Toward newborn screening of metachromatic leukodystrophy: results from analysis of over 27,000 newborn dried blood spots

PURPOSE

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused by the deficiency of arylsulfatase A (ARSA), which results in the accumulation of sulfatides. Newborn screening for MLD may be considered in the future as innovative treatments are advancing. We carried out a research study to assess the feasibility of screening MLD using dried blood spots (DBS) from de-identified newborns.

METHODS

To minimize the false-positive rate, a two-tier screening algorithm was designed. The primary test was to quantify C16:0-sulfatide in DBS by ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The screening cutoff was established based on the results from 15 MLD newborns to achieve 100% sensitivity. The secondary test was to measure the ARSA activity in DBS from newborns with abnormal C16:0-sulfatide levels. Only newborns that displayed both abnormal C16:0-sulfatide abundance and ARSA activity were considered screen positives.

RESULTS

A total of 27,335 newborns were screened using this two-tier algorithm, and 2 high-risk cases were identified. ARSA gene sequencing identified these two high-risk subjects to be a MLD-affected patient and a heterozygote.

CONCLUSION

Our study demonstrates that newborn screening for MLD is highly feasible in a real-world scenario with near 100% assay specificity.

Low-dose agalsidase beta treatment in male pediatric patients with Fabry disease: A 5-year randomized controlled trial

BACKGROUND

Fabry disease is a rare, X-linked, lifelong progressive lysosomal storage disorder. Severely deficient α-galactosidase A activity in males is associated with the classic phenotype with early-onset, multisystem manifestations evolving to vital organ complications during adulthood. We assessed the ability of 2 low-dose agalsidase beta regimens to lower skin, plasma, and urine globotriaosylceramide (GL-3) levels, and influence clinical manifestations in male pediatric Fabry patients.

METHODS

In this multicenter, open-label, parallel-group, phase 3b study, male patients aged 5-18 years were randomized to receive agalsidase beta at 0.5 mg/kg 2-weekly (n = 16) or 1.0 mg/kg 4-weekly (n = 15) for 5 years. All had plasma/urine GL-3 accumulation but no clinically evident organ involvement. The primary outcome was GL-3 accumulation in superficial skin capillary endothelium (SSCE).

RESULTS

The mean age was 11.6 (range: 5-18) years and all but one of the 31 patients had classic GLA mutations. In the overall cohort, shifts from non-0 to 0-scores for SSCE GL-3 were significant at years 1, 3, and 5, but results were variable. Plasma GL-3 normalized and urine GL-3 reduced substantially. Higher anti-agalsidase beta antibody titers were associated with less robust SSCE GL-3 clearance and higher urine GL-3 levels. Renal function remained stable and normal. Most Fabry signs and symptoms tended to stabilize; abdominal pain was significantly reduced (-26.3%; P = .0215). No new clinical major organ complications were observed. GL-3 accumulation and cellular and vascular injury were present in baseline kidney biopsies (n = 7). Treatment effects on podocyte GL-3 content and foot process width were highly variable. Fabry arteriopathy overall increased in severity. Two patients withdrew and 2 had their agalsidase beta dose increased.

CONCLUSIONS

Our findings increase the limited amount of available data on long-term effects of enzyme replacement therapy in pediatric, classic Fabry patients. The low-dose regimens studied here over a period of 5 years did not demonstrate a consistent benefit among the patients in terms of controlling symptomatology, urine GL-3 levels, and pathological histology. The current available evidence supports treatment of pediatric, classic male Fabry patients at the approved agalsidase beta dose of 1.0 mg/kg 2-weekly if these patients are considered for enzyme replacement therapy with agalsidase beta.

Validation of a therapeutic range for nitisinone in patients treated for tyrosinemia type 1 based on reduction of succinylacetone excretion

The drug nitisinone (NTBC; Orfadin, Vienna, Austria) has been used for the treatment of hereditary tyrosinemia type-1 since 1991. Nitisinone effectively blocks the metabolism of tyrosine to prevent the formation of the toxic compound succinylacetone (and precursor fumarylacetoacetate) in affected children. Monitoring of plasma drug levels and urine succinylacetone can be used to assess compliance and adequate dose of drug. We present retrospective data from patient monitoring for over 10 years that provide validation of a target therapeutic range for nitisinone of 40 to 60 μmol/L. The target nitisinone range is justified as valid based on reduction of succinylacetone excretion. There was no statistical significance in succinylacetone excretion in mmol/mol creatinine above a level of 40 μmol/L plasma NTBC (P > 0.05).

Detection of mucopolysaccharidosis III-A (Sanfilippo Syndrome-A) in dried blood spots (DBS) by tandem mass spectrometry

BACKGROUND

With ongoing efforts to develop improved treatments for Sanfilippo Syndrome Type A (MPS-IIIA), a disease caused by the inability to degrade heparan sulfate in lysosomes, we sought to develop an enzymatic activity assay for the relevant enzyme, sulfamidase, that uses dried blood spots (DBS).

METHODS

We designed and synthesized a new sulfamidase substrate that can be used to measure sulfamidase activity in DBS using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

RESULTS

Sulfamidase activity was readily detected in DBS using the new substrate and LC-MS/MS. Sulfamidase activity showed acceptable linearity proportional to the amount of enzyme and reaction time. Sulfamidase activity in 238 random newborns was well elevated compared to the range of activities measured in DBS from 8 patients previously confirmed to have MPS-IIIA.

CONCLUSIONS

This is the first report of an assay capable of detecting sulfamidase in DBS. The new assay could be useful in diagnosis and potentially for newborn screening of MPS-IIIA.

Multiplex tandem mass spectrometry assay for newborn screening of X-linked adrenoleukodystrophy, biotinidase deficiency, and galactosemia with flexibility to assay other enzyme assays and biomarkers

All States screen for biotinidase deficiency and galactosemia, and X-linked adrenoleukodystrophy (X-ALD) has recently been added to the Recommended Uniform Screening Panel (RUSP).We sought to consolidate these tests by combining them into a single multiplex tandem mass spectrometry assay as well as to improve the current protocol for newborn screening of galactosemia.A 3 mm punch of a dried blood spot (DBS) was extracted with organic solvent for analysis of the C26:0-lysophosphatidylcholine biomarker for X-ALD.An additional punch was used to assay galactose-1-phosphate uridyltransferase (GALT) and biotinidase.All assays were combined for a single injection for analysis by liquid chromatography-tandem mass spectrometry (LC-MS/MS) (2.3 min per sample).The GALT LC-MS/MS assay does not give a false positive for galactosemia if glucose-6-phosphate dehydrogenase is deficient.The multiplex assay shows acceptable reproducibility and provides for rapid analysis of X-ALD, biotinidase deficiency, and galactosemia.The throughput and ease of sample preparation are acceptable for newborn screening laboratories.We also show that the LC-MS/MS assay is expandable to include several other diseases including Pompe and Hurler diseases (enzymatic activities and biomarkers).Because of consolidation of assays, less manpower is needed compared to running individual assays on separate platforms.The flexibility of the LC-MS/MS platform allows each newborn screening laboratory to analyze the set of diseases offered in their panel.

Specific Substrate for the Assay of Lysosomal Acid Lipase

BACKGROUND

Deficiency of lysosomal acid lipase (LAL) causes Wolman disease and cholesterol ester storage disease. With the recent introduction of enzyme replacement therapy to manage LAL deficiency comes the need for a reliable assay of LAL enzymatic activity that can be applied to dried blood spots (DBS).

METHODS

We prepared and tested a library of analogs of palmitoyl 4-methylumbelifferyl esters to find a highly active and specific substrate for LAL in DBS. The LAL assay was optimized leading to both LC-MS/MS and fluorometric assay of LAL. We tested the new assay on DBS from healthy and LAL-deficient patients.

RESULTS

The ester formed between palmitic acid and 4-propyl-8-methyl-7-hydroxycoumarin (P-PMHC) was found to be >98% selective for LAL in DBS based on the sensitivity of its activity to the LAL-specific inactivator Lalistat-2 and the fact that the activity was close to zero using DBS from patients previously shown to be LAL-deficient. Use of P-PMHC and heavy isotope-labeled internal standard with optimized assay conditions led to an approximately 2-fold increase in the specific activity of LAL compared with the previously reported LAL assay. Patients deficient in LAL were readily distinguished from normal persons with the new LAL assay using UPLC-MS/MS or fluorometric assay platforms.

CONCLUSIONS

The new assay can measure LAL in DBS with a single measurement compared with the previous method involving 2 assays done in parallel.

