treated.

RESULTS

The Appaloosa was the most commonly represented breed and horses that had more than one base coat color represented the majority of the cases (53%). The coat colors of white, chestnut/sorrel and gray were the most commonly represented colors of the horses treated. Eight horses (21%) could not be assessed for tumor recurrence due to lack of two or more post-treatment examinations, and another horse was enucleated 6 days postoperatively due to progressive corneal ulceration. Twenty-four horses (63% of the entire study population; 83% of the followed cases) had a mean +/- SD of 1754 +/- 1319 days without tumor recurrence, ranging from 14 days to 5110 days. Five horses (13% of the entire study population; 17% of the assessed horses) had tumor recurrence at a mean +/- SD of 449 +/- 339 days with a range of 29 days to 900 days. For the five recurrences, treatment included local excision (n = 1), enucleation (n = 2), and additional strontium-90 therapy (n = 3).

CONCLUSIONS

The combination of superficial keratectomy, beta-irradiation and permanent bulbar conjunctival grafts for limbal, corneal or corneolimbal SCC in horses is effective in at least 83% of the horses. Recurrence occurred in about 17% of the horses. Multiple biannual re-examinations are recommended to observe for tumor recurrence.

Msh2 deficiency leads to chromosomal abnormalities, centrosome amplification, and telomere capping defect

Msh2 is a key mammalian DNA mismatch repair (MMR) gene and mutations or deficiencies in mammalian Msh2 gene result in microsatellite instability (MSI+) and the development of cancer. Here, we report that primary mouse embryonic fibroblasts (MEFs) deficient in the murine MMR gene Msh2 (Msh2(-/-)) showed a significant increase in chromosome aneuploidy, centrosome amplification, and defective mitotic spindle organization and unequal chromosome segregation. Although Msh2(-/-) mouse tissues or primary MEFs had no apparent change in telomerase activity, telomere length, or recombination at telomeres, Msh2(-/-) MEFs showed an increase in chromosome end-to-end fusions or chromosome ends without detectable telomeric DNA. These data suggest that MSH2 helps to maintain genomic stability through the regulation of the centrosome and normal telomere capping in vivo and that defects in MMR can contribute to oncogenesis through multiple pathways.

Microsatellite instability in endometrioid endometrial carcinoma: correlation with clinically relevant pathologic variables

This study of 218 patients with endometrioid endometrial carcinoma explores the relationship between microsatellite instability (MSI) as established by the BAT26 method and the common pathologic variables of prognostic and therapeutic significance. MSI was positively correlated with grade, associated endometrial atrophy, squamous metaplasia, isthmic involvement, depth of myoinvasion, vascular invasion-associated changes, extrauterine tumor spread, and extramyometrial angiolymphatic spread. There was no significant correlation with carcinoma developing in adenomyosis, mucinous metaplasia, tumor size, cornual involvement, cervical extension, uterine serosal involvement, and targeted lymphoid response. The positive correlations are discussed in terms of molecular genetics.

Microsatellite instability in endometrioid endometrial carcinoma: correlation with clinically relevant pathologic variables

This study of 218 patients with endometrioid endometrial carcinoma explores the relationship between microsatellite instability (MSI) as established by the BAT26 method and the common pathologic variables of prognostic and therapeutic significance. MSI was positively correlated with grade, associated endometrial atrophy, squamous metaplasia, isthmic involvement, depth of myoinvasion, vascular invasion–associated changes, extrauterine tumor spread, and extramyometrial angiolymphatic spread. There was no significant correlation with carcinoma developing in adenomyosis, mucinous metaplasia, tumor size, cornual involvement, cervical extension, uterine serosal involvement, and targeted lymphoid response. The positive correlations are discussed in terms of molecular genetics.

Variable MLH1 promoter methylation patterns in endometrial carcinomas of endometrioid subtype lacking DNA mismatch repair

A lack of DNA mismatch repair (MMR) is observed in approximately 20% of sporadic endometrial tumors, but few of these tumors have mutations in MSH2 or MLH1, the two major MMR genes. Promoter methylation is an important means of silencing transcription, and methylation of the MLH1 promoter has been reported as an important cause of MLH1 inactivation in endometrial cancers. Studies have shown that specific CpG sites within the MLH1 gene promoter are critical for gene expression, but other studies have shown that methylation of both more proximal and more distal sequences are important for MLH1 gene regulation. Here, we used a microsatellite instability assay and MLH1 immunohistochemistry to identify a subset of endometrial carcinomas of the endometrioid subtype lacking MMR. Sequencing of bisulphite-treated DNA from these tumors determined the methylation status of 42 CpG sites across the MLH1 promoter (spanning -204 to -702 bp upstream of the transcriptional start). Unlike the 4 normal endometrial samples that were unmethylated, 17 of 21 MMR-deficient samples showed complete or near-complete methylation and the remaining 4 MMR-deficient samples had a considerable degree of methylation (approximately 50% or greater). Five tumors demonstrated isolated unmethylated CpG sites, despite methylation throughout the rest of the promoter. This underscores the importance of examining the methylation status of at least several CpG sites within the promoter as methylation is not always consistent across DNA. Overall, our findings support the model that density of methylation of CpG sites across the MLH1 promoter is important in determining gene expression.

Profiles of matrix metalloproteinase activity in equine tear fluid during corneal healing in 10 horses with ulcerative keratitis

OBJECTIVE

Levels of tear film matrix metalloproteinases (MMPs) activity are significantly elevated in horses with ulcerative keratitis and contribute to the excessive breakdown of stromal collagen. Changes in the amount of proteolytic activity in horse tear film during corneal healing and stromal remodeling have not yet been reported, but we hypothesize they should decrease. In the present study we analyzed serial tear fluid from horses with ulcerative keratitis to identify any changes in MMP activity during corneal healing and stromal remodeling.

PROCEDURES

Samples of tear fluid were obtained from both eyes of 10 horses with ulcerative keratitis on the day of admission (day 1) at the hospital and then at various time points until complete healing of the cornea. Tear film MMP2 and MMP9 activity was determined by quantitative gelatin zymography. In all cases medical treatment included topical applications of equine serum, antibiotics, atropine and systemic administration of anti-inflammatory drugs. Surgical procedures were performed in several cases on day 2 in addition to the medical treatment.

