SOD1 gene screening in ALS - frequency of mutations, patients' attitudes to genetic information and transition to tofersen treatment in a multi-center program.

Objective: To report the frequency of pathogenic SOD1 gene variants in a screening program in amyotrophic lateral sclerosis (ALS), and the clinical practice of transition to an expanded access program (EAP) of tofersen treatment. Methods: From October 2021 to February 2024, at 11 ALS centers in Germany genetic testing for SOD1, FUS, TARDBP, and C9orf72 was performed. Patients were offered to opt for notification either about all genetic variants or SOD1 variants relevant for tofersen therapy. The transition to the EAP with tofersen was assessed. Results: 1935 patients were screened (94.7% sporadic ALS). 48.8% (n = 928) opted for notification of treatment-relevant information. Genetic variants were found as follows: SOD1 (likely) pathogenic variants (class 4/5) 1.8% (n = 34), variants of unknown significance (class 3) 0.8% (n = 16), FUS (class 4/5) 0.9% (n = 17), TARDBP (class 4/5) 1.3% (n = 25), C9orf72 hexanucleotide repeat expansion 7.0% (n = 135). In SOD1-ALS (encompassing class 3-5 variants, n = 50), 68.0% (n = 34) reported a negative family history. 74.0% (n = 37) of SOD1-ALS patients - which represent 1.9% of all participants of the screening program - were transitioned to tofersen. Median duration from start of genetic testing to treatment was 94 days (57 to 295 days). Eight patients declined treatment whereas five individuals died before initiation of therapy. Conclusion: The finding of SOD1 variants in patients with a negative family history underscores the need for a broad genetic screening in ALS. In SOD1-ALS, the treatment option with tofersen was mostly utilized. The wide range in the transition time to tofersen calls for a SOD1-ALS management program.

Higher precision, first tier newborn screening for metachromatic leukodystrophy using 16:1-OH-sulfatide.

Newborn screening (NBS) for metachromatic leukodystrophy (MLD) is based on first-tier measurement of sulfatides in dried blood spots (DBS) followed by second-tier measurement of arylsulfatase A in the same DBS. This approach is very precise with 0-1 false positives per ∼30,000 newborns tested. Recent data reported here shows that the sulfatide molecular species with an α-hydroxyl, 16­carbon, mono-unsaturated fatty acyl group (16:1-OH-sulfatide) is superior to the original biomarker 16:0-sulfatide in reducing the number of first-tier false positives. This result is consistent across 4 MLD NBS centers. By measuring 16:1-OH-sulfatide alone or together with 16:0-sulfatide, the estimated false positive rate is 0.048% and is reduced essentially to zero with second-tier arylsulfatase A activity assay. The false negative rate is predicted to be extremely low based on the demonstration that 40 out of 40 newborn DBS from clinically-confirmed MLD patients are detected with these methods. The work shows that NBS for MLD is extremely precise and ready for deployment. Furthermore, it can be multiplexed with several other inborn errors of metabolism already tested in NBS centers worldwide.

Long-Term Evaluation of Biomarkers in the Czech Cohort of Gaucher Patients.

A personalized treatment decision for Gaucher disease (GD) patients should be based on relevant markers that are specific to GD, play a direct role in GD pathophysiology, exhibit low genetic variation, reflect the therapy, and can be used for all patients. Thirty-four GD patients treated with enzyme replacement therapy (ERT) or substrate reduction therapy (SRT) were analyzed for platelet count, chitotriosidase, and tartrate-resistant acid phosphatase activity in plasma samples, and quantitative measurement of Lyso-Gb1 was performed in dried blood spots. In our ERT and SRT study cohorts, plasma lyso-GL1 correlated significantly with chito-triosidase (ERT: r = 0.55, p < 0.001; SRT: r = 0.83, p < 0.001) and TRAP (ERT: r = 0.34, p < 0.001; SRT: r = 0.88, p < 0.001), irrespective of treatment method. A platelet count increase was associated with a Lyso-Gb1 decrease in both treatment groups (ERT: p = 0.021; SRT: p = 0.028). The association of Lyso-Gb1 with evaluated markers was stronger in the SRT cohort. Our results indicate that ERT and SRT in combination or in a switch manner could offer the potential of individual drug effectiveness for particular GD symptoms. Combination of the key biomarker of GD, Lyso-Gb1, with other biomarkers can offer improved response assessment to long-term therapy.