Transformation in pretreatment manifestations of Gaucher disease type 1 during two decades of alglucerase/imiglucerase enzyme replacement therapy in the International Collaborative Gaucher Group (ICGG) Gaucher Registry

This study tests the hypothesis that the prevalence of severe clinical manifestations in Gaucher disease type 1 (GD1) patients at the time of treatment initiation has changed since alglucerase/imiglucerase enzyme replacement therapy (ERT) was approved in the United States (US) in 1991. US alglucerase/imiglucerase‐treated GD1 patients from the International Collaborative Gaucher Group Gaucher Registry clinicaltrials.gov NCT00358943 were stratified by age at ERT initiation (<18, 18 to <50, ≥50 years), era of ERT initiation (1991‐1995, 1996‐2000, 2001‐2005, 2006‐2009), and splenectomy status pre‐ERT. Prevalence of splenectomy decreased dramatically across the eras among all age groups. Bone manifestations were more prevalent in splenectomized patients than non‐splenectomized patients in all age groups. Prevalence of bone manifestations differed across eras in certain age groups: non‐splenectomized patients had a lower prevalence of ischemic bone events (pediatric patients) and bone crisis (pediatric patients and adults 18 to <50 years) in later eras; splenectomized adult (18 to <50 years) patients had a lower prevalence of ischemic bone events and bone crisis in later eras. Over two decades after the introduction of ERT, the prevalence of splenectomy and associated skeletal complications has declined dramatically. Concomitantly, the interval between diagnosis and initiation of ERT has decreased, most strikingly in pediatric patients who have the most severe disease. Together, these findings suggest that since the introduction of alglucerase/imiglucerase ERT, optimal standard of care has become established in the US to prevent destructive complications of GD1.

SNPs, linkage disequilibrium, and chronic mountain sickness in Tibetan Chinese

Chronic mountain sickness (CMS) is estimated at 1.2% in Tibetans living at the Qinghai-Tibetan Plateau. Eighteen single-nucleotide polymorphisms (SNPs) from nine nuclear genes that have an association with CMS in Tibetans have been analyzed by using pairwise linkage disequilibrium (LD). The SNPs included are the angiotensin-converting enzyme (rs4340), the angiotensinogen (rs699), and the angiotensin II type 1 receptor (AGTR1) (rs5186) from the renin-angiotensin system. A low-density lipoprotein apolipoprotein B (rs693) SNP was also included. From the hypoxia-inducible factor oxygen signaling pathway, the endothetal Per-Arnt-Sim domain protein 1 (EPAS1) and the egl nine homolog 1 (ENGL1) (rs480902) SNPs were included in the study. SNPs from the vascular endothelial growth factor (VEGF) signaling pathway included are the v-akt murine thymoma viral oncogene homolog 3 (rs4590656 and rs2291409), the endothelial cell nitric oxide synthase 3 (rs1007311 and rs1799983), and the (VEGFA) (rs699947, rs34357231, rs79469752, rs13207351, rs28357093, rs1570360, rs2010963, and rs3025039). An increase in LD occurred in 40 pairwise comparisons, whereas a decrease in LD was found in 55 pairwise comparisons between the controls and CMS patients. These changes were found to occur within and between signaling pathways, which suggests that there is an interaction between SNP alleles from different areas of the genome that affect CMS.

Newborn Screening for Pompe Disease

Started in 1963 by Robert Guthrie, newborn screening (NBS) is considered to be one of the great public health achievements. Its original goal was to screen newborns for conditions that could benefit from presymptomatic treatment, thereby reducing associated morbidity and mortality. With advances in technology, the number of disorders included in NBS programs increased. Pompe disease is a good candidate for NBS. Because decisions regarding which diseases should be included in NBS panels are made regionally and locally, programs and efforts for NBS for Pompe disease have been inconsistent both in the United States and globally. In this article, published in the "Newborn Screening, Diagnosis, and Treatment for Pompe Disease" guidance supplement, the Pompe Disease Newborn Screening Working Group, an international group of experts in both NBS and Pompe disease, review the methods used for NBS for Pompe disease and summarize results of current and ongoing NBS programs in the United States and other countries. Challenges and potential drawbacks associated with NBS also are discussed.

Lymphocyte Galactocerebrosidase Activity by LC-MS/MS for Post-Newborn Screening Evaluation of Krabbe Disease

BACKGROUND

Deficiency of the lysosomal enzyme galactosylcerebrosidase (GALC) causes Krabbe disease. Newborn screening for Krabbe disease is ongoing, but improved methods for follow-up analysis of screen-positive babies are needed to better advise families and to optimize treatment. We report a new assay for the enzymatic activity of GALC in lymphocytes.

METHODS

T lymphocytes were isolated from venous blood by magnetic bead technology. The assay used a close structural analog of the natural substrate and LC-MS/MS to quantify the amount of product with the aid of a chemically identical internal standard.

RESULTS

The analytical range of the assay (ratio of assay response for the QC high standard to that from all non-enzymatic-dependent processes) was 20-fold greater than that for the conventional radiometric GALC assay. The LC-MS/MS could distinguish cells that were null in GALC from those that contained traces of active enzyme (down to 0.3% of normal). There was a good correlation between the level of residual GALC activity in lymphocytes and the severity of Krabbe disease.

CONCLUSIONS

The new assay can measure small amounts of residual GALC activity in leukocytes with high accuracy compared to previous assays and can contribute, along with genotyping, biomarker analysis, and neurological imaging, a better plan for post-newborn screening follow-up for Krabbe disease.

Multiplex Tandem Mass Spectrometry Enzymatic Activity Assay for Newborn Screening of the Mucopolysaccharidoses and Type 2 Neuronal Ceroid Lipofuscinosis

BACKGROUND

We expanded the use of tandem mass spectrometry combined with liquid chromatography (LC-MS/MS) for multiplex newborn screening of seven lysosomal enzymes in dried blood spots (DBS). The new assays are for enzymes responsible for the mucopolysaccharidoses (MPS-I, -II, -IIIB, -IVA, -VI, and -VII) and type 2 neuronal ceroid lipofuscinosis (LINCL).

METHODS

New substrates were prepared and characterized for tripeptidyl peptidase 1 (TPP1), α-N-acetylglucosaminidase (NAGLU), and lysosomal β-glucuronidase (GUSB). These assays were combined with previously developed assays to provide a multiplex LC-MS/MS assay of 7 lysosomal storage diseases. Multiple reaction monitoring of ion dissociations for enzyme products and deuterium-labeled internal standards was used to quantify the enzyme activities.

RESULTS

Deidentified DBS samples from 62 nonaffected newborns were analyzed to simultaneously determine (run time 2 min per DBS) the activities of TPP1, NAGLU, and GUSB, along with those for α-iduronidase (IDUA), iduronate-2-sulfatase (I2S), N-acetylgalactosamine-6-sulfatase (GALNS), and N-acetylgalactosamine-4-sulfatase (ARSB). The activities measured in the 7-plex format showed assay response-to-blank-activity ratios (analytical ranges) of 102-909 that clearly separated healthy infants from affected children.

CONCLUSIONS

The new multiplex assay provides a robust comprehensive newborn screening assay for the mucopolysaccharidoses. The method has been expanded to include additional lysosomal storage diseases.

Urinary Podocyte Loss Is Increased in Patients with Fabry Disease and Correlates with Clinical Severity of Fabry Nephropathy

Chronic kidney disease is a major complication of Fabry disease. Podocytes accumulate globotriaosylceramide inclusions more than other kidney cell types in Fabry patients. Podocyte injury occurs early in age, and is progressive. Since injured podocytes detach into the urine (podocyturia), we hypothesized that podocyturia would increase in Fabry patients and correlate with clinical severity of Fabry nephropathy. Urine specimens from 39 Fabry patients and 24 healthy subjects were evaluated for podocyturia. Most of the Fabry patients and many healthy subjects had podocyturia. The number of podocytes per gram of urine creatinine (UPodo/g Cr) was 3.6 fold greater in Fabry patients (3,741 ± 2796; p = 0.001) than healthy subjects (1,040 ± 972). Fabry patients with normoalbuminuria and normoproteinuria had over 2-fold greater UPodo/g Cr than healthy subjects (p = 0.048). UPodo/gCr was inversely related to eGFR in male patients (r = -0.69, p = 0.003). UPodo/gCr was directly related to urine protein creatinine ratio (r = 0.33; p = 0.04) in all Fabry patients. These studies confirm increased podocyturia in Fabry disease, even when proteinuria and albuminuria are absent. Podocyturia correlates with clinical severity of Fabry nephropathy, and potentially may be of prognostic value.