RESULTS

The mean total MMP activity (+/- SD) measured in relative standard units (RSU) in the tear fluid of the ulcerated eye (2.44 +/- 1.44) of the 10 horses was significantly higher than the mean in the contralateral eye (0.81 +/- 0.68) (P = 0.006), on the day of admission at the VMTH. The mean MMP activity in these ulcerated eyes significantly decreased (-82.4%) between the first day of admission and the day when the ulcer had completely healed (P = 0.0002). The activity level in the healed eye (0.43 +/- 0.17) was not significantly different to the one in the contralateral eye (0.36 +/- 0.18) on the day of complete corneal healing (P = 0.374). The level of MMP activity in the contralateral eye also decreased from 0.81 +/- 0.68-0.36 +/- 0.18 but this decrease (56%) was not significant (P = 0.069).

CONCLUSIONS

Ulcerative keratitis in horses is associated with initially high levels of tear film proteolytic activity that decrease as the ulcers heal. The success of medical and surgical treatment of the corneal ulcers is reflected by the enzyme activity in tears. In horses successful treatment does lead to a rapid reduction in tear film proteolytic activity that corresponded with the improvement in the clinical signs of corneal ulceration. Measurement of MMP activity in the tear film might represent a way to monitor the progression of corneal healing in horses with ulcerative keratitis.

Changes in antibiotic resistance in equine bacterial ulcerative keratitis (1991-2000): 65 horses

OBJECTIVE

To document changes in antibiotic resistance of organisms in cases of equine bacterial ulcerative keratitis over a 10-year time period.

DESIGN

A retrospective study.

PARTICIPANTS

Medical records of equine patients with bacterial ulcerative keratitis seen at the University of Florida's VMTH for the years 1991-2000 were reviewed.

MATERIALS AND METHODS

All cases of equine bacterial ulcerative keratitis for the above mentioned years were examined. Bacterial isolates were identified and subjected to Kirby-Bauer disc diffusion method sensitivity tests. Antibiotics used in the sensitivity tests included bacitracin, ampicillin, gentamicin, chloramphenicol, polymyxin B, trimethoprim-sulfa, neomycin, kanamycin, carbenicillin, tobramycin and enrofloxacin.

RESULTS

A total of 65 bacterial isolates were subjected to sensitivity testing. Of these isolates, Pseudomonas aeruginosa accounted for 14 of the bacterial isolates (22%), Streptococcus equi subspecies zooepidemicus accounted for 13 of the bacterial isolates (20%), and Staphylococcus aureus accounted for four of the isolates (6%). A statistically significant increase in resistance of Pseudomonas aeruginosa isolates to the antibiotics gentamicin and tobramycin was found between the isolates from 1992 to 1998 and those from 1999 to 2000. An increase in resistance of Streptococcus equi subspecies zooepidemicus to gentamicin was found between the isolates from 1993 to 1997 and those from 1998 to 2000.

CONCLUSIONS

Streptococcus equi subspecies zooepidemicus and Pseudomonas aeruginosa were the most common organisms isolated from cases of equine bacterial keratitis referred to the University of Florida's VMTH for the years 1991-2000. There appears to be an increase in resistance of Streptococcus equi subspecies zooepidemicus to gentamicin over the past 10 years. In addition, there is a significant increase in resistance of Pseudomonas aeruginosa to both gentamicin and tobramycin over the same time period.

Effect of topical tropicamide on tear production as measured by Schirmer's tear test in normal dogs and cats

OBJECTIVE

To evaluate the effect of a single dose of topical 1% tropicamide on tear production as measured by the Schirmer tear test (STT) in the normal dog and cat.

MATERIAL AND METHODS

Twenty-eight dogs and 32 cats received 50 micro l : l of 1% tropicamide in one eye and the opposite eye served as the control. STTs were performed immediately before instillation of tropicamide and then at 1, 4, 8 and 24 h post drug instillation. STT results were compared between the control and treated eyes at the different times.

RESULTS

Aqueous tear production in dogs, measured by STT, was not significantly reduced. The mean +/- SEM STTs for the baseline time for control and tropicamide-treated eyes were 19.9 +/- 0.8 and 20.3 +/- 0.8 mm wetting/min, respectively. For the control eyes, the subsequent mean +/- SEM STT levels were 20.3 +/- 0.9 (1 h), 21.1 +/- 0.8 (4 h), 20.1 +/- 0.9 (8 h), and 18.7 +/- 0.7 (24 h). For the tropicamide-treated eyes, the subsequent mean +/- SEM STT levels were 19.4 +/- 0.9 (1 h), 19.3 +/- 0.9 (4 h), 20.0 +/- 0.9 (8 h), and 18.4 +/- 0.8 (24 h). Aqueous tear production of both eyes was significantly reduced in cats at 1 h but returned to baseline by 4 h post tropicamide instillation. The mean +/- SEM STT levels for the baseline time in cats for control and tropicamide-treated eyes were 14.9 +/- 0.8 and 14.7 +/- 0.8 mm wetting/min, respectively. Subsequent mean +/- SEM STT levels for the control eyes were 6.4 +/- 1.1 (1 h), 11.9 +/- 1.0 (4 h), 13.9 +/- 0.8 (8 h), and 16.4 +/- 1.0 (24 h). For the tropicamide-treated eyes, the subsequent mean +/- SEM STT levels were 5.3 +/- 0.8 (1 h), 10.2 +/- 0.8 (4 h), 14.7 +/- 1.0 (8 h), and 16.6 +/- 1.0 (24 h).

CONCLUSION

Single dose 1% tropicamide does not significantly lower tear production rates, as measured by the STT, in normal dogs. However, in normal cats single doses of 1% tropicamide in one eye cause significant reductions in tear production of both eyes at 1 h that recovered to baseline levels by 4 h.

Connective tissue growth factor in tear film of the horse: detection, identification and origin

BACKGROUND

Healing of corneal ulcers in horses is often associated with profound corneal stromal fibrosis and scar formation resulting in visual impairment. Connective tissue growth factor (CTGF) is a fibrogenic cytokine involved in wound healing and scarring. The purpose of this study was to determine whether CTGF was present in the tear fluid of normal horse eyes and the eyes of horses with corneal ulcers in order to evaluate the role of CTGF in corneal wound healing and corneal scar formation.

