Prospective study of the natural history of chronic acid sphingomyelinase deficiency in children and adults: eleven years of observation

Background

Acid sphingomyelinase deficiency (ASMD) (also known as Niemann-Pick disease types A and B) is a rare and debilitating lysosomal storage disorder. This prospective, multi-center, multinational longitudinal study aimed to characterize the clinical features of chronic forms of ASMD and disease burden over time in children and adults.

Results

Fifty-nine patients (31 males/28 females) ranging in age from 7 to 64 years with chronic ASMD types A/B and B and at least two disease symptoms participated from 5 countries. Disease characteristics were assessed at baseline, after 1 year, and at the final visit (ranging from 4.5 to 11 years). Thirty patients (51%) were < 18 years at baseline (median age 12 years), and 29 were adults (median age 32 years). Overall, 32/59 patients completed the final visit, 9 died, 9 discontinued, and 9 were lost to follow up. Common clinical characteristics that tended to worsen gradually with time were splenomegaly, hepatomegaly, interstitial lung disease, lung diffusion capacity (DL_CO), and dyslipidemia. Spleen volumes ranged from 4 to 29 multiples of normal at baseline, and splenomegaly was moderate or severe in 86%, 83%, and 90% of individuals at baseline, year 1, and final visits, respectively. The proportion of all individuals with interstitial lung disease was 66% (39/59) at baseline and 78% (25/32) at the final visit, while median % predicted DL_CO decreased by > 10% from baseline to the final visit. Nine patients died (15%), eight of causes related to ASMD (most commonly pneumonia); of these eight patients, five (63%) had symptom onset at or before age 2. Overall, six of the nine deaths occurred before age 50 with three occurring before age 20. Individuals with either severe splenomegaly or prior splenectomy were ten times more likely to have died during the follow-up period than those with smaller or intact spleens (odds ratio 10.29, 95% CI 1.7, 62.7). Most children had growth deficits that persisted into adulthood.

Conclusions

This study provides important information about the natural history of chronic ASMD and provides a longitudinal view of the spectrum of disease manifestations and major morbidities in children and adults and supports the selection of clinically meaningful endpoints in therapeutic trials.

Disease manifestations and burden of illness in patients with acid sphingomyelinase deficiency (ASMD)

Acid sphingomyelinase deficiency (ASMD), a rare lysosomal storage disease, is an autosomal recessive genetic disorder caused by different SMPD1 mutations. Historically, ASMD has been classified as Niemann-Pick disease (NPD) types A (NPD A) and B (NPD B). NPD A is associated with a uniformly devastating disease course, with rapidly progressing psychomotor degeneration, leading to death typically by the age of 3 years, most often from respiratory failure. In contrast, the clinical phenotype and life expectancy of patients with NPD B may vary widely. Almost all patients have hepatosplenomegaly and an atherogenic lipid profile, and most patients have interstitial lung disease with progressive impairment of pulmonary function and hematologic abnormalities including cytopenias. Other common clinical manifestations include liver dysfunction, heart disease, skeletal abnormalities and growth delays. Some patients with ASMD who survive beyond early childhood have intermediate phenotypes (variant NPD B) characterized by combinations of non-neurologic and mild to severe neurologic symptoms. The physical and psychosocial burden of illness in patients with NPD B is substantial. Common symptoms include shortness of breath, joint or limb pain, abdominal pain, bleeding and bruising. The disease often leads to chronic fatigue, limited physical or social activity and difficulties in performing daily activities or work. Many patients die before or in early adulthood, often from pneumonia/respiratory failure or liver failure. Available treatments are limited to symptom management and supportive care. An enzyme replacement therapy currently in clinical development is expected to be the first treatment addressing the underlying pathology of the disease. Early diagnosis and appropriate management are essential for reducing the risk of complications. While knowledge about ASMD is evolving, more evidence about ASMD and the natural history across the disease spectrum is needed, to improve disease recognition, timely diagnosis and appropriate disease management.

Lyso-sphingomyelin is elevated in dried blood spots of Niemann-Pick B patients

Niemann-Pick disease type B (NPD-B) is caused by a partial deficiency of acid sphingomyelinase activity and results in the accumulation of lysosomal sphingomyelin (SPM) predominantly in macrophages. Notably, SPM is not significantly elevated in the plasma, whole blood, or urine of NPD-B patients. Here, we show that the de-acylated form of sphingomyelin, lyso-SPM, is elevated approximately 5-fold in dried blood spots (DBS) from NPD-B patients and has no overlap with normal controls, making it a potentially useful biomarker.

Skeletal manifestations in pediatric and adult patients with Niemann Pick disease type B

INTRODUCTION

Niemann-Pick disease (NPD) due to acid sphingomyelinase deficiency is a lipid storage disease resulting from the accumulation of sphingomyelin, predominantly within cells of the monocyte-macrophage system. In contrast to other lysosomal storage disorders, skeletal involvement in NPD has not been systematically studied.

METHODS

Pediatric and adult NPD-B patients underwent medical histories and physical examinations, DEXA scans to measure bone mineral content (BMC), and bone mineral density (BMD) and computed tomography scan or MRI of the abdomen for spleen volume. Z and/or T scores were calculated for the DEXA results. For the pediatric patients adjusted mean BMC (g) and BMD (g/cm(2)) of the lumbar spine, hip, and femoral neck was compared to control subjects. For determination of the relationship between spleen volume and lumbar spine BMD Z score, linear correlation analyses were performed.

RESULTS

Lumbar spine Z scores for pediatric patients ranged from 0.061 to -4.879. Statistically significant decreases were observed for the adjusted mean BMC and BMD at the lumbar spine, hip, and femoral neck between the pediatric NPD-B cohort and control subjects. Most NPD-B adults were osteopenic or osteoporotic at one or more sites according the WHO classification of BMD. In NPD-B patients, the degree of splenomegaly was inversely correlated with lumbar spine BMD Z scores.

CONCLUSION

Skeletal involvement is a common and previously unrecognized manifestation of NPD-B. The association between splenomegaly and BMD lends further support to spleen size as an indicator of disease severity.

Randomized trial of a decision aid for BRCA1/BRCA2 mutation carriers: impact on measures of decision making and satisfaction

OBJECTIVE

Genetic testing is increasingly part of routine clinical care for women with a family history of breast cancer. Given their substantially elevated risk for breast cancer, BRCA1/BRCA2 mutation carriers must make the difficult decision whether or not to opt for risk reducing mastectomy. To help BRCA1/2 carriers make this decision, the authors developed a computer-based interactive decision aid that was tested against usual care in a randomized controlled trial.