Treatment of Fabry's Disease with the Pharmacologic Chaperone Migalastat

BACKGROUND

Fabry's disease, an X-linked disorder of lysosomal α-galactosidase deficiency, leads to substrate accumulation in multiple organs. Migalastat, an oral pharmacologic chaperone, stabilizes specific mutant forms of α-galactosidase, increasing enzyme trafficking to lysosomes.

METHODS

The initial assay of mutant α-galactosidase forms that we used to categorize 67 patients with Fabry's disease for randomization to 6 months of double-blind migalastat or placebo (stage 1), followed by open-label migalastat from 6 to 12 months (stage 2) plus an additional year, had certain limitations. Before unblinding, a new, validated assay showed that 50 of the 67 participants had mutant α-galactosidase forms suitable for targeting by migalastat. The primary end point was the percentage of patients who had a response (≥50% reduction in the number of globotriaosylceramide inclusions per kidney interstitial capillary) at 6 months. We assessed safety along with disease substrates and renal, cardiovascular, and patient-reported outcomes.

RESULTS

The primary end-point analysis, involving patients with mutant α-galactosidase forms that were suitable or not suitable for migalastat therapy, did not show a significant treatment effect: 13 of 32 patients (41%) who received migalastat and 9 of 32 patients (28%) who received placebo had a response at 6 months (P=0.30). Among patients with suitable mutant α-galactosidase who received migalastat for up to 24 months, the annualized changes from baseline in the estimated glomerular filtration rate (GFR) and measured GFR were -0.30±0.66 and -1.51±1.33 ml per minute per 1.73 m(2) of body-surface area, respectively. The left-ventricular-mass index decreased significantly from baseline (-7.7 g per square meter; 95% confidence interval [CI], -15.4 to -0.01), particularly when left ventricular hypertrophy was present (-18.6 g per square meter; 95% CI, -38.2 to 1.0). The severity of diarrhea, reflux, and indigestion decreased.

CONCLUSIONS

Among all randomly assigned patients (with mutant α-galactosidase forms that were suitable or not suitable for migalastat therapy), the percentage of patients who had a response at 6 months did not differ significantly between the migalastat group and the placebo group. (Funded by Amicus Therapeutics; ClinicalTrials.gov numbers, NCT00925301 [study AT1001-011] and NCT01458119 [study AT1001-041].).

Silent Tyrosinemia Type I Without Elevated Tyrosine or Succinylacetone Associated with Liver Cirrhosis and Hepatocellular Carcinoma

Tyrosinemia type I (TYRSN1, TYR I) is caused by fumarylacetoacetate hydrolase (FAH) deficiency and affects approximately one in 100,000 individuals worldwide. Pathogenic variants in FAH cause TYRSN1, which induces cirrhosis and can progress to hepatocellular carcinoma (HCC). TYRSN1 is characterized by the production of a pathognomonic metabolite, succinylacetone (SUAC) and is included in the Recommended Uniform Screening Panel for newborns. Treatment intervention is effective if initiated within the first month of life. Here, we describe a family with three affected children who developed HCC secondary to idiopathic hepatosplenomegaly and cirrhosis during infancy. Whole exome sequencing revealed a novel homozygous missense variant in FAH (Chr15(GRCh38):g.80162305A>G; NM_000137.2:c.424A > G; NP_000128.1:p.R142G). This novel variant involves the catalytic pocket of the enzyme, but does not result in increased SUAC or tyrosine, making the diagnosis of TYRSN1 problematic. Testing this novel variant using a rapid, in vivo somatic mouse model showed that this variant could not rescue FAH deficiency. In this case of atypical TYRSN1, we show how reliance on SUAC as a primary diagnostic test can be misleading in some patients with this disease. Augmentation of current screening for TYRSN1 with targeted sequencing of FAH is warranted in cases suggestive of the disorder.

Pilot study of newborn screening for six lysosomal storage diseases using Tandem Mass Spectrometry

BACKGROUND

There is current expansion of newborn screening (NBS) programs to include lysosomal storage disorders because of the availability of treatments that produce an optimal clinical outcome when started early in life.

OBJECTIVE

To evaluate the performance of a multiplex-tandem mass spectrometry (MS/MS) enzymatic activity assay of 6 lysosomal enzymes in a NBS laboratory for the identification of newborns at risk for developing Pompe, Mucopolysaccharidosis-I (MPS-I), Fabry, Gaucher, Niemann Pick-A/B, and Krabbe diseases.

METHODS AND RESULTS

Enzyme activities (acid α-glucosidase (GAA), galactocerebrosidase (GALC), glucocerebrosidase (GBA), α-galactosidase A (GLA), α-iduronidase (IDUA) and sphingomyeline phosphodiesterase-1 (SMPD-1)) were measured on ~43,000 de-identified dried blood spot (DBS) punches, and screen positive samples were submitted for DNA sequencing to obtain genotype confirmation of disease risk. The 6-plex assay was efficiently performed in the Washington state NBS laboratory by a single laboratory technician at the bench using a single MS/MS instrument. The number of screen positive samples per 100,000 newborns were as follows: GAA (4.5), IDUA (13.6), GLA (18.2), SMPD1 (11.4), GBA (6.8), and GALC (25.0).

DISCUSSION

A 6-plex MS/MS assay for 6 lysosomal enzymes can be successfully performed in a NBS laboratory. The analytical ranges (enzyme-dependent assay response for the quality control HIGH sample divided by that for all enzyme-independent processes) for the 6-enzymes with the MS/MS is 5- to 15-fold higher than comparable fluorimetric assays using 4-methylumbelliferyl substrates. The rate of screen positive detection is consistently lower for the MS/MS assay compared to the fluorimetric assay using a digital microfluidics platform.

Dataset and standard operating procedure for newborn screening of six lysosomal storage diseases: By tandem mass spectrometry

In this data article we provide a detailed standard operating procedure for performing a tandem mass spectrometry, multiplex assay of 6 lysosomal enzymes for newborn screening of the lysosomal storage diseases Mucopolysaccharidosis-I, Pompe, Fabry, Niemann-Pick-A/B, Gaucher, and Krabbe, (Elliott, et al., 2016) [1]. We also provide the mass spectrometry peak areas for the product and internal standard ions typically observed with a dried blood spot punch from a random newborn, and we provide the daily variation of the daily mean activities for all 6 enzymes.

Sulfatide Analysis by Mass Spectrometry for Screening of Metachromatic Leukodystrophy in Dried Blood and Urine Samples

BACKGROUND

Metachromatic leukodystrophy (MLD) is an autosomal recessive disorder caused by deficiency in arylsulfatase A activity, leading to accumulation of sulfatide substrates. Diagnostic and monitoring procedures include demonstration of reduced arylsulfatase A activity in peripheral blood leukocytes or detection of sulfatides in urine. However, the development of a screening test is challenging because of instability of the enzyme in dried blood spots (DBS), the widespread occurrence of pseudodeficiency alleles, and the lack of available urine samples from newborn screening programs.

METHODS

We measured individual sulfatide profiles in DBS and dried urine spots (DUS) from MLD patients with LC-MS/MS to identify markers with the discriminatory power to differentiate affected individuals from controls. We also developed a method for converting all sulfatide molecular species into a single species, allowing quantification in positive-ion mode upon derivatization.

RESULTS

In DBS from MLD patients, we found up to 23.2-fold and 5.1-fold differences in total sulfatide concentrations for early- and late-onset MLD, respectively, compared with controls and pseudodeficiencies. Corresponding DUS revealed up to 164-fold and 78-fold differences for early- and late-onset MLD patient samples compared with controls. The use of sulfatides converted to a single species simplified the analysis and increased detection sensitivity in positive-ion mode, providing a second option for sulfatide analysis.

CONCLUSIONS

This study of sulfatides in DBS and DUS suggests the feasibility of the mass spectrometry method for newborn screening of MLD and sets the stage for a larger-scale newborn screening pilot study.

Tandem Mass Spectrometry Has a Larger Analytical Range than Fluorescence Assays of Lysosomal Enzymes: Application to Newborn Screening and Diagnosis of Mucopolysaccharidoses Types II, IVA, and VI

BACKGROUND

There is interest in newborn screening and diagnosis of lysosomal storage diseases because of the development of treatment options that improve clinical outcome. Assays of lysosomal enzymes with high analytical range (ratio of assay response from the enzymatic reaction divided by the assay response due to nonenzymatic processes) are desirable because they are predicted to lead to a lower rate of false positives in population screening and to more accurate diagnoses.