METHODS

Tear fluid samples were collected from 65 eyes of 44 horses; 32 samples from normal eyes, 21 samples from eyes with corneal ulceration, and 12 samples from the unaffected contralateral eyes of horses with ulcers. CTGF levels in the tears were determined by enzyme immunoassay using goat IgG against human CTGF. Antigenetic similarity of human and horse CTGF was established in a bio-equivalence assay. The identity of horse CTGF was confirmed by western blot. Lacrimal and nictitating membrane glands were investigated by immunohistochemistry in the attempt to clarify the origin of tear fluid CTGF.

RESULTS

CTGF was detected in tear film of 23 normal unaffected eyes (72%) and 8 normal contralateral eyes (67%), with the mean CTGF levels (+/- SEM) being 51.5+/-19.2 and 13.4+/-3.9 ng/ml respectively. CTGF was found in 8 eyes with corneal ulcers (38%) with the mean CTGF concentration of 26.3+/-14.8 ng/ml. Western blot identified the protein detected as CTGF. The identification of CTGF in lacrimal glands suggests a major role of these glands in the presence of CTGF in tears.

CONCLUSIONS

CTGF is present in horse tear fluid and derives, at least partly, from the lacrimal gland. Equine CTGF has strong antigenic similarity with human CTGF. Corneal disease leads to a decrease of CTGF concentrations in tears. The possible role of CTGF in the healing process of ocular surface requires further investigation.

Corneal thickness and endothelial cell density measured by non-contact specular microscopy and pachymetry in Rhesus macaques (Macaca mulatta) with laser-induced ocular hypertension

PURPOSE

Sustained increase in intraocular pressure (IOP) in humans results in a loss of corneal endothelial cells and an increase of corneal thickness. The effects of chronically elevated IOP on the corneal endothelium of monkeys with laser-induced ocular hypertension, a commonly used animal model of human glaucoma have not been documented. This study examined the central corneal thickness (CCT), the corneal endothelial cell density (ECD), and the corneal endothelial cell size (ACS) in Rhesus monkeys with experimental ocular hypertension. Materials and methods. Ten male monkeys with argon laser-induced ocular hypertension in one eye for an average duration of 2.4+/-0.7 years, were sedated with ketamine hydrochloride, and the CCT, ECD, and ACS measured at the center of the cornea of both eyes with a Topcon SP-2000P non-contact specular microscope (Topcon America Corporation((R)), Paramus, NJ, USA). CCT was also measured using a DHG-500 Pachette ultrasonic pachymeter (DHG Technology Inc., Exton, PA, USA). Mean and standard deviation (S.D.) of CCT, ECD and ACS for each eye was calculated and statistically compared.Results. Mean CCT in the hypertensive and normal eyes measured by specular microscopy was 0.477+/-0.023mm and 0.468+/-0.020 mm, respectively. Mean ECD in the hypertensive and normal eyes was 2601.7+/-631.8 and 3990.2+/-402.9 cells mm(-2), respectively. The mean size of the endothelial cells was 252.4+/-23.9 micro m(2) in the normal eye and 408.7+/-115.0 microm m(2) in the hypertensive eye. No significant difference in the measurement of CCT was observed between the specular microscope and the pachymeter (p=0.46). No significant difference in the mean CCT was observed between the two eyes (p=0.4820), whereas the mean ECD was significantly lower in the hypertensive eye than in the normal eye (p<0.001). The ECD was inversely related to the length of IOP elevation (p<0.001).

CONCLUSIONS

No difference in the corneal thickness measurement was observed between the specular microscopy and the pachymetry techniques. Chronic ocular hypertension did not significantly affect the CCT, but caused a significant loss of endothelial cells in the center of the cornea of the laser treated eyes compared to the normotensive eyes. The duration of elevated IOP was the most important factor affecting the ECD.

DNA instability and human disease

It is now well established that non-Mendelian examples of DNA instability are associated with human disease. Most malignancies are associated with various chromosomal instabilities, such as aneuploidy, gene amplification, and chromosomal deletion. Furthermore, widespread microsatellite instability (MSI) is associated with a variety of tumors, and instability at specific dynamic repeat expansions underlies a family of neurologic disorders. Inactivation of DNA mismatch repair genes results in genomic instabilities affecting microsatellite regions. Mutations in genes involved in DNA polymerization or Okazaki fragment processing are also associated with MSI. Such instabilities convey a 'mutator' phenotype which is pathogenic. The mechanisms controlling trinucleotide repeat expansions are less well understood. Why this type of genomic instability is particularly pathogenic to neurons is also not clear. An understanding of what normally maintains stability is the first step towards preventing such loss of control and maintaining health.

Corneoconjunctival transposition for the treatment of feline corneal sequestra: a retrospective study of 17 cases (1990-1998)

OBJECTIVE

To describe and evaluate the use of corneoconjunctival transposition as a surgical treatment for corneal sequestra in cats.

ANIMALS STUDIED

Fifteen cats (17 eyes) of various breeds and ages with corneal sequestra treated surgically with corneoconjunctival transposition.

PROCEDURE

Retrospective medical records study.

RESULTS

Median age was 39 months. Eight patients were castrated males, five were spayed females, and two were intact females. The left eye was affected in eight cats, the right eye was affected in five cats, and both eyes were affected in two cats. Mean (+/- SD) sequestrum diameter was 4.4 +/- 2.4 mm (range 1-10 mm) and the sequestrum was located centrally in all cases. Sequestra occurred in the superficial stroma in 8/17 corneas, in the anterior one third of the stroma in 3/17 corneas, and in the middle one third of the stroma in 6/17 corneas. Median duration of clinical signs prior to presentation was 39 days (range 14-1095 days). Median time to healing, defined as the number of days from surgery to cessation of medical treatment, was 34 days (range 25-86 days). There were no recurrences and minimal scar formation.

CONCLUSIONS

Corneoconjunctival transposition is a valid procedure for surgical treatment of corneal sequestra in cats. The procedure results in excellent cosmesis and functional vision with no recurrences.

Enrofloxacin-associated retinal degeneration in cats

OBJECTIVE

The objective of this study was to evaluate the possible relationship between the administration of parenteral and/or oral [corrected] enrofloxacin and the onset of acute retinal degeneration in cats. The animals studied included 17 cats that received systemic enrofloxacin and developed retinal degeneration soon thereafter.

PROCEDURES

In this retrospective clinical study, cats that received parenteral and/or oral [corrected] enrofloxacin and developed acute blindness were identified. Parameters recorded included breed, age, sex, enrofloxacin dosage (daily dose and number of days administered), medical condition for which the antibiotic had been prescribed, ophthalmic signs, examination results, and the visual outcome. Fundus photographs were obtained in seven cats, and electroretinography was performed in five cats. Histopathology was performed on two eyes from one cat (case 1) that received enrofloxacin 5 months previously and developed retinal degeneration.