DESIGN

After the completion of genetic counseling, 214 female (aged 21-75) BRCA1/BRCA2 mutation carriers were randomized to Usual Care (UC; N = 114) or Usual Care plus Decision Aid (DA; N = 100) arms. UC participants received no additional intervention. DA participants were sent the CD-ROM DA to view at home.

MAIN OUTCOME MEASURES

The authors measured final management decision, decisional conflict, decisional satisfaction, and receipt of risk reducing mastectomy at 1-, 6-, and 12-months postrandomization.

RESULTS

Longitudinal analyses revealed that the DA was effective among carriers who were initially undecided about how to manage their breast cancer risk. Within this group, the DA led to an increased likelihood of reaching a management decision (OR = 3.09, 95% CI = 1.62, 5.90; p < .001), decreased decisional conflict (B = -.46, z = -3.1, p <002), and increased satisfaction (B = .27, z = 3.1, p = .002) compared to UC. Among carriers who had already made a management decision by the time of randomization, the DA had no benefit relative to UC.

CONCLUSION

These results demonstrate that BRCA1/BRCA2 mutation carriers who are having difficulty making a breast cancer risk management decision can benefit from adjunct decision support.

Clinician perspectives about molecular genetic testing for heritable conditions and development of a clinician-friendly laboratory report

The use of molecular genetic tests for heritable conditions is expected to increase in medical settings, where genetic knowledge is often limited. As part of a project to improve the clarity of genetic test result reports to minimize misunderstandings that could compromise patient care, we sought input about format and content from practicing primary care clinicians. In facilitated workgroup discussions, clinicians from pediatric, obstetrics-gynecology, and family practice provided their perspectives about molecular genetic testing with a focus on the laboratory reporting of test results. Common principles for enhancing the readability and comprehension of test result reports were derived from these discussions. These principles address the presentation of patient- and test-specific information, the test result interpretation, and guidance for future steps. Model test result reports for DNA-based cystic fibrosis testing are presented that were developed based on workgroup discussions, previous studies, and professional guidelines. The format of these model test reports, which are applicable to a variety of molecular genetic tests, should be useful for communicating essential information from the laboratory to health care professionals.

A prospective, cross-sectional survey study of the natural history of Niemann-Pick disease type B

OBJECTIVE

The objective of this study was to characterize the clinical features of patients with Niemann-Pick disease type B and to identify efficacy end points for future clinical trials of enzyme-replacement therapy.

METHODS

Fifty-nine patients who had Niemann-Pick disease type B, were at least 6 years of age, and manifested at least 2 disease symptoms participated in this multicenter, multinational, cross-sectional survey study. Medical histories; physical examinations; assessments of cardiorespiratory function, clinical laboratory data, and liver and spleen volumes; radiographic evaluation of the lungs and bone age; and quality-of-life assessments were obtained during a 2- to 3-day period.

RESULTS

Fifty-three percent of the patients were male, 92% were white, and the median age was 17.6 years. The R608del mutation accounted for 25% of all disease alleles. Most patients initially presented with splenomegaly (78%) or hepatomegaly (73%). Frequent symptoms included bleeding (49%), pulmonary infections and shortness of breath (42% each), and joint/limb pain (39%). Growth was markedly delayed during adolescence. Patients commonly had low levels of platelets and high-density lipoprotein, elevated levels of low-density lipoprotein, very-low-density lipoprotein, triglycerides, leukocyte sphingomyelin, and serum chitotriosidase, and abnormal liver function test results. Nearly all patients had documented splenomegaly and hepatomegaly and interstitial lung disease. Patients commonly showed restrictive lung disease physiology with impaired pulmonary gas exchange and decreased maximal exercise tolerance. Quality of life was only mildly decreased by standardized questionnaires. The degree of splenomegaly correlated with most aspects of disease, including hepatomegaly, growth, lipid profile, hematologic parameters, and pulmonary function.

CONCLUSIONS

This study documents the multisystem involvement and clinical variability of Niemann-Pick B disease. Several efficacy end points were identified for future clinical treatment studies. Because of its correlation with disease severity, spleen volume may be a useful surrogate end point in treatment trials, whereas biomarkers such as chitotriosidase also may play a role in monitoring patient treatment responses.

Association between temporal orientation and attitudes about BRCA1/2 testing among women of African descent with family histories of breast cancer

OBJECTIVE

Previous studies have identified specific attitudes (pros and cons) about BRCA testing held by women of African descent that are associated with decisions to participate in testing. These testing attitudes may be determined, in part, by temporal orientation, or how one perceives the significance of events and the consequences of their actions in terms of past, present, and future. The current study explored the relationship between temporal orientation and pros and cons of BRCA testing among 140 women of African descent with a family history suggestive of a genetic mutation predisposing to breast cancer.

METHODS

Participants completed measures of temporal orientation and genetic testing attitudes.

RESULTS

Multivariate analyses indicated that future orientation was positively associated with perceived pros of testing. Additional analyses revealed significant associations between temporal orientation and specific item subsets related to the negative and positive impact of testing on family and personal control over one's health.

CONCLUSION

These results support an association between temporal orientation and attitudes about BRCA testing among women of African descent with family histories of breast cancer.

PRACTICE IMPLICATIONS

Findings support exploration of temporal orientation in future research on BRCA testing decisions among women of African descent and this construct's importance in developing decision aids and tailoring genetic counseling.

Ordering molecular genetic tests and reporting results: practices in laboratory and clinical settings

Previous studies have suggested that patient care may be compromised as a consequence of poor communication between clinicians and laboratory professionals in cases in which molecular genetic test results are reported. To understand better the contributing factors to such compromised care, we investigated both pre- and postanalytical processes using cystic fibrosis mutation analysis as our model. We found that although the majority of test requisition forms requested patient/family information that was necessary for the proper interpretation of test results, in many cases, these data were not provided by the individuals filling out the forms. We found instances in which result reports for simulated diagnostic testing described individuals as carriers where only a single mutation was found with no comment pertaining to a diagnosis of cystic fibrosis. Similarly, reports based on simulated scenarios for carrier testing were problematic when no mutations were identified, and the patient's race/ethnicity and family history were not discussed in reference to residual risk of disease. Remarkably, a pilot survey of obstetrician-gynecologists revealed that office staff, including secretaries, often helped order genetic tests and reported test results to patients, raising questions about what efforts are undertaken to ensure personnel competency. These findings are reviewed in light of what efforts should be taken to improve the quality of test-ordering and result-reporting practices.

Report of an International Survey of Molecular Genetic Testing Laboratories

OBJECTIVE

To collect data on the practices of molecular genetic testing (MGT) laboratories for the development of national and international policies for quality assurance (QA).

METHODS

A web-based survey of MGT laboratory directors (n = 827; response rate 63%) in 18 countries on 3 continents. QA and reporting indices were developed and calculated for each responding laboratory.