METHODS

We designed new tandem mass spectrometry (MS/MS) assays that give the largest analytical ranges reported to date for the use of dried blood spots (DBS) for detection of mucopolysaccharidoses type II (MPS-II), MPS-IVA, and MPS-VI. For comparison, we carried out fluorometric assays of 6 lysosomal enzymes using 4-methylumbelliferyl (4MU)-substrate conjugates.

RESULTS

The MS/MS assays for MPS-II, -IVA, and -VI displayed analytical ranges that are 1-2 orders of magnitude higher than those for the corresponding fluorometric assays. The relatively small analytical ranges of the 4MU assays are due to the intrinsic fluorescence of the 4MU substrates, which cause high background in the assay response.

CONCLUSIONS

These highly reproducible MS/MS assays for MPS-II, -IVA, and -VI can support multiplex newborn screening of these lysosomal storage diseases. MS/MS assays of lysosomal enzymes outperform 4MU fluorometric assays in terms of analytical range. Ongoing pilot studies will allow us to gauge the impact of the increased analytical range on newborn screening performance.

Fluorimetric assays for N-acetylgalactosamine-6-sulfatase and arylsulfatase B based on the natural substrates for confirmation of mucopolysaccharidoses types IVA and VI

BACKGROUND

Treatments have been developed for mucopolysaccharidoses IVA (MPS IVA) and MPS VI suggesting the need for eventual newborn screening. Biochemical enzyme assays are important for diagnosis. Previously reported fluorimetric assays of the relevant enzymes are based on substrates with poor activity or specificity.

METHODS

We developed new fluorimetric assays for N-acetylgalactosamine-6-sulfatase (GALNS) and arylsulfatase B (ARSB) based on the natural substrates, N-acetylgalactosamine-6-sulfate (and 4-sulfate), which have improved activity and specificity toward the relevant enzymes. The new substrates were tested on dried blood spots on newborn screening cards, and assays showed acceptable linearity in response with the amount of enzyme present (using quality control samples).

RESULTS

When tested on dried blood spots from random newborns and affected patients, the assays showed good discrimination between the 2 sample groups.

CONCLUSIONS

The analytical range of the new fluorimetric assays, defined as the ratio of enzyme-dependent-to-enzyme-independent assay response, is likely to be insufficient to use these assays for newborn screening. Rather, these new fluorimetric assays should be useful in a diagnostic lab to confirm a diagnosis via biochemical enzyme testing.

Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors

The objective of this document is to provide recommendations for genetic counseling and screening for consanguineous couples (related as second cousins or closer) and their offspring with the goals of1. providing preconception reproductive options2. improving pregnancy outcome and identifying reproductive choices3. reducing morbidity and mortality in the 1st years of life, and4. respecting psychosocial and multicultural issues.The recommendations are the opinions of a multicenter working group (the Consanguinity Working Group (CWG)) with expertise in genetic counseling, medical genetics, biochemical genetics, genetic epidemiology, pediatrics, perinatology, and public health genetics, which was convened by the National Society of Genetic Counselors (NSGC). The consensus of the CWG and NSGC reviewers is that beyond a thorough medical family history with follow-up of significant findings, no additional preconception screening is recommended for consanguineous couples. Consanguineous couples should be offered similar genetic screening as suggested for any couple of their ethnic group. During pregnancy, consanguineous couples should be offered maternal-fetal serum marker screening and high-resolution fetal ultrasonography. Newborns should be screened for impaired hearing and detection of treatable inborn errors of metabolism. These recommendations should not be construed as dictating an exclusive course of management, nor does use of such recommendations guarantee a particular outcome. The professional judgment of a health care provider, familiar with the facts and circumstances of a specific case, will always supersede these recommendations.

Long-term treatment outcomes in Gaucher disease

Following the treatment of the first Gaucher disease patient with enzyme replacement therapy (ERT), it was clear that ERT had the potential to be transformative with dramatic improvements in systemic manifestations of the disease within 2 years. Following over 20 years existence of the International Collaborative Gaucher Group Gaucher Registry and evidence from ∼6000 patients, the long-term effects of therapy have been documented. It has been shown that ERT can result in improvements in all clinical and laboratory parameters of nonneuronopathic disease. However, different aspects of the disease, such as hematologic parameters, organ volumes and bone disease do not necessarily respond to therapy at the same rate or to the same extent, and this has had major implications for disease monitoring and for the establishment of therapeutic goals for ERT. Response may be affected by factors such as the timing of therapy initiation, the presence of irreversible complications such as osteonecrosis, and by enzyme dose. It is also apparent that ERT has no impact on neurological aspects of disease and highlights the need for additional or alternative treatment strategies able to meet the needs of patients with neuronopathic disease.

Detecting the effects of Fabry disease in the adult human brain with diffusion tensor imaging and fast bound-pool fraction imaging

BACKGROUND

To identify quantitative MRI parameters associated with diffusion tensor imaging (DTI) and fast bound-pool fraction imaging (FBFI) that may detect alterations in gray matter and/or white matter in adults with Fabry disease, a lysosomal storage disorder.

MATERIALS AND METHODS

Twelve healthy controls (mean age ± standard deviation: 48.0 ± 12.4 years) and 10 participants with Fabry disease (46.7 ± 12.9 years) were imaged at 3.0 Tesla. Whole-brain parametric maps of diffusion tensor metrics (apparent diffusion coefficient [ADC] and fractional anisotropy [FA]) and the bound-pool fraction (f) were acquired. Mean voxel values of parametric maps from regions-of-interest within gray and white matter structures were compared between cases and controls using the independent t-test. Spearman's rho was used to identify associations between parametric maps and age.

RESULTS

Compared with controls, the left thalamus of Fabry participants had an increase in FA (0.29 ± 0.02 versus 0.33 ± 0.05, respectively; P = 0.030) and a trend toward an increase in ADC (0.73 ± 00.02 versus 0.76 ± 0.03 μm(2) /s, respectively; P = 0.082). The left posterior white matter demonstrated a reduction in f (10.45 ± 0.37 versus 9.00 ± 1.84%, respectively; P = 0.035), an increase in ADC (0.78 ± 0.04 versus 0.94 ± 0.19 μm(2) /s, respectively; P = 0.024), and a trend toward a reduction in FA (0.42 ± 0.07 versus 0.36 ± 0.08, respectively; P = 0.052). Among all parameters, only f measured in the left posterior white matter was significantly associated with age in Fabry participants (rho = -0.71; P = 0.022).

CONCLUSION

Parameters derived from DTI and FBFI detect Fabry-related changes in the adult human brain, particularly in the posterior white matter where reductions in myelin density as measured by FBFI appear age related.

Characterization of early disease status in treatment-naive male paediatric patients with Fabry disease enrolled in a randomized clinical trial

Trial Design

This analysis characterizes the degree of early organ involvement in a cohort of oligo-symptomatic untreated young patients with Fabry disease enrolled in an ongoing randomized, open-label, parallel-group, phase 3B clinical trial.

Methods

Males aged 5–18 years with complete α-galactosidase A deficiency, without symptoms of major organ damage, were enrolled in a phase 3B trial evaluating two doses of agalsidase beta. Baseline disease characteristics of 31 eligible patients (median age 12 years) were studied, including cellular globotriaosylceramide (GL-3) accumulation in skin (n = 31) and kidney biopsy (n = 6; median age 15 years; range 13–17 years), renal function, and glycolipid levels (plasma, urine).

Results

Plasma and urinary GL-3 levels were abnormal in 25 of 30 and 31 of 31 patients, respectively. Plasma lyso-GL-3 was elevated in all patients. GL-3 accumulation was documented in superficial skin capillary endothelial cells (23/31 patients) and deep vessel endothelial cells (23/29 patients). The mean glomerular filtration rate (GFR), measured by plasma disappearance of iohexol, was 118.1 mL/min/1.73 m² (range 90.4–161.0 mL/min/1.73 m²) and the median urinary albumin/creatinine ratio was 10 mg/g (range 4.0–27.0 mg/g). On electron microscopy, renal biopsy revealed GL-3 accumulation in all glomerular cell types (podocytes and parietal, endothelial, and mesangial cells), as well as in peritubular capillary and non-capillary endothelial, interstitial, vascular smooth muscle, and distal tubules/collecting duct cells. Lesions indicative of early Fabry arteriopathy and segmental effacement of podocyte foot processes were found in all 6 patients.