RESULTS

All cats were the domestic shorthair breed; seven were females (one neutered) and ten were males (seven castrated). Ages ranged from 3 to 16 years old (mean +/- SD; 8.8 +/- 4.6 years). The medical disorders for which enrofloxacin was administered ranged from lymphoma and pancreatitis to otitis and dermatitis, and eight cats had urinary diseases. The daily and total dosage of enrofloxacin and number of days of administration were also highly variable. Presenting clinical signs were most often mydriasis and acute blindness. All cats had diffuse retinal degeneration as evidenced by increased tapetal reflectivity and retinal vascular attenuation. Absence of recordable electroretinographic responses suggested diffuse and extensive outer retinal disease. Vision returned in a few cats, but the retinal degeneration persisted or even progressed. Histopathology of two eyes revealed primarily outer retinal degeneration, with diffuse loss of the outer nuclear and photoreceptor layers, and hypertrophy and proliferation of the retinal pigment epithelium.

CONCLUSION

Parenteral and/or oral [corrected] enrofloxacin is potentially retinotoxic in some cats, and may result in acute and diffuse retinal degeneration. Blindness often results, but some cats may regain vision. Practitioners should adhere closely to the manufacturer's current enrofloxacin dosage recommendation (5 mg/kg q 24 h), and continue clinical observations for this drug toxicity in cats.

Density of corneal endothelial cells and corneal thickness in eyes of euthanatized horses

OBJECTIVE

To determine density of corneal endothelial cells and corneal thickness in eyes of euthanatized horses.

SAMPLE POPULATION

52 normal eyes from 26 horses.

PROCEDURE

Eyes were enucleated after horses were euthanatized. Eyes were examined to determine that they did not have visible ocular defects. Noncontact specular microscopy was used to determine density of corneal endothelial cells. Corneal thickness was measured, using ultrasonic pachymetry or specular microscopy.

RESULTS

Mean density of corneal endothelial cells was 3,155 cells/mm2. Cell density decreased with age, but sex did not affect cell density. Values did not differ significantly between right and left eyes from the same horse. Cell density of the ventral quadrant was significantly less than cell density of the medial and temporal quadrants. Mean corneal thickness was 893 microm. Sex or age did not affect corneal thickness. Dorsal and ventral quadrants were significantly thicker than the medial and temporal quadrants and central portion of the cornea. We did not detect a correlation between corneal thickness and density of endothelial cells in normal eyes of horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Density of corneal endothelial cells decreases with age, but corneal thickness is not affected by age or sex in normal eyes of horses. The technique described here may be useful for determining density of endothelial cells in the cornea of enucleated eyes. This is clinically relevant for analyzing corneal donor tissue prior to harvest and use for corneal transplantation.

Ocular manifestations of feline herpesvirus

Feline herpesvirus-1 (FHV-1) infection is ubiquitous in the domestic cat population worldwide. The most common clinical ocular manifestations of infection with FHV-1 are conjunctivitis and keratitis. This paper reviews the pathogenesis of feline herpesvirus-1 and discusses the various clinical ocular manifestations, diagnostic techniques and treatment of FHV-1-induced diseases. Ocular manifestations include: conjunctivitis, keratitis, stromal keratitis, keratoconjunctivitis sicca, ophthalmia neonatorium, symblepharon, corneal sequestrum, eosinophilic keratitis and anterior uveitis. Diagnostic techniques discussed include: virus isolation, fluorescent antibody testing, serum neutralising titers, ELISA and polymerase chain reaction. Various therapies are also discussed.

The "flap" endonuclease gene FEN1 is excluded as a candidate gene implicated in the CAG repeat expansion underlying Huntington disease

At least 12 disorders including Huntington disease (HD) are associated with expansion of a trinucleotide repeat (TNR). Factors contributing to the risk of expansion of TNRs and the mechanism of expansion have not been elucidated. Data from Saccharomyces cerevisiae suggest that the flap endonuclease FEN1 plays a role in expansion of repetitive DNA tracts. It has been hypothesized that insufficiency of FEN1 or a mutant FEN1 might contribute to the occurrence of expansion events of long repetitive DNA tracts after polymerase slippage events during lagging strand synthesis. The expression pattern of FEN1 was determined, and ubiquitous tissue expression, including germ cells, suggested that FEN1 has the potential to be involved in HD. Fifteen HD parent/child pairs that demonstrated intergenerational increases in CAG length of greater than 10 repeats were examined for possible mutations or polymorphisms within the FEN1 gene that could underlie the saltatory repeat expansions seen in these individuals. No alterations were observed compared to 50 controls, excluding FEN1 as a trans-acting factor underlying TNR expansion. The identification of a candidate gene(s) in HD or other CAG-expansion disorders implicated in TNR instability will elucidate the mechanism of expansion for this growing family of neurological disorders.

Candidate mutator genes in mismatch repair-deficient thymic lymphomas: no evidence of mutations in the DNA polymerase delta gene

DNA mismatch repair (MMR) proteins recognize nucleotides that are incorrectly paired. Deficiencies in MMR lead to increased genomic instability reflected in an increased mutation frequency and predisposition to tumorigenesis. Mice lacking the MMR gene, Msh2, develop thymic lymphomas that exhibit much higher mutational frequencies than other Msh2(-/-) tumours and Msh2(-/-) normal thymic tissue, suggesting that an additional mutator may have been acquired in a tissue-specific manner. Clustered mutations observed exclusively in the thymic lymphomas suggests that a gene(s) associated with the replication machinery might have become altered during tumorigenesis. Based on mutation studies in Saccharomyces cerevisiae lacking Msh2 and DNA polymerase delta (DNA pol delta), we hypothesized that the acquisition of mutations in DNA pol delta could contribute to the hypermutator phenotype and tumorigenesis in Msh2(-/-) thymic tissue. Furthermore, previous reports have suggested that genes containing mononucleotide repeats are non-random mutational targets in the absence of MMR. Therefore, we sequenced all 26 exons of the DNA pol delta catalytic subunit, including the six exons containing mononucleotide repeats of >5 bp, from nine Msh2(-/-) thymic lymphomas and two wild-type controls. No DNA pol delta pathogenic mutations were found in the thymic lymphomas, although several DNA base differences compared with published DNA pol delta sequences were observed. We conclude, therefore, that inactivating mutations in DNA pol delta are not a contributing factor in the development of the hypermutator phenotype in MMR-deficient murine thymic lymphomas.