RESULTS

Laboratory setting varied among and within countries, as did qualifications of the directors. Respondents in every country indicated that their laboratory receives specimens from outside their national borders (64%, n = 529). Pair-wise comparisons of the QA index revealed a significant association with the director having formal training in molecular genetics (p < 0.005), affiliation with a genetics unit (p = 0.003), accreditation of the laboratory (p < 0.005) and participation in proficiency testing (p < 0.005). Research labs had a lower mean report score compared to all other settings (p < 0.05) as did laboratories accessioning <150 samples per year.

CONCLUSION

MGT is provided under widely varying conditions and regulatory frameworks. The data provided here may be a useful guide for policy action at both governmental and professional levels.

Acid sphingomyelinase deficiency: prevalence and characterization of an intermediate phenotype of Niemann-Pick disease

OBJECTIVE

To document the prevalence of neurologic disease in Niemann-Pick disease (NPD) NPD-B.

STUDY DESIGN

Sixty-four patients with NPD-B had detailed neurologic and ophthalmologic evaluations. The presence of neurologic abnormalities was compared with genotype.

RESULTS

Nineteen of 64 patients (30%) had neurologic abnormalities, which were minor and nonprogressive in 14 (22%), and global and progressive in 5 (8%). In these five patients, the onset of neurologic difficulties occurred between 2 and 7 years of age and was associated with peripheral neuropathy, retinal abnormalities, and the Q292K mutation. No patients with at least one copy of DeltaR608 had neurologic involvement.

CONCLUSIONS

The majority of patients with NPD-B have no neurologic abnormalities. In patients with neurologic abnormalities, the findings can be minor and static or severe and progressive. The latter phenotype follows a course distinct from that of classic NPD-A and is associated with the Q292K mutation and characteristic retinal findings. Thus, similar to other lysosomal storage disorders, there is a broad spectrum of neurologic abnormalities in acid sphingomyelinase deficiency, which makes the current classification scheme inaccurate.

Imprinting at the SMPD1 locus: implications for acid sphingomyelinase-deficient Niemann-Pick disease

Acid sphingomyelinase (ASM) is the lipid hydrolase that is deficient in types A and B Niemann-Pick disease (NPD). Here, we demonstrate that the gene encoding ASM (SMPD1) is paternally imprinted and that differential expression of the mutant alleles in patients with ASM-deficient NPD and in carriers influences the disease phenotype. Comparison of the results of genomic sequencing versus reverse-transcriptase polymerase chain reaction sequencing for several patients with NPD revealed preferential expression of one mutant allele. Further analysis of one family showed that the expressed allele was maternally inherited and that the distinct clinical presentations of the individual patients were correlated with the amount of residual ASM activity expressed from the maternal mutation. Treatment of NPD cell lines with 5-aza-2'-deoxycytidine enhanced the expression of the paternal SMPD1 allele, and bisulfite genomic sequencing identified which CpG dinucleotides within the SMPD1 promoter were methylated. In a related set of studies, we identified a carrier individual who had approximately 15% of normal ASM activity and clinical features of ASM-deficient NPD. DNA sequencing confirmed that this individual carried a single SMPD1 mutation and that this mutant allele was preferentially expressed. These data thus demonstrate, for the first time, imprinting at the SMPD1 gene and reveal the influence of this epigenetic modification on the presentation of ASM-deficient NPD.

Referral and experience with genetic testing among women with early onset breast cancer

The purpose of this study was to determine whether physicians refer women with early onset breast cancer for genetic testing for BRCA1 and BRCA2, and how women respond to being offered testing and use the results. A web-based survey was distributed to 1221 women with early onset breast cancer. The survey included 158 questions divided into the following sections: demographics, family history of cancer, medical history, treatment history, and experience with genetic testing. Of 551 women diagnosed since 1993 who responded to the survey (45.1%), less than half (45%, n = 246) had ever discussed genetic testing with their physician and/or been referred to see a genetic counselor. Women with a family history of cancer (53%) and Ashkenazi Jewish women (81%) were more likely to have been referred. Of those who had discussed testing, 60% had undergone or were interested in testing. Overall 92 women were tested and 19 (20.6%) of these tested positive for a deleterious BRCA1 or BRCA2 mutation. Fourteen (74%) who tested positive subsequently underwent prophylactic surgery. Satisfaction with counseling and the decision to be tested was high. Among women who were not offered testing, the fact that the test had not been offered by their physician (89%), and fear of discrimination (83%) were the two most frequently cited factors for lack of interest in testing. A substantial number of women are not being referred to genetic counseling and/or testing after a diagnosis of early onset breast cancer. Among those who were tested, there was high interest in prophylactic surgery after confirmation of a BRCA1/2 mutation.

Natural history of Type A Niemann-Pick disease: possible endpoints for therapeutic trials

OBJECTIVE

To describe the disease course and natural history of Type A Niemann-Pick disease (NPD).

METHODS

Ten patients with NPD-A (six male, four female; age range at entry: 3 to 6 months) were serially evaluated including clinical neurologic, ophthalmologic, and physical examinations, and assessment of development. Laboratory analyses, abdominal and brain ultrasounds, and chest radiographs also were obtained and information on intercurrent illnesses and cause of mortality was collected.

RESULTS

All affected infants had a normal neonatal course and early development. The first symptom detected in all patients was hepatosplenomegaly. Developmental age did not progress beyond 10 months for adaptive behavior, 12 months for expressive language, 9 months for gross motor skills, and 10 months for fine motor skills. Non-neurologic symptoms included frequent vomiting, failure to thrive, respiratory infections, irritability, and sleep disturbance. Neurologic examination at the time of presentation was normal in most patients. Later neurologic examinations revealed progressive hypotonia with loss of the deep tendon reflexes. All patients had cherry red spots by 12 months. The median time from diagnosis to death was 21 months. The cause of death was respiratory failure in nine patients and complications from bleeding in the tenth.

CONCLUSIONS

The clinical course in Type A Niemann-Pick disease is similar among affected patients and is characterized by a relentless neurodegenerative course that leads to death, usually within 3 years.

Use of standardized patients in, undergraduate medical genetics education

BACKGROUND AND PURPOSE

To study the effectiveness of a standardized patient (SP) program in increasing the competence of medical students in assessing genetic risks and communicating genetic information to patients.

METHODS

Third-year medical students at the Mount Sinai School of Medicine had two encounters from 2001 to 2003 with the same SP, who portrayed a woman at risk for hereditary breast cancer. Assessment instruments included student self-assessment of skills, SP assessment of student communication skills, an observer checklist, grading of the student-drawn pedigree, and a knowledge test. Students also completed an evaluation form after the debriefing session at the end of each of the SP sessions.