Conclusions

These data reveal that in this small cohort of children with Fabry disease, histological evidence of GL-3 accumulation, and cellular and vascular injury are present in renal tissues at very early stages of the disease, and are noted before onset of microalbuminuria and development of clinically significant renal events (e.g. reduced GFR). These data give additional support to the consideration of early initiation of enzyme replacement therapy, potentially improving long-term outcome.

Trial Registration

ClinicalTrials.gov NCT00701415

Ten-year outcome of enzyme replacement therapy with agalsidase beta in patients with Fabry disease

Background

Fabry disease results from deficient α-galactosidase A activity and globotriaosylceramide accumulation causing renal insufficiency, strokes, hypertrophic cardiomyopathy and early demise. We assessed the 10-year outcome of recombinant α-galactosidase A therapy.

Methods

The outcomes (severe clinical events, renal function, cardiac structure) of 52/58 patients with classic Fabry disease from the phase 3 clinical trial and extension study, and the Fabry Registry were evaluated. Disease progression rates for patients with low renal involvement (LRI, n=32) or high renal involvement (HRI, n=20) at baseline were assessed.

Results

81% of patients (42/52) did not experience any severe clinical event during the treatment interval and 94% (49/52) were alive at the end of the study period. Ten patients reported a total of 16 events. Patients classified as LRI started therapy 13 years younger than HRI (mean 25 years vs 38 years). Mean slopes for estimated glomerular filtration rate for LRI and HRI were −1.89 mL/min/1.73 m²/year and −6.82 mL/min/1.73 m²/year, respectively. Overall, the mean left ventricular posterior wall thickness and interventricular septum thickness remained unchanged and normal. Patients who initiated treatment at age ≥40 years exhibited significant increase in left ventricular posterior wall thickness and interventricular septum thickness. Mean plasma globotriaosylceramide normalised within 6 months.

Conclusions

This 10-year study documents the effectiveness of agalsidase beta (1 mg/kg/2 weeks) in patients with Fabry disease. Most patients remained alive and event-free. Patients who initiated treatment at a younger age and with less kidney involvement benefited the most from therapy. Patients who initiated treatment at older ages and/or had advanced renal disease experienced disease progression.

Ethical considerations of population screening for late-onset genetic disease

Population-based genetic screening has been a mainstay of public health in the United States for many years. The goal of genetic screening is to identify individuals at increased risk for treatable diseases. The evolution of genetic testing to include multi-disease panels allows for new screening applications which challenge the traditional model of clinical genetics care by the identification of late-onset disorders in an asymptomatic fetus, child, or adult. We present two unique examples of individuals referred to a biochemical genetics clinic due to the detection of late-onset Pompe disease by population-based screening modalities. We review early experiences in counseling and management of pre-symptomatic individuals and highlight some of the primary ethical factors warranting consideration as we enter the era of genomic medicine.

Actionable exomic incidental findings in 6503 participants: challenges of variant classification

Recommendations for laboratories to report incidental findings from genomic tests have stimulated interest in such results. In order to investigate the criteria and processes for assigning the pathogenicity of specific variants and to estimate the frequency of such incidental findings in patients of European and African ancestry, we classified potentially actionable pathogenic single-nucleotide variants (SNVs) in all 4300 European- and 2203 African-ancestry participants sequenced by the NHLBI Exome Sequencing Project (ESP). We considered 112 gene-disease pairs selected by an expert panel as associated with medically actionable genetic disorders that may be undiagnosed in adults. The resulting classifications were compared to classifications from other clinical and research genetic testing laboratories, as well as with in silico pathogenicity scores. Among European-ancestry participants, 30 of 4300 (0.7%) had a pathogenic SNV and six (0.1%) had a disruptive variant that was expected to be pathogenic, whereas 52 (1.2%) had likely pathogenic SNVs. For African-ancestry participants, six of 2203 (0.3%) had a pathogenic SNV and six (0.3%) had an expected pathogenic disruptive variant, whereas 13 (0.6%) had likely pathogenic SNVs. Genomic Evolutionary Rate Profiling mammalian conservation score and the Combined Annotation Dependent Depletion summary score of conservation, substitution, regulation, and other evidence were compared across pathogenicity assignments and appear to have utility in variant classification. This work provides a refined estimate of the burden of adult onset, medically actionable incidental findings expected from exome sequencing, highlights challenges in variant classification, and demonstrates the need for a better curated variant interpretation knowledge base.

Newborn screening for lysosomal storage diseases

BACKGROUND

There is worldwide interest in newborn screening for lysosomal storage diseases because of the development of treatment options that give better results when carried out early in life. Screens with high differentiation between affected and nonaffected individuals are critical because of the large number of potential false positives.

CONTENT

This review summarizes 3 screening methods: (a) direct assay of enzymatic activities using tandem mass spectrometry or fluorometry, (b) immunocapture-based measurement of lysosomal enzyme abundance, and (c) measurement of biomarkers. Assay performance is compared on the basis of small-scale studies as well as on large-scale pilot studies of mass spectrometric and fluorometric screens.

SUMMARY

Tandem mass spectrometry and fluorometry techniques for direct assay of lysosomal enzymatic activity in dried blood spots have emerged as the most studied approaches. Comparative mass spectrometry vs fluorometry studies show that the former better differentiates between nonaffected vs affected individuals. This in turn leads to a manageable number of screen positives that can be further evaluated with second-tier methods.

Congenital sucrase-isomaltase deficiency: identification of a common Inuit founder mutation

BACKGROUND

Congenital sucrase-isomaltase deficiency is a rare hereditary cause of chronic diarrhea in children. People with this condition lack the intestinal brush-border enzyme required for digestion of di- and oligosaccharides, including sucrose and isomaltose, leading to malabsorption. Although the condition is known to be highly prevalent (about 5%-10%) in several Inuit populations, the genetic basis for this has not been described. We sought to identify a common mutation for congenital sucrase-isomaltase deficiency in the Inuit population.

METHODS

We sequenced the sucrase-isomaltase gene, SI, in a single Inuit proband with congenital sucrase-isomaltase deficiency who had severe fermentative diarrhea and failure to thrive. We then genotyped a further 128 anonymized Inuit controls from a variety of locales in the Canadian Arctic to assess for a possible founder effect.

RESULTS

In the proband, we identified a novel, homozygous frameshift mutation, c.273_274delAG (p.Gly92Leufs*8), predicted to result in complete absence of a functional protein product. This change was very common among the Inuit controls, with an observed allele frequency of 17.2% (95% confidence interval [CI] 12.6%-21.8%). The predicted Hardy-Weinberg prevalence of congenital sucrase-isomaltase deficiency in Inuit people, based on this single founder allele, is 3.0% (95% CI 1.4%-4.5%), which is comparable with previous estimates.

INTERPRETATION

We found a common mutation, SI c.273_274delAG, to be responsible for the high prevalence of congenital sucrase-isomaltase deficiency among Inuit people. Targeted mutation testing for this allele should afford a simple and minimally invasive means of diagnosing this condition in Inuit patients with chronic diarrhea.

Tandem mass spectrometry assays of palmitoyl protein thioesterase 1 and tripeptidyl peptidase activity in dried blood spots for the detection of neuronal ceroid lipofuscinoses in newborns

We report new substrates for quantitative enzyme activity measurements of human palmitoyl protein thioesterase (PPT1) and tripeptidyl peptidase (TPP1) in dried blood spots from newborns using tandem mass spectrometry. Deficiencies in these enzyme activities due to inborn errors of metabolism cause neuronal ceroid lipofuscinoses. The assays use synthetic compounds that were designed to mimic the natural substrates. Incubation produces nanomole quantities of enzymatic products per a blood spot that are quantified by tandem mass spectrometry using synthetic internal standards and selected reaction monitoring. The assays utilize a minimum steps for sample workup and can be run in a duplex format for the detection of neuronal ceroid lipofuscinoses or potentially multiplexed with other mass spectrometry-based assays for newborn screening of lysosomal storage disorders.

Quantification of sulfatides in dried blood and urine spots from metachromatic leukodystrophy patients by liquid chromatography/electrospray tandem mass spectrometry

BACKGROUND

Treatments are being developed for metachromatic leukodystrophy (MLD), suggesting the need for eventual newborn screening. Previous studies have shown that sulfatide molecular species are increased in the urine of MLD patients compared to samples from non-MLD individuals, but there is no data using dried blood spots (DBS), the most common sample available for newborn screening laboratories.