Feline infectious peritonitis

Feline infectious peritonitis is a noncurable viral disease affecting cats worldwide. Recent evidence suggests that the FIPV has evolved as a deletion mutation of FECV. Immune complex deposition and vasculitis with pyogranulomatous lesions are the hallmark of FIP. The only definitive antemortem diagnostic test for FIP is histopathologic examination of tissue. Ocular manifestations occur commonly with noneffusive FIP. The most common clinical sign is a bilateral granulomatous anterior uveitis often accompanied by chorioretinitis. Treatment of ocular FIP is symptomatic, and the mainstay of palliative therapy is topical or systemic corticosteroids or both.

Mutagenesis in PMS2- and MSH2-deficient mice indicates differential protection from transversions and frameshifts

DNA mismatch repair (MMR) deficiency leads to an increased mutation frequency and a predisposition to neoplasia. 'Knockout' mice deficient in the MMR proteins Msh2 and Pms2 crossed with mutation detection reporter (supF, lacI and cII) transgenic mice have been used to facilitate a comparison of the changes in mutation frequency and spectra. We find that the mutation frequency was consistently higher in Msh2-deficient mice than Pms2-deficient mice. The lacI target gene, which is highly sensitive to point mutations, demonstrated that both Msh2- and Pms2-deficient mice accumulate transition mutations as the predominant mutation. However, when compared with Msh2(-/-) mice, lacI and cII mutants from Pms2-deficient mice revealed an increased proportion of +/-1 bp frameshift mutations and a corresponding decrease in transversion mutations. The supF target gene, which is sensitive to frameshift mutations, and the cII target gene revealed a strong tendency for -1 bp deletions over +1 bp insertions in Msh2(-/-) compared with Pms2(-/-) mice. These data indicate that Msh2 and Pms2 deficiency have subtle but differing effects on mutation avoidance which may contribute to the differences in tumor spectra observed in the two 'knockout' mouse models. These variances in mutation accumulation may also play a role, in part, in the differences seen in prevalence of MSH2 and PMS2 germline mutations in hereditary non-polyposis colorectal cancer patients.

Development of glaucoma after cataract surgery in dogs: 220 cases (1987-1998)

OBJECTIVE

To examine postoperative ocular hypertension (POH) and other variables as predictors of the risk of developing glaucoma after cataract surgery in dogs.

DESIGN

Retrospective study.

ANIMALS

220 dogs that had cataract surgery.

PROCEDURE

Medical records of 220 dogs (346 eyes) that had extracapsular cataract removal or phacoemulsification of cataracts were reviewed. With respect to glaucoma development, 8 variables were analyzed, which included development of POH, breed, sex, age at time of surgery, eye (right vs left), phacoemulsification time, intraocular lens (IOL) placement (yes or no), and stage of cataract development. Eyes developed glaucoma within 6 or 12 months of surgery or did not have signs of glaucoma at least 6 or 12 months after cataract surgery.

RESULTS

Of 346 eyes, 58 (16.8%) developed glaucoma after surgery. At 6 months, 32 of 206 (15.5%) eyes examined had glaucoma; at 12 months, 44 of 153 (28.8%) eyes examined had glaucoma. Median follow-up time was 5.8 months (range, 0.1 to 48 months). Mixed-breed dogs were at a significantly lower risk for glaucoma, compared with other breeds. Eyes with IOL placement were at a significantly lower risk for glaucoma, compared with eyes without IOL placement. Eyes with hypermature cataracts were at a significantly higher risk for glaucoma, compared with eyes with mature or immature cataracts.

CONCLUSIONS AND CLINICAL RELEVANCE

Multiple factors appear to contribute to the onset of glaucoma in dogs after cataract surgery. Complications prohibiting IOL placement during cataract surgery may lead to a high risk of glaucoma development.

Tumors arising in DNA mismatch repair-deficient mice show a wide variation in mutation frequency as assessed by a transgenic reporter gene

We reported previously that thymic lymphomas arising in mice lacking the DNA mismatch repair (MMR) gene, Msh2(-/-), exhibited striking elevations in the mutation frequency of a transgenic lacI reporter gene when compared with normal Msh2(-/-) tissues. To investigate whether hypermutation was a feature of all tumors arising in MMR-deficient mice, lacI transgene mutation frequencies were obtained from several different mouse tumors deficient for PMS2 and/or MSH2. While lacI gene hypermutation was again clearly evident in Msh2 +/- ms2(-/-) and Msh2(-/-)Pms2(-/-) thymic lymphomas, three non-thymic MSH2-deficient tumors failed to show lacI gene mutation frequency elevations when compared with a normal tissue of MMR-deficient mice. The elevated mutation frequencies in the lymphoid tumors, and the finding of multiple clustered mutations in lacI genes rescued from these tumors, suggest that they are possibly generated by a lymphoma-specific hypermutational mechanism.

Comparison of selectable and plaque assay systems to detect menadione- and UV-induced lacI mutations in mammalian cells

We have compared the spontaneous mutation frequency and spectrum of lacI genes recovered from a rat embryonic fibroblast line transfected with a lambda-phage shuttle vector (Rat2lambdalacI) using both the traditional plaque assay as well as a positive selection assay. In addition, mutation frequencies and spectrum were determined after treatment of the cells with either the intracellular superoxide-generating compound, menadione, or UVC light. The differences in mutation frequency between the two systems suggested that the selectable assay was better at discerning relatively small mutation frequency increases, more rapidly and at lower cost, than the plaque assay method. Some novel lacI mutations were observed in mutants derived from the selectable assay. This indicates that the selectable assay system may be a useful tool for assessing the mutagenic potential of different agents.

Tumors of DNA mismatch repair-deficient hosts exhibit dramatic increases in genomic instability

DNA mismatch repair (MMR) deficiency is associated with an increased mutational burden and predisposition to certain malignancies. Relatively little is known, however, about gene-specific mutation frequencies within MMR-deficient primary tumors. Thymic lymphomas from Msh2(-/-) mice were thus analyzed by using a lacI-based transgenic shuttle-phage mutation detection system. All tumors exhibited greatly elevated lacI gene mutation frequencies, ranging from 3.2- to 17.4-fold above the approximately 15-fold elevations present within normal Msh2(-/-) thymi. In addition, lacI genes harboring multiple changes, including clusters of mutations, were found in thymic tumor DNA. The results suggest that an additional mutator activity, such as an error-prone DNA polymerase, leads to increased genomic instability in these MMR-deficient tumors.