RESULTS

The SP program was completed by 136 students. The student self-evaluation of skills instrument revealed that students who completed the SP program felt more competent in their ability to draw a pedigree, assess genetic risks based on family history and pedigree information, and communicate genetic risks compared to students at the same level of training who did not participate in the SP program. Of participating students, 90% agreed that the program allowed them to identify areas for improvement in their skills, and 95% agreed that the exercise increased their confidence for having a similar patient interaction in the future.

CONCLUSIONS

The use of SPs in undergraduate medical genetics education may be one means for increasing the confidence of medical students in skills that are related to genetic encounters.

Type B Niemann-Pick Disease: Findings at Chest Radiography, Thin-Section CT, and Pulmonary Function Testing

PURPOSE

To evaluate findings at radiography, computed tomography (CT), and pulmonary function testing in patients with type B Niemann-Pick disease.

MATERIALS AND METHODS

The study was approved by the institutional review board or ethics committee at each study site and was compliant with HIPAA at the U.S. site. Written informed consent was obtained from each patient or guardian and minor assent was obtained from all children before any study-related procedures. Pulmonary involvement in 53 patients (27 male and 26 female patients; age range, 7-65 years; mean age, 23.3 years) with type B Niemann-Pick disease was evaluated with imaging and pulmonary function tests. All patients underwent chest radiography and thin-section CT, and images were independently interpreted by one of two radiologists. Spirometry (forced vital capacity [FVC] and forced expiratory volume in 1 second [FEV1]) was performed and diffusing capacity of lung for carbon monoxide (Dlco) was evaluated in all patients who could comply. A score for the degree of interstitial lung disease was derived at both radiography and CT, and the CT scores were then compared with results of pulmonary function testing and patient age by means of linear regression. CT scores were compared between the upper and lower lung zones by using the Wilcoxon signed rank test.

RESULTS

Chest radiography and CT, respectively, revealed interstitial lung disease in 47 (90%) and 51 (98%) of the 52 patients who completed both imaging examinations. There was a basilar predominance of interstitial lung disease at CT. Six patients had pulmonary nodules, one of which was calcified at chest radiography. There were no statistically significant correlations between interstitial lung disease score at CT and age or percentage predicted FVC, FEV1, or Dlco values.

CONCLUSION

Although pulmonary function test indexes may be abnormal, imaging findings do not necessarily correlate with pulmonary function in patients with type B Niemann-Pick disease.

The natural history of type B Niemann-Pick disease: results from a 10-year longitudinal study

OBJECTIVES

Type B Niemann-Pick disease (NPD-B) caused by acid sphingomyelinase deficiency is a rare, autosomal recessive, lysosomal storage disorder with a broad range of disease severity. The objectives of this study were to document the natural history of the disease in a large, clinically heterogeneous patient population that was followed for a period of 10 years and to determine how genotype influences phenotype.

METHODS

Twenty-nine patients with NPD-B had serial evaluations at least 9 months apart. Organ volumes, hematologic indices, lipid concentrations, pulmonary function, and hepatic activity were studied, and individual phenotypic severity was compared with genotype.

RESULTS

All patients with intact spleens had splenomegaly (mean value: 12.7 multiples of normal [MN]; range: 4.5-27.3 MN), and all but 1 had hepatomegaly (mean volume: 1.91 MN; range: 0.93-3.21 MN). At initial visit, 39% had thrombocytopenia and 3% had leukopenia. At final visit, the percentages increased to 54% and 34%, respectively. Mean annual decreases in platelet count and leukocyte count were 7 x 10(3) and 0.2 x 10(3) per mm3, respectively. The typical atherogenic lipid profile was worse in older patients. A total of 69% of patients had low diffusion capacity for carbon monoxide, and more than one third had low forced expiratory volume in 1 second, forced vital capacity, and forced expiratory volume in 1 second/forced vital capacity at initial visit. All measurements of pulmonary function showed a gradual deterioration over time. Liver dysfunction was characterized by stable elevation of hepatic transaminases and bilirubin. Homozygotes for DeltaR608, P323A, and P330R had milder disease than patients with all other genotypes.

CONCLUSIONS

The natural history of NPD-B is characterized by hepatosplenomegaly with progressive hypersplenism, worsening atherogenic lipid profile, gradual deterioration in pulmonary function, and stable liver dysfunction.

Ocular manifestations of Niemann–Pick disease type B

PURPOSE

To investigate the ocular manifestations in Niemann-Pick disease type B (NPD-B).

DESIGN

Observational case series.

PARTICIPANTS

Forty-five patients (23 male and 22 female) with NPD-B from 37 unrelated families.

METHODS

Serial clinical evaluations were carried out over a 2- to 14-year period, including a complete physical examination, neurologic assessment, and ophthalmologic examination. Genotyping of the specific mutations in the acid sphingomyelinase (ASM) gene was performed when possible for genotype-phenotype correlations.

MAIN OUTCOME MEASURES

Fundus photographs to evaluate the retina, ASM genotype, and neurologic examination findings.

RESULTS

Ophthalmoscopic examination revealed retinal stigmata in 15 of 45 patients, 3 with macular halos and 12 with cherry red maculae. Neurologic examinations did not reveal any evidence of neurodegeneration, and there was no consistent relationship between retinal findings and genotype.

CONCLUSIONS

The presence of macular halos and/or cherry red maculae is not an absolute predictor of neurodegeneration, but should prompt a thorough evaluation to determine the underlying etiology and the precise diagnosis.

Lipid abnormalities in children with types A and B Niemann Pick disease

OBJECTIVE

To characterize the lipid profiles in patients with types A and B Niemann Pick disease (NPD) and determine if lipid abnormalities are associated with evidence of early cardiovascular disease or correlate with genotype.

STUDY DESIGN

The study was a cross-sectional analysis of 10 patients with NPD type A and 30 patients with NPD type B that was carried out in the General Clinical Research Center. For each patient, fasting lipid profile and glucose, T4, height or length, weight, resting blood pressure, and acid sphingomyelinase deficiency genotype were measured. In type B patients, electrocardiograhic-gated helical computed tomography of the heart also was obtained.

RESULTS

Lipid abnormalities included low (<35 mg/dL) high-density lipoprotein cholesterol in 100% of patients and hypertriglyceridemia and increased low-density lipoprotein cholesterol in 62% (25/40) and 67% (27/40) of patients, respectively. Coronary artery calcium scores were positive (>1.0) in 10 of 18 type B patients studied. There was no correlation of the Delta R608 genotype with a milder phenotype for the lipid abnormalities, as has been observed for a number of other NPD manifestations.