METHODS

We used ultra-high performance liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS) to quantify sulfatides in DBS and dried urine spots from 14 MLD patients and 50 non-MLD individuals.

RESULTS

Several sulfatide molecular species were increased in dried urine samples from all MLD samples compared to non-MLD samples. Sulfatides, especially low molecular species, were increased in DBS from MLD patients, but the sulfatide levels were relatively low. There was good separation in sulfatide levels between MLD and non-MLD samples when dried urine spots were used, but not with DBS, because DBS from non-MLD individuals have measurable levels of sulfatides.

CONCLUSION

Sulfatide accumulation studies in urine, but not in DBS, emerges as the method of choice if newborn screening is to be proposed for MLD.

Improved reagents for newborn screening of mucopolysaccharidosis types I, II, and VI by tandem mass spectrometry

Tandem mass spectrometry for the multiplex and quantitative analysis of enzyme activities in dried blood spots on newborn screening cards has emerged as a powerful technique for early assessment of lysosomal storage diseases. Here we report the design and process-scale synthesis of substrates for the enzymes α-l-iduronidase, iduronate-2-sulfatase, and N-acetylgalactosamine-4-sulfatase that are used for newborn screening of mucopolysaccharidosis types I, II, and VI. The products contain a bisamide unit that is hypothesized to readily protonate in the gas phase, which improves detection sensitivity by tandem mass spectrometry. The products contain a benzoyl group, which provides a useful site for inexpensive deuteration, thus facilitating the preparation of internal standards for the accurate quantification of enzymatic products. Finally, the reagents are designed with ease of synthesis in mind, thus permitting scale-up preparation to support worldwide newborn screening of lysosomal storage diseases. The new reagents provide the most sensitive assay for the three lysosomal enzymes reported to date as shown by their performance in reactions using dried blood spots as the enzyme source. Also, the ratio of assay signal to that measured in the absence of blood (background) is superior to all previously reported mucopolysaccharidosis types I, II, and VI assays.

Actionable, pathogenic incidental findings in 1,000 participants' exomes

The incorporation of genomics into medicine is stimulating interest on the return of incidental findings (IFs) from exome and genome sequencing. However, no large-scale study has yet estimated the number of expected actionable findings per individual; therefore, we classified actionable pathogenic single-nucleotide variants in 500 European- and 500 African-descent participants randomly selected from the National Heart, Lung, and Blood Institute Exome Sequencing Project. The 1,000 individuals were screened for variants in 114 genes selected by an expert panel for their association with medically actionable genetic conditions possibly undiagnosed in adults. Among the 1,000 participants, 585 instances of 239 unique variants were identified as disease causing in the Human Gene Mutation Database (HGMD). The primary literature supporting the variants' pathogenicity was reviewed. Of the identified IFs, only 16 unique autosomal-dominant variants in 17 individuals were assessed to be pathogenic or likely pathogenic, and one participant had two pathogenic variants for an autosomal-recessive disease. Furthermore, one pathogenic and four likely pathogenic variants not listed as disease causing in HGMD were identified. These data can provide an estimate of the frequency (∼3.4% for European descent and ∼1.2% for African descent) of the high-penetrance actionable pathogenic or likely pathogenic variants in adults. The 23 participants with pathogenic or likely pathogenic variants were disproportionately of European (17) versus African (6) descent. The process of classifying these variants underscores the need for a more comprehensive and diverse centralized resource to provide curated information on pathogenicity for clinical use to minimize health disparities in genomic medicine.

Identification of infants at risk for developing Fabry, Pompe, or mucopolysaccharidosis-I from newborn blood spots by tandem mass spectrometry

OBJECTIVE

To assess the performance of a tandem mass spectrometry (MS/MS) technology in a newborn screening laboratory to simultaneously measure α-galactosidase, acid-α-glucosidase, and α-L-iduronidase for the detection of infants at risk to develop Fabry, Pompe, or mucopolysaccharidosis (MPS)-I diseases.

STUDY DESIGN

Enzyme activity was assayed from a 3.2-mm punch from 100,000+ anonymous newborn blood spots. Punches with low enzyme activity were further evaluated by nucleotide sequence analysis of the responsible gene. Confirmation of affected infants was dependent on identification of mutations compatible with diminished enzyme activity.

RESULTS

The technology for simultaneously measuring multiple enzyme activities by MS/MS was successful. The confirmation of diagnosis for Fabry, Pompe, or MPS-I, by DNA sequencing estimated the prevalence of Fabry disease at 1/7800 males (95% CI 1/17,800-1/3600); Pompe disease at 1/27,800 newborns (95% CI 1/90,000-1/10,200); and MPS-I at 1/35,500 newborns (95% CI 1/143,000-1/11,100). These estimates of prevalence are 2 to 4 times greater than the prevalence estimated by clinical diagnosis. The combined prevalence for the 3 disorders was 1/7500 newborns (95% CI 1/13,500-1/4500).

CONCLUSIONS

MS/MS for the simultaneous assay of multiple lysosomal enzymes can be successfully introduced into a routine newborn screening laboratory. The technology has a positive predictive value equal to, or better, than methods currently used for the detection of nonlysosomal disorders. Using newborn blood spots, the combined prevalence of Fabry, Pompe, and MPS-I is estimated at 1/7500 newborns based on low-enzyme activity and confirmation by mutation analysis.

VEGFA SNPs and transcriptional factor binding sites associated with high altitude sickness in Han and Tibetan Chinese at the Qinghai-Tibetan Plateau

Mountain sickness (MS) occurs among humans visiting or inhabiting high altitude environments. We conducted genetic analyses of seven single nucleotide polymorphisms (SNPs) in the promoter region of VEGFA gene for lowland (Han) and highland (Tibetan) Chinese. The seven SNPs were evaluated in Han and Tibetan patients with acute (A) and chronic (C) MS. We compared 64 patients with AMS with 64 Han unaffected with MS, as well as 48 CMS patients with 32 unaffected Tibetans. The SNPs studied are rs699947, rs34357231, rs79469752, rs13207351, rs28357093, rs1570360, and rs2010963 which are found in the promoter ranging from -2,578 to -634 bp from the transcriptional start site (TSS), respectively. Direct sequencing was used to identify individual genotypes for these SNPs. Arterial oxygen saturation of hemoglobin (SaO2) was found to be significantly associated with the rs699947, rs34357231, rs13207351, and rs1570360 SNPs in Han patients with AMS, while the rs2010963 SNP was found to approach significance in the AMS study group, but found to be significantly associated in the normal Tibetan study group. The Han and Tibetan control groups were found to diverge significantly for the rs28357093 and rs2010963 SNPs, as measured by genetic distances of 0.073 and 0.054, respectively. All the SNPs are found in transcriptional factor binding sites (TFBS), and their possible role in gene regulation was evaluated with regard to MS. MS was found to be significantly associated with these SNPs compared with their Han and Tibetan control groups, indicating that these nucleotide substitutions result in TFBS changes which apparently have a physiological effect on the development of high altitude sickness.

High-throughput assay of 9 lysosomal enzymes for newborn screening

BACKGROUND

There is interest in newborn screening of lysosomal storage diseases (LSDs) because of the availability of treatments. Pilot studies have used tandem mass spectrometry with flow injection of samples to achieve multiplex detection of enzyme products. We report a multiplexing method of 9 enzymatic assays that uses HPLC-tandem mass spectrometry (MS/MS).

METHODS

The assay of 9 enzymes was carried out in 1 or 2 buffers with a cassette of substrates and internal standards and 1 or 2 punches of a dried blood spot (DBS) from a newborn screening card as the source of enzymes. The pre-HPLC-MS/MS sample preparation required only 4 liquid transfers before injection into a dual-column HPLC equipped with switching valves to direct the flow to separation and column equilibration. Product-specific and internal standard-specific ion fragmentations were used for MS/MS quantification in the selected reaction monitoring mode.

RESULTS

Analysis of blood spots from 58 random newborns and lysosomal storage disease-affected patients showed that the assay readily distinguished affected from nonaffected individuals. The time per 9-plex analysis (1.8 min) was sufficiently short to be compatible with the workflow of newborn screening laboratories.

CONCLUSIONS

HPLC-MS/MS provides a viable alternative to flow-injection MS/MS for the quantification of lysosomal enzyme activities. It is possible to assay 9 lysosomal enzymes using 1 or 2 reaction buffers, thus minimizing the number of separate incubations necessary.