Equine ulcerative keratomycosis: visual outcome and ocular survival in 39 cases (1987-1996)

The medical records of 39 horses treated for ulcerative keratomycosis over a 10 year period were reviewed. Records were evaluated to determine the medical and/or surgical treatment protocol, visual outcome, globe survival and whether the outcome was influenced by the fungal species isolated. Stromal abscesses and iris prolapses caused by fungi were not included. Twenty of the horses underwent medical treatment only, and 19 horses had combined medical and surgical treatment. Most horses had been treated with topical antibiotics (n = 32) and atropine sulphate (n = 23) prior to referral; topical antifungals had been employed less frequently (n = 14). Fungi were identified by cytology (n = 31), culture (n = 33) and/or surgical histopathology (n = 6). Aspergillus (n = 13) and Fusarium (n = 10) were the most commonly isolated fungi. Miconazole (n = 35) was the most common topical antifungal medication utilised. Median duration of treatment was 48 days (range 31-192 days). Associated bacterial infection (n = 13) was frequently encountered. Visual outcome was favourable in 36/39 (92.3%) eyes. All eyes (20/20) retained vision following medical management only, and 16/19 (84%) retained vision following combined medical and surgical therapy. All medically treated horses (20/20), and 17/19 (89%) of those treated medically and surgically retained their globes. Overall ocular survival was favourable in 37/39 (94.9 %) eyes. Aggressive therapy can result in successful results for equine ulcerative keratomycosis.

Tissues of MSH2-deficient mice demonstrate hypermutability on exposure to a DNA methylating agent

The mutational response of mismatch repair-deficient animals to the alkylating agent N-methyl-N-nitrosourea was evaluated by using a transgenic lacI reporter system. Although the mutations detected in MSH2 heterozygotes were similar to those of controls, MSH2-/- animals demonstrated striking increases in mutation frequency in response to this agent. G:C to A:T transitions at GpG sites, as opposed to CpG sites, dominated the mutational spectrum of both MSH2+/+ and MSH2-/- N-methyl-N-nitrosourea -treated animals. Extrapolating to humans with hereditary non-polyposis colorectal cancer, the results suggest that MSH2 heterozygotes are unlikely to be at increased risk of mutation, even when exposed to potent DNA methylating agents. In contrast, mismatch repair-deficient cells spontaneously arising within individuals with hereditary non-polyposis colorectal cancer would likely exhibit hypermutability in response to such mutagens, an outcome predicted to accelerate the pace of tumorigenesis.

Antimicrobial susceptibility patterns of fungi isolated from horses with ulcerative keratomycosis

OBJECTIVES

To evaluate in vitro susceptibility to topical antifungal medications, as measured by minimum inhibitory concentration (MIC) and 50% inhibitory concentration (IC50%), of fungal isolates from horses with ulcerative keratomycosis in Florida; to compare results with those of other studies to identify differences in susceptibility patterns among fungi isolated from horses in different geographic regions; and to note indications of fungal resistance to drugs tested in other studies.

SAMPLE POPULATION

Corneal fungal cultures from client-owned horses from Florida with ulcerative keratomycosis (n = 22).

PROCEDURE

Fungal cultures were plated on Emmons modified Sabouraud dextrose agar and mycobiotic agar, examined weekly for growth, and kept for a total of 30 days. In vitro MIC and IC50% of fluconazole, itraconazole, ketoconazole, miconazole, and natamycin were measured for each fungal isolate.

RESULTS

Aspergillus (n = 9; 41%), Fusarium (7; 32%), Penicillium (2; 9%), Cylindrocarpon (1; 4%), Scytalidium (1; 4%), and Torulopsis (1; 4%) spp and an unidentified yeast (1; 4%) were isolated. Fungi were most susceptible to antifungal drugs in the following order: natamycin and miconazole equally, itraconazole, and ketoconazole, although no significant difference was found among drugs. Fungi were significantly less susceptible to fluconazole (P < 0.0001) than to the other 4 drugs.

CONCLUSIONS

Initial antifungal therapy with topically applied natamycin, miconazole, itraconazole, or ketoconazole is recommended for ulcerative keratomycosis in horses in the subtropical environment of Florida.

CLINICAL RELEVANCE

Specific antifungal treatment of horses with ulcerative keratomycosis should be based on history, results of ophthalmic examination, cytologic findings, isolation of the pathogenic fungus, and known prevalence of unique ocular fungi in specific geographic areas. In vitro antifungal susceptibility testing may be most beneficial in aiding documentation of pharmacologic susceptibility patterns of fungi in specific geographic regions.

Role of DNA mismatch repair in the cytotoxicity of ionizing radiation

The DNA mismatch repair (MMR) system in mammalian cells not only serves to correct base mispairs and other replication errors, but it also influences the cellular response to certain forms of DNA damage. Cells that are deficient in MMR are relatively resistant to alkylation damage because, in wild-type cells, the MMR system is thought to promote toxicity via futile repair of alkylated mispairs. Conversely, MMR-deficient cells are sensitive to UV light, possibly due to the requirement for MMR factors in transcription-coupled repair of active genes. MMR deficiency has been associated with familial and sporadic carcinomas of the colon and other sites, and so, we sought to determine the influence of MMR status on cellular response to ionizing radiation, an agent commonly used for cancer therapy. Fibroblast cell lines were established from transgenic mice carrying targeted disruptions of one of three MMR genes in mammalian cells: Pms2, Mlh1, or Msh2. In comparison to wild-type cell lines from related mice, the Pms2-, Mlh1-, or Msh2-nullizygous cell lines were found to exhibit higher levels of clonogenic survival following exposure to ionizing radiation. Because ionizing radiation generates a variety of lesions in DNA, the differences in survival may reflect a role for MMR in processing a subset of these lesions, such as damaged bases. These results both identify a new class of DNA-damaging agents whose effects are modulated by the MMR system and may help to elucidate pathways of radiation response in cancer cells.