CONCLUSIONS

Lipid abnormalities are part of the phenotype in types A and B NPD and may be associated with early atherosclerotic heart disease.

Attitudes and psychosocial adjustment of unaffected siblings of patients with phenylketonuria

Sibling illness may contribute to an increased risk of adjustment problems in healthy siblings. Previous studies have reported a variety of effects on healthy individuals who have an ill sibling, but the psychosocial effects of treatable inherited disease on healthy siblings have not yet been investigated. We report the results of a survey study conducted in families with both unaffected and affected children with classic phenylketonuria (PKU), an inherited inborn error of metabolism. The survey included a knowledge test about PKU, and four previously validated instruments designed to assess psychosocial adjustment of unaffected siblings compared to age and sex matched norms. The responses revealed that unaffected adolescent and adult siblings had gaps in their knowledge about the genetic basis of PKU, and had evidence for the presence of adverse psychosocial sequelae. These findings suggest a role for genetic services providers, including genetic counselors, in assisting all members of a family adjust, when the diagnosis of an inborn error of metabolism has been made.

Interaction of genetic counselors with molecular genetic testing laboratories: implications for non-geneticist health care providers

The availability of molecular genetic tests for the identification of mutant gene carriers, and for assessing individual genetic response to pharmacologic agents, infectious agents, and other environmental exposures, is expected to result in the increased use of the molecular genetic testing laboratory by primary care physicians. However, a number of concerns have been raised about such testing including the need for safeguards to protect patient privacy, and if the interface between genetic testing laboratories and the ordering physician facilitates the appropriate clinical use of the test result. In this study, genetic counselors were surveyed to determine their practices with regard to the clinical issues of informed consent and confidentiality in the context of genetic testing, and to assess their level of satisfaction with the reporting practices of molecular genetic testing laboratories. The results of this survey revealed that there is variability in the practices of genetic counselors with regard to obtaining informed consent, and that there are areas for improvement with regard to molecular genetic test reports, particularly in terms of interpretation of results.

Biochemical effect of intravenous arginine butyrate in X-linked adrenoleukodystrophy

OBJECTIVES

To determine the biochemical and clinical effects of intravenous arginine butyrate in X-linked Adrenoleukodystrophy (X-ALD).

STUDY DESIGN

Arginine butyrate was intravenously infused over a 4-month period in a patient with the rapid cerebral form of X-ALD. Very long chain fatty acids (VLCFA), complete blood counts, and serum chemistries were monitored, and serial MRI of the brain and clinical neurologic examinations were performed.

RESULTS

All blood chemical and hematologic values remained within the normal range for age throughout the therapy. After completion of the first day of infusion, the C 26:0 value fell from 1.01 microg/mL to 0.445 microg/mL, which is below the mean value for an X-ALD heterozygote. Throughout the remainder of the trial, all C26:0 levels fell below the mean -1 SD for X-ALD hemizygotes (mean, 1.18 microg/mL, 1 SD = 0.53), ranging from 0.321 to 0.565 microg/mL. Despite reduction of the plasma VLCFA, the patient continued to deteriorate neurologically.

CONCLUSIONS

Intravenous arginine butyrate resulted in a rapid decrease in plasma VLCFA but no effect on the neurologic progression of the disease in this patient. Additional studies are needed to determine minimum effective dosage and interval, what proportion of patients respond, and whether the agent can prevent neurologic degeneration.

Growth restriction in children with type B Niemann-Pick disease

OBJECTIVES

To compare growth of children with type B Niemann-Pick disease (NPD) with disease variables including genotype, organomegaly, bone age, and serum insulin-like growth factor-1 (IGF-1).

STUDY DESIGN

A cross-sectional analysis of growth was performed in 23 children and adolescents with enzymatically and genotypically confirmed NPD. Liver and spleen volumes were measured by quantitative computed tomography and skeletal age by a wrist radiograph.

RESULTS

The mean Z scores for height and weight were -1.24 (29th percentile) and -0.75 (34th percentile). The mean liver and spleen volumes were 2.06 and 13.46 times normal for weight, respectively. Skeletal age was delayed by an average of 2.5 years, and serum IGF-1 level was at or below the 2nd percentile in 8 of 12 patients. Short stature and low weight were significantly correlated with large organ volumes, delayed bone age, and low IGF-1 levels. In contrast to patients with other mutations, individuals homozygous for the DeltaR608 mutation had normal height and weight, markedly less hepatosplenomegaly and bone age delay, and normal IGF-1 levels.

CONCLUSIONS

Abnormal linear growth and delayed skeletal maturation are common in children and adolescents with type B NPD; however, homozygosity for DeltaR608 is associated with normal growth.

Personnel standards and quality assurance practices of biochemical genetic testing laboratories in the United States

CONTEXT

It has been suggested that specific regulation of laboratories performing genetic testing may be needed to ensure standards and quality assurance, and to safeguard the rights of patients with regard to confidentiality and providing informed consent. Previously, a comprehensive analysis of current practices of molecular genetic testing laboratories was conducted, the results of which have assisted in the assessment of the need for regulation and its impact on access to testing. However, a study designed to determine clinical laboratory practices with regard to biochemical genetic testing has not been carried out.

OBJECTIVE

To collect and analyze data regarding availability of clinical biochemical genetic testing, personnel standards, and laboratory quality assurance practices.

DESIGN

A mail survey of biochemical genetic testing laboratory directors and assignment of a quality assurance score based on responses to genetic testing process items.

SETTING

Hospital-based, independent, and research-based biochemical genetic testing laboratories in the United States.

PARTICIPANTS

Directors of biochemical genetic testing laboratories (n = 133; response rate 68.5%).

MAIN OUTCOME MEASURE

Laboratory process quality assurance score based on the standards defined by the American College of Medical Genetics Laboratory Practice Committee.

RESULTS

Personnel qualifications varied, although all directors had doctoral degrees. The mean quality assurance score was 77% (range 28%-100%). Higher scores were associated with the following variables: test director having an MD degree versus PhD degree (P = .002), director board certification in biochemical genetics (P = .002), research and hospital laboratory versus independent laboratory setting (P < .001), and participation in a proficiency testing program (P = .03). Twelve percent of participants had a confidentiality policy, and 19% required informed consent before testing.

CONCLUSION

The finding that a number of laboratories had quality assurance scores that may reflect suboptimal laboratory practices, particularly with regard to reporting practices, suggests that personnel qualification and laboratory practice standards may be in need of improvement to ensure quality in clinical biochemical genetic testing laboratories, as well as the appropriate clinical use of the test results.