Effect of nitisinone (NTBC) treatment on the clinical course of hepatorenal tyrosinemia in Québec

BACKGROUND

Hepatorenal tyrosinemia (HT1, fumarylacetoacetate hydrolase deficiency, MIM 276700) can cause severe hepatic, renal and peripheral nerve damage. In Québec, HT1 is frequent and neonatal HT1 screening is practiced. Nitisinone (NTBC, Orfadin ®) inhibits tyrosine degradation prior to the formation of toxic metabolites like succinylacetone and has been offered to HT1 patients in Québec since 1994.

METHODS

We recorded the clinical course of 78 Québec HT1 patients born between 1984 and 2004. There were three groups: those who never received nitisinone (28 patients), those who were first treated after 1 month of age (26 patients) and those treated before 1 month (24 patients). Retrospective chart review was performed for events before 1994, when nitisinone treatment began, and prospective data collection thereafter.

FINDINGS

No hospitalizations for acute complications of HT1 occurred during 5731 months of nitisinone treatment, versus 184 during 1312 months without treatment (p<0.001). Liver transplantation was performed in 20 non-nitisinone-treated patients (71%) at a median age of 26 months, versus 7 late-treated patients (26%, p<0.001), and no early-treated patient (p<0.001). No early-treated patient has developed detectable liver disease after more than 5 years. Ten deaths occurred in non-nitisinone treated patients versus two in treated patients (p<0.01). Both of the latter deaths were from complications of transplantation unrelated to HT1. One probable nitisinone-related event occurred, transient corneal crystals with photophobia.

INTERPRETATION

Nitisinone treatment abolishes the acute complications of HT1. Some patients with established liver disease before nitisinone treatment eventually require hepatic transplantation. Patients who receive nitisinone treatment before 1 month had no detectable liver disease after more than 5 years.

AKT3, ANGPTL4, eNOS3, and VEGFA associations with high altitude sickness in Han and Tibetan Chinese at the Qinghai-Tibetan Plateau

Mountain sickness (MS) occurs among humans visiting or inhabiting high altitude environments. We conducted genetic analyses of the AKT3, ANGPTL4, eNOS3 and VEGFA genes in lowland (Han) and highland (Tibetan) Chinese. Ten single nucleotide polymorphisms (SNPs) were evaluated in Han and Tibetan patients with acute (A) and chronic (C) MS. We compared 74 patients with AMS to 79 Han unaffected with MS, as well as 48 CMS patients to 31 unaffected Tibetans. The ten SNPs studied are AKT3 (rs4590656, rs2291409), ANGPTL4 (rs1044250), eNOS3 (rs1007311, rs1799983) and VEGFA (rs79469752, rs13207351, rs28357093, rs1570360, rs3025039). Direct sequencing was used to identify individual genotypes for these SNPs. Hemoglobin (Hb), hematocrit (Hct), and red blood cell count (RBC) were found to be significantly associated with the AKT3 SNP (rs4590656), Hb was found to be associated with the eNOS3 SNP (rs1007311), and RBC was found to be significantly associated with the VEGFA SNP (rs1570360) in Tibetan patients with CMS. CMS patients were found to diverge significantly for both eNOS3 SNPs as measured by genetic distance (0.042, 0.047) and for the VEGFA SNP (rs28357093) with a genetic distance of 0.078 compared to their Tibetan control group. Heart rate (HR) was found to be significantly associated with the eNOS3 SNP (rs1799983) and arterial oxygen saturation of hemoglobin (SaO2) was found to be significantly associated with the VEGFA SNPs (rs13207351, rs1570360) in Han patients with AMS. The Han and Tibetan control groups were found to diverge significantly for the ANGPTL4 SNP and VEGFA SNP (rs28357093), as measured by genetic distances of 0.049 and 0.073, respectively. Seven of the SNPs from non-coding regions are found in the transcriptional factor response elements and their possible role in gene regulation was evaluated with regard to MS. AMS and CMS were found to be significantly associated with the four genes compared to their Han and Tibetan control groups, respectively, indicating that these nucleotide alterations have a physiological effect for the development of high altitude sickness.

EPAS1 and EGLN1 associations with high altitude sickness in Han and Tibetan Chinese at the Qinghai-Tibetan Plateau

High altitude sickness (HAS) occurs among humans visiting or inhabiting high altitude environments. Genetic differences in the EPAS1 and EGLN1 genes have been found between lowland (Han) and highland (Tibetan) Chinese. Three SNPs within EPAS1 and EGLN1 were evaluated in Han and Tibetan patients with acute mountain sickness (AMS) and chronic mountain sickness (CMS). We compared 85 patients with AMS to 79 Han unaffected with mountain sickness (MS) as well as 45 CMS patients to 34 unaffected Tibetan subjects. The three SNPs studied were EPAS1 [ch2: 46441523 (hg18], EGLN1 (rs480902) and (rs516651). Direct sequencing was used to identify individual genotypes for the three SNPs. Age was found to be significantly associated with the EPAS1 SNP in the CMS patients while heart rate (HR) and oxygen saturation level of hemoglobin (SaO(2)) were found to be significantly associated with the EGLN1 (rs480902) SNP in the Han patients with AMS. The individuals with CMS were found to diverge significantly for the EPAS1 SNP compared to their Tibetan control group as measured by genetic distance (0.123) indicating positive selection of the EPAS-G allele with age and illness. The EGLN1 (rs480902) SNP had a significant correlation with hematocrit (HCT), HR and SaO(2) in AMS patients. AMS and CMS were found to be significantly associated with the EPAS1 and EGLN1 SNPs compared to their Han and Tibetan control groups, respectively, indicating these nucleotide alterations have a physiological effect for the development of high altitude sickness.

New substrates and enzyme assays for the detection of mucopolysaccharidosis III (Sanfilippo Syndrome) types A, B, C, and D by tandem mass spectrometry

The clinical phenotype of Sanfilippo Syndrome is caused by one of four enzyme deficiencies that are associated with a defect in mucopolysaccharide metabolism. The four subtypes (A, B, C, and D) are each caused by an enzyme deficiency involved in the degradation of heparan sulfate. We have developed a highly efficient synthesis of the substrates and internal standards required for the enzymatic assay of each of the four enzymes. The synthesis of the substrates involves chemical modification of a common intermediate. The substrates and internal standards allow the measurement of the enzymes relevant to heparan N-sulfatase (type A); N-acetyl-α-glucosaminidase (type B); acetyl-CoA:α-glucosamide N-acetyltransferase (type C); and N-acetylglucosamine 6-sulfatase (type D). The internal standards are similar to the substrates and allow for the accurate quantification of the enzyme assays using tandem mass spectrometry. The synthetic substrates incorporate a coumarin moiety and can also be used in fluorometric enzyme assays. We confirm that all four substrates can detect the appropriate Sanfilippo Syndrome in fibroblast lysates, and the measured enzyme activities are distinctly lower by a factor of 10 when compared to fibroblast lysates from unaffected persons.

Comparative triplex tandem mass spectrometry assays of lysosomal enzyme activities in dried blood spots using fast liquid chromatography: application to newborn screening of Pompe, Fabry, and Hurler diseases

We report a comparative study of triplex tandem mass spectrometry (MS/MS) based assays of lysosomal enzymes in dried blood spots for the early detection of Pompe, Fabry, and Hurler diseases in newborns. Four methods have been evaluated that differed in sample handling and the equipment used. A newly developed method uses assay quenching with acetonitrile to precipitate blood proteins followed by analysis on an LC-electrospray/MS/MS system capable of multiple consecutive sample injections on two parallel chromatographic columns. This method requires 1.5 min per a triplex analysis of enzyme products and internal standards, which matches the throughput of the previously reported flow injection method. LC separation reduces matrix effects and allows for more facile sample workup. The new LC-based method showed figures of merit that were superior to those of the currently used method based on liquid-liquid extraction into ethyl acetate and flow injection into the mass spectrometer. The other methods we investigated for comprehensive comparison involved liquid-liquid extraction into ethyl acetate followed by LC-ESI-MS/MS and acetonitrile quenching followed by direct flow injection. Both methods using acetonitrile quenching were found to be robust and provide good quality data while requiring fewer liquid transfer steps and less disposable material and labor than did the extraction methods. The individual merits of the new methods are discussed to present an evaluated alternative approach to high-throughput analysis in newborn screening laboratories.