Base transitions dominate the mutational spectrum of a transgenic reporter gene in MSH2 deficient mice

Tumors derived from individuals with hereditary nonpolyposis colorectal cancer syndrome frequently demonstrate mutations in both alleles of hMSH2, a key gene in DNA mismatch repair (MMR). Sporadic tumors also frequently exhibit MMR deficiency. In keeping with the role of MMR in the maintenance of genome integrity, mice deficient in MSH2 via gene targeting demonstrate a high incidence of thymic lymphomas and small intestinal adenocarcinomas. To investigate the effects of MSH2 deficiency in normal tissues, mice containing a retrievable transgenic lacI reporter gene for mutation detection were crossed with MSH2-/- mice. Mice homozygous for MSH2 deficiency revealed 4.8, 11.0 and 15.2-fold elevations in spontaneous mutation frequency in DNA obtained from brain, small intestine, and thymus, respectively, as compared to heterozygous or wild-type mice. Mutations most frequently recovered from MSH2-/- mice were single base substitutions (77%), particularly base transitions (64%). Frameshifts occurred less frequently (19%) and fell within very short (3-5 bp) mononucleotide runs. Thus the number of key growth control genes potentially impacted by MMR deficiency extends beyond those containing repetitive sequences. These results highlight the capacity for MSH2 deficiency to serve as a potent driving force during the multi-step evolution of tumors.

MutS interaction with mismatch and alkylated base containing DNA molecules detected by optical biosensor

An optical biosensor was used to monitor interactions between the Escherichia coli DNA mismatch repair molecule MutS and various immobilized oligonucleotides. While associating poorly with single-stranded DNA, MutS was capable of rapid association/dissociation from homoduplex DNA. The interaction of MutS with oligonucleotide 30-mers containing single site mismatches demonstrated that during the dissociation phase, MutS binding was greatest to a G-G mismatch, followed by G-T > A-A > C-T, A-C. Binding to A-G, T-T and C-C mispairs was marginally higher than that seen between MutS and homoduplex DNA. The ability of MutS to interact with 30-mers containing alkylated bases was also tested. While binding to O6-methyl-G-C, or to O4-methyl-T-A base pairs was similar to that of homoduplex DNA, strong binding was seen to a O6-methyl-G-T mispair. O4-methyl-T-G, however, was poorly recognized by MutS, with relative binding affinity similar to homoduplex DNA, predicting poor in vivo recognition of O4-methyl-T-G by MutS. Interestingly, MutS demonstrated a relatively high affinity for an 1,N6-etheno-A-T containing homoduplex. Thus, in allowing rapid evaluation of interactions between such molecules, the biosensor will be useful to structure-function analyses.

A novel lacI transgenic mutation-detection system and its application to establish baseline mutation frequencies in the scid mouse

To assess DNA mutations in vivo, we have established a new transgenic mouse line, BC-1, carrying a lacI target gene for mutation detection within a bacteriophage shuttle-vector. The lacI gene was positioned within sequences derived from a rearranged murine immunoglobulin gene locus, a feature that distinguishes the BC-1 transgene from other shuttle vector systems. As mutations in lacI transgenes likely reflect mutations occurring throughout the genome, these systems have been successfully used to investigate spontaneous and induced mutations in a variety of tissues. An important additional application of the transgenic systems is the characterization of lacI mutations occurring in murine strains having specific DNA repair defects. For this study, scid (severe combined immunodeficiency) mice were selected as animals with this mutation have a defect in double-strand DNA break repair. To determine what impact the scid mutation might have on spontaneous mutation frequencies within DNA recovered from various tissues, these mice were crossed with the BC-1 line. Interestingly, mutation frequencies within BC-1/scid mouse DNA were not significantly different from those of BC-1 control mice. Furthermore, spontaneous lacI mutations obtained from BC-1 and from BC-1/scid liver DNA were similar in spectrum. As spontaneous BC-1 liver mutations were similar to those reported previously for other lacI systems, such as the Big Blue transgenic line, this suggested that the nature of the DNA sequences flanking the reporter gene did not modify lacI mutation rate or character.

Origins and evolution of Huntington disease chromosomes

Huntington disease (HD) is one of five neurodegenerative disorders resulting from an expansion of a CAG repeat located within the coding portion of a novel gene. CAG repeat expansion beyond a particular repeat size has been shown to be a specific and sensitive marker for the disease. A strong inverse correlation is evident between CAG length and age of onset. Sporadic cases of HD have been shown to arise from intermediate sized alleles in the unaffected parent. The biochemical pathways underlying the relationship between CAG repeat length and specific cell death are not yet known. However, there is an increasing understanding of how and why specific chromosomes and not others expand into the disease range. Haplotype analysis has demonstrated that certain normal chromosomes, with CAG lengths at the high range of normal, are prone to further expansion and eventually result in HD chromosomes. New mutations preferentially occur on normal chromosomes with these same haplotypes associated with higher CAG lengths. The distribution of different haplotypes on control chromosomes in different populations is thus one indication of the frequency of new mutations for HD within that population. Analysis of normal chromosomes in different populations suggests that genetic factors contribute to expansion and account for the variation in prevalence rates for HD worldwide.

Ancestral differences in the distribution of the delta 2642 glutamic acid polymorphism is associated with varying CAG repeat lengths on normal chromosomes: insights into the genetic evolution of Huntington disease

This study addresses genetic factors associated with normal variation of the CAG repeat in the Huntington disease (HD) gene. To achieve this, we have studied patterns of variation of three trinucleotide repeats in the HD gene including the CAG and adjacent CCG repeats as well as a GAG polymorphism at residue 2642 (delta 2642). We have previously demonstrated that variation in the CCG repeat is associated with variation of the CAG repeat length on normal chromosomes. Here we show that differences in the GAG trinucleotide polymorphism at residue 2642 is also significantly correlated with CAG size on normal chromosomes. The B allele which is associated with higher CAG repeat lengths on normal chromosomes is markedly enriched on affected chromosomes. Furthermore, this glutamic acid polymorphism shows significant variation in different ancestries and is absent in chromosomes of Japanese, Black and Chinese descent. Haplotype analysis of both the CCG and delta 2642 polymorphisms have indicated that both are independently associated with differences in CAG length on normal chromosomes. These findings lead to a model for the genetic evolution of new mutations for HD preferentially occurring on normal chromosomes with higher CAG repeat lengths and a CCG repeat length of seven and/or a deletion of the glutamic acid residue at delta 2642. This study also provides additional evidence for genetic contributions to demographic differences in prevalence rates for HD.