The demographics and distribution of type B Niemann-Pick disease: novel mutations lead to new genotype/phenotype correlations

We have collected demographic and/or mutation information on a worldwide sample of 394 patients with type B Niemann-Pick disease (NPD). The disorder is panethnic, with the highest incidence occurring in individuals of Turkish, Arabic, and North African descent. Only five of the 394 patients were Ashkenazi Jewish, revealing that, unlike the type A form of NPD, type B NPD does not occur frequently within this population. Mutation analysis of the acid sphingomyelinase (ASM) gene (designated "SMPD1") was performed on 228 patients (324 unique alleles), and several novel, "common" mutations were found. Among these were the L137P, fsP189, and L549P mutations, which accounted for approximately 75% of the alleles in Turkish patients, the H421Y and K576N mutations, which accounted for approximately 85% of the alleles in Saudi Arabian patients, the S379P, R441X, R474W, and F480L mutations, which accounted for approximately 55% of the alleles in Portuguese/Brazilian patients, and the A196P mutation, which accounted for approximately 42% of the alleles in Scottish/English patients. The previously reported DeltaR608 mutation occurred on approximately 12% of the alleles studied. Overall, a total of 45 novel mutations were found, and several new genotype/phenotype correlations were identified. In particular, the L137P, A196P, and R474W mutations were consistent with a less severe form of type B NPD, whereas the H421Y and K576N mutations led to an early-onset, more severe form that was specific to Saudi Arabia. These data provide the first extensive demographic assessment of this disorder and describe several new mutations that can be used to predict phenotypic outcome and to gain new insights into the structure and function of ASM.

Attitudes of pediatric residents toward ethical issues associated with genetic testing in children

OBJECTIVE

To assess the attitudes of pediatric residents toward molecular genetic testing and some associated ethical issues that accompany its use in pediatric patients.

METHODS

A questionnaire study of pediatric residents (n = 160; response rate: 40%) enrolled in training programs at 3 New York metropolitan area hospitals was designed to determine their attitudes toward genetic testing in children and adolescents. The study instrument presented 2 clinical vignettes that described scenarios where a pediatric patient was at risk for being affected with or a carrier of a genetic disorder. Residents returned the questionnaire anonymously.

RESULTS

Most pediatric residents recognized the importance of educating family members (95%) and at-risk individuals (89%) about the inheritance pattern of the disorders they were at risk for. However, a substantial number would order predictive testing for Huntington disease in a child at the request of a parent (39% at age 10 and 52% at age 17), and more than half would order fragile X carrier studies on the cognitively normal sister of an affected male (56%).

CONCLUSIONS

An effort must be made to educate all physicians during their residency training about the ethical issues associated with genetic testing. For pediatric residents, such education must include instruction on the complexities associated with offering such testing in this age group.

A fluorescence-based, high-throughput sphingomyelin assay for the analysis of Niemann-Pick disease and other disorders of sphingomyelin metabolism

Sphingomyelin is an important lipid component of cell membranes and lipoproteins that can be hydrolyzed by sphingomyelinases into ceramide and phosphorylcholine. The Type A and B forms of Niemann-Pick disease (NPD) are lipid storage disorders due to the deficient activity of the enzyme acid sphingomyelinase and the resultant accumulation of sphingomyelin in cells, tissues, and fluids. In this paper we report a new, enzymatic method to quantify the levels of sphingomyelin in plasma, urine, or tissues from NPD patients and mice. In this assay, bacterial sphingomyelinase is first used to hydrolyze sphingomyelin to phosphorylcholine and ceramide. Alkaline phosphatase then generates choline from the phosphorylcholine, and the newly formed choline is then used to generate hydrogen peroxide in a reaction catalyzed by choline oxidase. Finally, with peroxidase as a catalyst, hydrogen peroxide reacts with the Amplex Red reagent to generate a highly fluorescent product, resorufin. These enzymatic reactions are carried out simultaneously in a single 100-microl reaction mixture for 20 min. Use of a 96-well microtiter plate permits automated and sensitive quantification using a plate reader and fluorescence detector. This procedure allowed quantification of sphingomyelin over a broad range from 0.02 to 10 nmol, similar in sensitivity to a recently described radioactive method using diacylglycerol kinase and 50 times more sensitive than a colorimetric, aminoantipyrine/phenol-based assay. To validate this new assay method, we quantified sphingomyelin in plasma, urine, and tissues from normal individuals and from NPD mice and patients. The sphingomyelin content in adult homozygous or heterozygous NPD mouse plasma and urine was significantly elevated compared to that of normal mice. Moreover, the accumulated sphingomyelin in the tissues of NPD mice was 4 to 15 times higher than that in normal mice depending on the tissue analyzed. The sphingomyelin levels in plasma from several Type B NPD patients also was significantly elevated compared to normal individuals of the same age. Based on these results, we propose that this new, fluorescence-based procedure can provide simple, fast, sensitive, and reproducible sphingomyelin quantification in tissues and fluids from normal individuals and NPD patients. It could also be a useful tool for the study of other sphingomyelin-related diseases and in a variety of research settings where sphingomyelin quantification is required.

Lethal osteosclerotic skeletal dysplasia with intracellular inclusion bodies

We report an apparently previously undescribed form of lethal osteosclerotic skeletal dysplasia in a 30-week male fetus with micromelic shortness of the limbs. Radiographic findings at necropsy included increased density in all bones, most marked in the skull, mandible, and pubis. The ribs were very short, abnormally modeled, and wide anteriorly. The vertebrae were posteriorly hypoplastic and wedged, particularly in the cervical and lumbar regions. The femora and tibiae were short with wide distal metaphyses, undermodeled diaphyses, and coxa vara. The humeri, radii, and ulnae were also short and undermodeled with proximal and distal flare. Chondro-osseous morphology showed short chondrocyte columns, extension of hypertrophic cells into the metaphysis, and overgrowth of perichondral bone. In the resting cartilage there were large chondrocytes containing a homogeneous material staining pink with von Kossa trichrome, gray with toluidine blue, and black with silver methenamine. The cortical bone was lacking and the trabecular bone was hypercellular, thick, and coarse. Ultrastructurally, the resting zone chondrocytes were large and round with condensed chromatin and dilated loops of rough endoplasmic reticulum. The radiographic and histopathologic findings in this case are unique and differ from those seen in other reported lethal osteosclerotic skeletal dysplasias.

Quality assurance in molecular genetic testing laboratories

CONTEXT

Specific regulation of laboratories performing molecular genetic tests may be needed to ensure standards and quality assurance (QA) and safeguard patient rights to informed consent and confidentiality. However, comprehensive analysis of current practices of such laboratories, important for assessing the need for regulation and its impact on access to testing, has not been conducted.

OBJECTIVE

To collect and analyze data regarding availability of clinical molecular genetic testing, including personnel standards and laboratory practices.