Tandem mass spectrometry for the direct assay of lysosomal enzymes in dried blood spots: application to screening newborns for mucopolysaccharidosis II (Hunter Syndrome)

We have developed a tandem mass spectrometry based assay of iduronate-2-sulfatase (IdS) activity for the neonatal detection of mucopolysaccharidosis II (MPS-II, Hunter Syndrome). The assay uses a newly designed synthetic substrate (IdS-S) consisting of α-L-iduronate-2-sulfate, which is glycosidically conjugated to a coumarin and a linker containing a tert-butyloxycarbamido group. A short synthesis of the substrate has been developed that has the potential of being scaled to multigram quantities. Sulfate hydrolysis of IdS-S by IdS found within a 3 mm dried blood spot specifically produces a nonsulfated product (IdS-P) which is detected by electrospray tandem mass spectrometry and quantified using a deuterium-labeled internal standard, both carried out in positive ion mode. Analysis of DBS from 75 random human newborns showed IdS activities in the range of 4.8-16.2 (mean 9.1) μmol/(h L of blood), which were clearly distinguished from the activities measured for 14 MPS-II patients at 0.17-0.52 (mean 0.29) μmol/(h L of blood). The assay shows low blank activity, 0.15 ± 0.03 μmol/(h L of blood). The within-assay coefficient of variation (CV) was 3.1% while the interassay CV was 15%.

Direct assay of delta-aminolevulinic acid dehydratase in heme biosynthesis for the detection of porphyrias by tandem mass spectrometry

We report a new assay of human delta-aminolevulinic acid dehydratase (ALAD), an enzyme converting delta-aminolevulinic acid (ALA) into porphobilinogen. The assay is developed for use in the clinical diagnosis of delta-aminolevulinic acid dehydratase-deficient porphyria, a rare enzymatic deficiency of the heme biosynthetic pathway. The assay involves the incubation of erythrocyte lysate with the natural substrate, ALA, followed by quantitative in situ conversion of porphobilinogen to its butyramide, and liquid-liquid extraction into a mass spectrometer-friendly solvent. Quantitation of the butyrylated porphobilinogen is done by electrospray ionization tandem mass spectrometry, using a deuterium labeled internal standard. The assay stays well within the range wherein ALAD activity is linear with time. The K(m) of ALAD for ALA was measured as 333 microM, and the V(max) was 19.3 microM/h. Average enzyme activity among a random sample of 36 anonymous individuals was 277 micromol/L erythrocyte lysate/hour with a standard deviation of 90 micromol/L erythrocyte lysate/hour. The tandem mass spectrometric assay should easily detect the enzyme deficiency, which causes a reduction of activity by 95-99%. The assay shows good reproducibility and low background, requires a simple workup, and uses a commercially available substrate.

Tandem mass spectrometry for the direct assay of lysosomal enzymes in dried blood spots: application to screening newborns for mucopolysaccharidosis VI (Maroteaux-Lamy syndrome)

We report a new assay of N-acetylgalactosamine-4-sulfatase (aryl sulfatase B) activity in dried blood spots (DBS) for the early detection of mucopolysaccharidosis VI (Maroteaux-Lamy syndrome) in newborn screening. The assay uses a synthetic substrate consisting of N-acetylgalactosamine-4-sulfate moiety glycosidically linked to a hydrophobic residue and furnished with a tert-butyloxycarbamido group as a marker for specific mass spectrometric fragmentation. Incubation with aryl sulfatase B present in DBS converts the substrate to a desulfated product which is detected by electrospray tandem mass spectrometry and quantified using a homologous internal standard. Assay and workup procedures were optimized to be compatible with the work flow in newborn screening laboratories. Analysis of DBS from human newborns showed clear distinction of aryl sulfatase B activity from 89 healthy individuals where it ranged between 1.4 and 16.9 μmol/(h L of blood), with an average activity of 7.4 μmol/(h L of blood), and an MPS-VI patient that had an activity of 0.12 μmol/(h L of blood). Results are also reported for the aryl sulfatase B assay in DBS from groups of normal felines and felines affected with MPS-VI.

Tandem mass spectrometry for the direct assay of lysosomal enzymes in dried blood spots: application to screening newborns for mucopolysaccharidosis IVA

BACKGROUND

Treatments are being developed for an increasing number of mucopolysaccharidoses, and early diagnosis is expected to be necessary to maximize the benefits of therapy. Therefore, we developed an assay for N-acetylgalactosamine-6-sulfate sulfatase (GALNS), the enzyme deficient in mucopolysaccharidosis IVA (Morquio A syndrome), that is applicable for clinical diagnosis.

METHODS

A novel substrate for GALNS was synthesized for a new enzyme activity assay that is based on tandem mass spectrometry and uses dried blood spots (DBSs) as the enzyme source. We optimized the assay conditions, including the substrate concentration, reaction pH, lead formate concentration, incubation time, punch size of the DBS, and mass spectrometer conditions. We also assessed inter- and intraassay variation.

RESULTS

The assay uses either solid-phase or liquid-phase extraction before analysis by mass spectrometry. An evaluation of blood spots from 90 randomly chosen healthy newborns and 9 patients with Morquio A syndrome showed a well-defined interval between their respective enzyme activities. Inter- and intraassay imprecision was <10%.

CONCLUSIONS

This tandem mass spectrometry assay requires a minimal number of sample-preparation steps, thus making it easy to implement. The assay has the potential to be adopted for early diagnosis of Morquio A syndrome. We believe this assay could be performed in a multiplex fashion with assays for other lysosomal enzymes.

A tandem mass spectrometry triplex assay for the detection of Fabry, Pompe, and mucopolysaccharidosis-I (Hurler)

BACKGROUND

We sought to develop a tandem mass spectrometry assay in which the enzymatic activities of 3 lysosomal enzymes (α-glucosidase, α-galactosidase A, and α-l-iduronidase) could be quantified in dried blood spots by using a single assay buffer.

METHODS

A 3-mm dried blood spot punch was incubated in a single assay buffer with 3 different substrates and internal standards. The sample was processed by a simple liquid-liquid extraction by using ethyl acetate. The extract was dried down and resuspended in solvent for injection into the tandem mass spectrometer. Products and internal standards were monitored by multiple reaction monitoring.

RESULTS

Assay for the 3 lysosomal enzymes was successfully achieved with acceptable statistics. The assay can be performed by using a minimal quantity of disposable supplies and equipment. The entire procedure fits into a 48-h cycle including data analysis. Data from 5990 anonymous newborn dried blood spots showed an approximate bell-shaped distribution of enzymatic activities (mean values of 19.0, 11.5, and 3.5 μmol · h(-1) · (L blood)(-1) for α-glucosidase, α-galactosidase A, and α-l-iduronidase, respectively. Blank values obtained in the absence of blood were 0.13, 0.24, and 0.45 μmol · h(-1) · (L blood)(-1), respectively). By assaying 3 enzymes at once, problematic samples are spotted for reanalysis if enzyme activity values are low for all enzymes (for example, if insufficient blood is present in the assay).

CONCLUSIONS

This method demonstrates that a triplex assay in a single buffer and with minimal supplies and labor can be adapted to a high-throughput newborn screening laboratory for the analysis of Pompe, Fabry, and mucopolysaccharidosis-I (Hurler) diseases.

Tandem Mass Spectrometry for the Direct Assay of Lysosomal Enzymes in Dried Blood Spots: Application to Screening Newborns for Mucopolysaccharidosis I

Background: Treatments now available for mucopolysaccharidosis I require early detection for optimum therapy. Therefore, we have developed an assay appropriate for newborn screening of the activity of the relevant enzyme, α-L-iduronidase.

Methods: We synthesized a new α-L-iduronidase substrate that can be used to assay the enzyme by use of tandem mass spectrometry together with an internal standard or by fluorometry. The assay uses a dried blood spot on a newborn screening card as the enzyme source. The assay protocol uses a simple liquid-liquid extraction step before mass spectrometry. We optimized enzyme reaction conditions and procedures for the assay, including the concentration of substrate, the reaction pH, the incubation time, and mass spectrometer operation. We also assessed inter- and intraassay imprecision.

Results: When the assay was tested on dried blood spots, the α-L-iduronidase activity measured for 5 patients with mucopolysaccharidosis I was well below the interval found for 10 randomly chosen newborns. Inter- and intraassay imprecision were &lt;10%. The synthesis of the α-L-iduronidase substrate is practical for use on a scale needed to support newborn screening demands.

Conclusions: This newly developed tandem mass spectrometry assay has the potential to be adopted for newborn screening of mucopolysaccharidosis I. This assay has advantages over a previously reported assay also developed in this laboratory and has the potential to be performed in a multiplex fashion to measure several lysosomal enzymes relevant to treatable lysosomal storage diseases.