DNA haplotype analysis of Huntington disease reveals clues to the origins and mechanisms of CAG expansion and reasons for geographic variations of prevalence

This study of allelic association using three intra- and two extragenic markers within 150 kb of the Huntington disease (HD) mutation has provided evidence for linkage disequilibrium for four of five markers. Haplotype analysis of 67 HD families using markers in strong linkage disequilibrium with HD identified two haplotypes underlying 77.6% of HD chromosomes. Normal chromosomes with these two haplotypes had a mean number of CAG repeats significantly larger than and an altered distribution of CAG repeats compared with other normal chromosomes. Furthermore, haplotype analysis of five new mutation families reveals that HD has arisen on these same two chromosomal haplotypes. These findings suggest that HD arises more frequently on chromosomes with specific DNA haplotypes and higher CAG repeat lengths. We then studied CAG and CCG repeat lengths in the HD gene on 896 control chromosomes from different ancestries to determine whether the markedly reduced frequency of HD in Finland, Japan, China and African Blacks is associated with an altered frequency of DNA haplotypes and subsequently lower CAG lengths on control chromosomes compared to populations of Western European descent. The results show a highly significant inverse relationship between CAG and CCG repeat lengths. In populations with lowered prevalence rates of HD, CAG repeat lengths are smaller and the distribution of CCG alleles is markedly different from Western European populations. These findings suggest that, in addition to European emigration, new mutations make a contribution to geographical variation of prevalence rates and is consistent with a multistep model of HD developing from normal chromosomes with higher CAG repeat lengths.

A worldwide study of the Huntington's disease mutation. The sensitivity and specificity of measuring CAG repeats

BACKGROUND

Huntington's disease is associated with an expanded sequence of CAG repeats in a gene on chromosome 4p16.3. However, neither the sensitivity of expanded CAG repeats in affected persons of different ethnic origins nor the specificity of such repeats for Huntington's disease as compared with other neuropsychiatric disorders has been determined.

METHODS

We studied 1007 patients with diagnosed Huntington's disease from 565 families and 43 national and ethnic groups. In addition, the length of the CAG repeat was determined in 113 control subjects with a family history of Alzheimer's disease (44 patients), schizophrenia (39), major depression (16), senile chorea (5), benign hereditary chorea (5), neuroacanthocytosis (2), and dentatorubropallidoluysian atrophy (2). The number of CAG repeats was also assessed in 1595 control chromosomes, with the size of adjacent polymorphic CCG trinucleotide repeats taken into account.

RESULTS

Of 1007 patients with signs and symptoms compatible with a diagnosis of Huntington's disease, 995 had an expanded CAG repeat that included from 36 to 121 repeats (median, 44) (sensitivity, 98.8 percent; 95 percent confidence interval, 97.7 to 99.4 percent). There were no significant differences among national and ethnic groups in the number of repeats. No CAG expansion was found in the 110 control subjects with other neuropsychiatric disorders (specificity, 100 percent; 95 percent confidence interval, 95.2 to 100 percent). In 1581 of the 1595 control chromosomes (99.1 percent), the number of CAG repeats ranged from 10 to 29 (median, 18). In 12 control chromosomes (0.75 percent), intermediate-sized CAG sequences with 30 to 35 repeats were found, and 2 normal chromosomes unexpectedly had expanded CAG sequences, of 39 and 37 repeats.

CONCLUSIONS

CAG trinucleotide expansion is the molecular basis of Huntington's disease worldwide and is a highly sensitive and specific marker for inheritance of the disease mutation.

Huntington disease without CAG expansion: phenocopies or errors in assignment?

Huntington disease (HD) has been shown to be associated with an expanded CAG repeat within a novel gene on 4p16.3 (IT15). A total of 30 of 1,022 affected persons (2.9% of our cohort) did not have an expanded CAG in the disease range. The reasons for not observing expansion in affected individuals are important for determining the sensitivity of using repeat length both for diagnosis of affected patients and for predictive testing programs and may have biological relevance for the understanding of the molecular mechanism underlying HD. Here we show that the majority (18) of the individuals with normal sized alleles represent misdiagnosis, sample mix-up, or clerical error. The remaining 12 patients represent possible phenocopies for HD. In at least four cases, family studies of these phenocopies excluded 4p16.3 as the region responsible for the phenotype. Mutations in the HD gene that are other than CAG expansion have not been excluded for the remaining eight cases; however, in as many as seven of these persons, retrospective review of these patients' clinical features identified characteristics not typical for HD. This study shows that on rare occasions mutations in other, as-yet-undefined genes can present with a clinical phenotype very similar to that of HD.

Somatic and gonadal mosaicism of the Huntington disease gene CAG repeat in brain and sperm

Huntington disease is associated with an unstable and expanded (CAG) trinucleotide repeat. We have analysed the CAG expansion in different tissues from 12 affected individuals. All tissues examined were found to display some repeat mosaicism, with the greatest levels detected in brain and sperm. Regions within the brain showing most obvious neuropathology, such as the basal ganglia and the cerebral cortex, displayed the greatest mosaicism, whereas the cerebellar cortex, which is seldom involved, displayed the lowest degree of CAG instability. In two cases of childhood onset disease we detected differences of 8 and 13 trinucleotides between the cerebellum and other regions of the brain. Our results provide evidence for tissue specific instability of the CAG repeat, with the largest CAG repeat lengths in affected regions of the brain.

A CCG repeat polymorphism adjacent to the CAG repeat in the Huntington disease gene: implications for diagnostic accuracy and predictive testing

The polymorphic CAG repeat that is expanded on Huntington disease (HD) chromosomes is flanked by a CCG repeat. Here we show that this CCG tract, previously assumed to be invariant at seven CCG repeats, is also polymorphic. We have identified five CCG alleles from 205 normal chromosomes, with 137 (67%) having alleles of seven repeats, five (2%) with nine repeats, 61 (30%) with 10 repeats, one (0.5%) with 11 repeats and one (0.5%) with 12 repeats. In contrast, analysis of 113 HD chromosomes revealed that the majority (105 chromosomes, 93%) contained seven CCG repeats, while the remaining eight chromosomes (7%) had allele sizes of 10 CCG repeats. Despite evidence that both CAG and CCG are polymorphic on normal chromosomes, we have found that it is only the CAG length that has a significant impact on age of onset. The discovery of larger sized CCG alleles, however, has significant implications for the assessment of CAG repeat length, particularly for persons with estimated CAG size of 36-42 repeats, since an overestimation of CAG length in this range could result in erroneous information being imparted to patients.