DESIGN

A mail survey in June 1997 of molecular genetic testing laboratory directors and assignment of a QA score based on responses to genetic testing process items.

SETTING

Hospital-based, independent, and research-based molecular genetic testing laboratories in the United States.

PARTICIPANTS

Directors of molecular genetic testing laboratories (n = 245; response rate, 74.9%).

MAIN OUTCOME MEASURE

Laboratory process QA score, using the American College of Medical Genetics Laboratory Practice Committee standards.

RESULTS

The 245 responding laboratories reported availability of testing for 94 disorders. Personnel qualifications varied, although all directors had doctoral degrees. The mean QAscore was 90% (range, 44%-100%) with 36 laboratories (15%) scoring lower than 70%. Higher scores were associated with test menu size of more than 4 tests (P = .01), performance of more than 30 analyses annually (P = .01), director having a PhD vs MD degree (P = .002), director board certification (P = .03), independent (P <.001) and hospital (P = .01) laboratories vs research laboratory, participation in proficiency testing (P<.001), and Clinical Laboratory Improvement Amendment certification (P = .006). Seventy percent of laboratories provided access to genetic counseling, 69% had a confidentiality policy, and 45% required informed consent prior to testing.

CONCLUSION

The finding that a number of laboratories had QA scores that may reflect suboptimal laboratory practices suggests that both personnel qualification and laboratory practice standards are most in need of improvement to ensure quality in clinical molecular genetic testing laboratories.

Acceptability of chorionic villi sampling for prenatal diagnosis

The factors that influence women in choosing between first-trimester chorionic villi sampling and second-trimester amniocentesis for prenatal diagnosis were investigated. Five hundred twenty women of advanced maternal age who had previously undergone prenatal diagnosis by amniocentesis and were delivered of a normal infant were requested to complete a questionnaire concerning their attitudes toward amniocentesis and chorionic villi sampling. The majority of respondents indicated that the time at which chorionic villi sampling is performed (76%), the rapid availability of diagnostic results (72%), and the type of abortion procedure available (68%) would make them choose this method. In contrast, the factors that influenced women to choose amniocentesis included the known low risk of spontaneous abortion (76%) and confidence in the skill of the obstetrician who would perform the procedure (56%). When all factors were considered together, 68% of the respondents chose amniocentesis based on the known low risk of spontaneous abortion, whereas for those who chose chorionic villi sampling (32%), the major criterion was the fact that the procedure is performed in the first trimester. However, 87% of women who preferred amniocentesis indicated that if the risk of spontaneous abortion associated with chorionic villi sampling. These results indicate that for many women of advanced maternal age, the acceptability and the use of chorionic villi sampling will be dependent on the demonstration that the risk of fetal loss is low, approaching that of amniocentesis.

Animal model studies of allelism: characterization of arylsulfatase B mutations in homoallelic and heteroallelic (genetic compound) homozygotes with feline mucopolysaccharidosis VI

The identification of a second structural gene mutation at the feline arylsulfatase B locus (MPS VIb) provided the opportunity to investigate the expression of allelism by characterization of the residual enzymatic activity in feline mucopolysaccharidosis VI, an animal analogue of human Maroteaux-Lamy syndrome. Matings were designed to produce affected homozygotes who were homoallelic for the MPS VIa and MPS VIb mutations or heteroallelic genetic compounds (MPS VIa/VIb). The physicokinetic and immunological properties of the partially purified residual hepatic arylsulfatase B isozymes from the affected homoallelic and heteroallelic cats were compared to those of the normal feline enzyme. The residual hepatic arylsulfatase B activities from the inbred MPS VIa and MPS VIb homozygotes were distinguished by differences in physicokinetic and immunological properties. The newly identified mutant isozyme b had abnormal kinetic properties toward artificial and natural substrates, normal cryo- and thermostabilities, a normal molecular weight and an altered electrophoretic mobility. Polyacrylamide gel electrophoresis demonstrated that the mutant b subunits formed dimers with normal subunits in obligate heterozygotes (MPS VI+/b). In contrast, mutant isozyme a subunits from obligate MPS VIa/+ heterozygotes did not dimerize with the normal subunit, and the mutant and normal isozymes could be separated by anion exchange chromatography and polyacrylamide gel electrophoresis. Characterization of the partially purified residual hepatic arylsulfatase B activity from the heteroallelic homozygotes revealed the presence of both mutant isozymes a and b. The demonstration of two allelic mutations in the feline arylsulfatase B gene documented the occurrence of genetic heterogeneity in feline mucopolysaccharidosis VI and permitted characterization of the enzymatic defect in homoallelic and heteroallelic (genetic compound) homozygotes, providing a model for the study of allelism in human genetic disorders.

Bone-marrow transplantation in the Maroteaux-Lamy syndrome (mucopolysaccharidosis type VI). Biochemical and clinical status 24 months after transplantation

A 13-year-old girl with the severe form of the Maroteaux-Lamy syndrome (mucopolysaccharidosis Type VI, arylsulfatase B deficiency) has had successful reconstitution with bone marrow from her HLA-MLC-matched sister who had normal arylsulfatase B activity. Full engraftment has been present for 24 months. The following biochemical and clinical changes have occurred: arylsulfatase B activity in peripheral lymphocytes and granulocytes increased to normal levels, and the activity in serial liver-biopsy specimens increased from about 3 per cent of the mean normal level 43 days after transplantation to about 16 per cent at 600 days. Urinary excretion of acid mucopolysaccharide decreased. Ultrastructural evidence of accumulated dermatan sulfate was no longer detectable in bone-marrow cells; in peripheral-blood lymphocytes, granulocytes, or platelets; or in Ito cells of liver. Twenty-four months after engraftment, hepatosplenomegaly was substantially decreased and cardiopulmonary function was normal. Visual acuity and joint mobility were also improved. The patient returned to school and continued to perform well in academic studies. Thus, bone-marrow transplantation provided a source of enzymatically normal cells, which have altered the metabolic and clinical course of the disease.

Purification and properties of human hepatic aspartylglucosaminidase

Aspartylglucosaminidase (EC 3.5.1.26), the lysosomal enzyme which hydrolyzes the N-acetylglucosamine-asparagine linkages in glycoproteins, was purified from human liver to homogeneity. The purification procedure included chromatography on DEAE-cellulose and concanavalin A-Sepharose, gel filtration on Sephadex G-200, and high performance liquid chromatography. The purified enzyme had a final specific activity of 1,200,000 units/mg of protein, a pH optimum of 6.1, a pI of 5.7, a Vmax of 1,240,000 units/mg, and a Km of 1.25 mM toward a natural substrate, aspartylglucosamine. The purified enzyme was remarkably thermostable, retaining 90% of initial activity after 1 h at 60 degrees C. The molecular weight of the native enzyme was estimated to be 80,000 by gel filtration and 84,000 by analytical polyacrylamide gel electrophoresis. Under denaturing conditions, the molecular weight was 76,000, indicating that the native enzyme was a monomer. Amino acid composition revealed only 2 methionine residues/enzyme molecule.

Tissue culture of cat retinal pigment epithelium

To culture retinal pigment epithelial (RPE) cells from normal cats, the cells were enzymatically dissociated from the eyecup and grown in either Ham's F-10 Nutrient Mixture or Eagle's Minimum Essential Media supplemented with 20% fetal calf serum. Cultures reached confluency between 6 and 10 days and contained monolayers of polygonally shaped cells. Light and electron microscopy demonstrated that most of the normal morphological characteristics of cat RPE cells in vivo were maintained in vitro; these included apical microvilli, apicolateral junctional specializations, basal infoldings and intracellular organelles. Pigment granules appeared to be diluted by cell division. No evidence of a basal membrane formation was seen; however, a fine granular or fibrillar extracellular matrix was observed in some cultures and was located between the culture plate surface and the basal surface of the RPE. Primary cultures were viable for up to 145 days. The activities of two lysosomal hydrolases (arylsulfatase A and arylsulfatase B) involved in the metabolism of sulfatide and dermatan sulfate were measured in confluent cultures. Mean arylsulfatase A activity was 1297 nmol nitrocatechol/mg protein/hr and arylsulfatase B activity was 553 nmol nitrocatechol/mg protein/hr. These activities were approximately 5 to 10-fold higher than present in cat peripheral leukocytes and skin fibroblasts in vitro. This in vitro system will facilitate studies on normal function and in conditions where the RPE has been compromised by inherited diseases (i.e. gyrate atrophy, mucopolysaccharidosis I and VI).

Purification and properties of feline and human arylsulfatase B isozymes. Evidence for feline homodimeric and human monomeric structures

Normal feline and human arylsulfatase B isozymes were purified to homogeniety from liver. The specific activities of the feline and human enzymes toward p-nitrocatechol sulfate were 1,100,000 and 800,000 units/mg of protein, and toward UDP-N-acetylgalactosamine-4-sulfate were 5,500 and 4,000 units/mg of protein, respectively. Although both enzymes had the same pH optimum (5.7), the feline enzyme was more electronegative than the human enzyme when electrophoresed on polyacrylamide gels. Compared to the human isozyme, feline arylsulfatase B had a lower Km toward p-nitrocatechol sulfate (1.2 versus 3.6 mM), was more thermostable at 60 degrees C (68 versus 30 min), and had a slightly lower pI (7.8 versus 8.0). The native molecular weight of the feline enzyme was estimated to be about twice that the human isozyme by gel filtration, analytical polyacrylamide gel electrophoresis, and sucrose density-gradient centrifugation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed single protein bands of Mr = 41,000 and 38,000 for the feline and human isozymes, respectively. Alkylation and cross-linking experiments were consistent with the feline enzyme being a homodimer and the human enzyme a monomer. Amino acid compositional analyses revealed few significant differences between the two isozymes.

Enhancement of residual arylsulfatase B activity in feline mucopolysaccharidosis VI by thiol-induced subunit association

The molecular pathology of the deficient arylsulfatase B activity in feline mucopolysaccharidosis (MPS) VI was investigated. Compared with the highly purified normal feline hepatic enzyme, the purified MPS VI residual activity had a 100-fold higher Michaelis constant (K(m)), an altered electrophoretic mobility, half the apparent native molecular weight, and markedly decreased thermo-, cryo-, and pH stabilities. Molecular weight and alkylation studies were consistent with the normal enzyme being a homodimer and the residual MPS VI enzyme a monomer. When incubated with various sulfhydryl reagents, the residual specific activity was enhanced several-fold, whereas the activity of the purified normal enzyme was un-affected or slightly inhibited. In the presence of dithiothreitol (DTT) and cysteamine, a lysosomotropic aminothiol, the residual activity had an electrophoretic mobility and native molecular weight similar to those of the normal feline enzyme. These findings suggested that the monomeric residual enzyme was dimerized in the presence of thiol-reducing agents. To determine if thiol-induced subunit association could therapeutically increase the residual activity and degrade the accumulated dermatan sulfate, in vitro and in vivo experiments were undertaken. When 2 mM DTT or cysteamine was incubated with heparinized whole blood from an MPS VI cat, the leukocyte residual arylsulfatase B activity increased 11- and 20-fold, respectively, and the accumulated dermatan sulfate was degraded in the presence of both thiol reagents. Intravenous administration of DTT (50 mg/kg) effected an immediate, but transient, increase in leukocyte residual activity; however, the substrate levels were not significantly decreased. In contrast, intravenous administration of cysteamine (15 mg/kg) increased leukocyte residual activity more than sixfold 30 min postinfusion; concomitantly, the leukocyte substrate was decreased to 35% of the initial level immediately after infusion and to about 45% of preinfusion values during the 120-min period studied. These results suggest that the defective residual activity in feline MPS VI can be therapeutically manipulated by thiol-induced subunit association. Furthermore, this animal analog provides a prototype for the investigation of human inborn errors of metabolism resulting from enzymatic defects that might be amenable to enzyme manipulation therapy.

Feline mucopolysaccharidosis VI: purification and characterization of the resident arylsulfatase B activity

Hepatic arylsulfatase B (ASB) from normal and mucopolysaccharidosis VI (MPS VI) cats was purified over 2,800- and 1,800-fold, respectively, and their physical and kinetic properties were characterized. In contrast to the normal feline enzyme, the partially purified MPS VI residual activity had a 100-fold greater Km value and was markedly less stable to thermal, cryo-, and pH-inactivation. In addition, the MPS VI enzyme had a more negative charge as determined by its migration on polyacrylamide gel electrophoresis and its elution profile on cation exchange chromatography. Finally, the MPS VI activity had approximately half the apparent molecular weight of the normal feline enzyme, which was a homodimer, suggesting that the genetic mutation in feline MPS VI altered the subunit association as well as the kinetic and stability properties of the mutant protein.

An improved method for heterozygote identification in feline and human mucopolysaccharidosis VI, arylsulfatase-B deficiency

An improved method has been developed for the detection of heterozygotes for feline and human mucopolysaccharidosis VI. Arylsulfatase-A and -B activities were assayed in leukocyte extracts following separation of the enzymes by batch chromatography on DEAE cellulose. Determination of arylsulfatase-B specific activities did not permit accurate heterozygote identification, whereas the arylsulfatase-A to arylsulfatase-B activity ratio discriminated all 16 obligate heterozygotes for the feline and human disorders.