Importance to include differential diagnostics for acid sphingomyelinase deficiency (ASMD) in patients suspected to have to Gaucher disease.

The clinical manifestation of sphingolipidosis leads often to misclassification between acid sphingomyelinase deficiency (ASMD) and Gaucher disease. In this multicenter, prospective study, we investigated a cohort of 31,838 individuals suspected to have Gaucher disease, due to clinical presentation, from 61 countries between 2017 and 2022. For all samples, both Acid-ß-glucocerebrosidase and acid sphingomyelinase enzyme activities were measured in dried blood spot specimens by tandem mass spectrometry followed by genetic confirmatory testing in potential positive cases. In total, 5933 symptomatic cases showed decreased enzyme activities and were submitted for genetic confirmatory testing. 1411/5933 (24%) cases were finally identified with Gaucher disease and 550/5933 (9%) with ASMD. Most of the confirmed ASMD cases were newborns and children below 2 years of age (63%). This study reveals that one in four cases suspected for Gaucher disease is diagnosed with ASMD. An early appropriate diagnostic work-up is essential because of the availability of a recently approved enzyme replacement therapy for ASMD. In conclusion, a diagnostic strategy using differential biochemical testing including genetic confirmatory testing in clinically suspected cases for sphingolipidosis is highly recommended.

Molecular basis of mucopolysaccharidosis IVA (Morquio A syndrome): A review and classification of GALNS gene variants and reporting of 68 novel variants.

Mucopolysaccharidosis IVA (MPS IVA, Morquio A syndrome) is a rare autosomal recessive lysosomal storage disorder caused by mutations in the N-acetylgalactosamine-6-sulfatase (GALNS) gene. We collected, analyzed, and uniformly summarized all published GALNS gene variants, thus updating the previous mutation review (published in 2014). In addition, new variants were communicated by seven reference laboratories in Europe, the Middle East, Latin America, Asia, and the United States. All data were analyzed to determine common alleles, geographic distribution, level of homozygosity, and genotype-phenotype correlation. Moreover, variants were classified according to their pathogenicity as suggested by ACMG. Including those previously published, we assembled 446 unique variants, among which 68 were novel, from 1190 subjects (including newborn screening positive subjects). Variants' distribution was missense (65.0%), followed by nonsense (8.1%), splicing (7.2%), small frameshift deletions(del)/insertions(ins) (7.0%), intronic (4.0%), and large del/ins and complex rearrangements (3.8%). Half (50.4%) of the subjects were homozygous, 37.1% were compound heterozygous, and 10.7% had only one variant detected. The novel variants underwent in silico analysis to evaluate their pathogenicity. All variants were submitted to ClinVar (https://www.ncbi.nlm.nih.gov/clinvar/) to make them publicly available. Mutation updates are essential for the correct molecular diagnoses, genetic counseling, prenatal and preimplantation diagnosis, and disease management.

At-Risk Testing for Pompe Disease Using Dried Blood Spots: Lessons Learned for Newborn Screening.

Pompe disease (GSD II) is an autosomal recessive disorder caused by deficiency of the lysosomal enzyme acid-α-glucosidase (GAA, EC 3.2.1.20), leading to generalized accumulation of lysosomal glycogen especially in the heart, skeletal, and smooth muscle, and the nervous system. It is generally classified based on the age of onset as infantile (IOPD) presenting during the first year of life, and late onset (LOPD) when it presents afterwards. In our study, a cohort of 13,627 samples were tested between January 2017 and December 2018 for acid-α-glucosidase (GAA, EC 3.2.1.20) deficiency either by fluorometry or tandem mass spectrometry (MS). Testing was performed for patients who displayed conditions of unknown etiology, e.g., CK elevations or cardiomyopathy, in the case of infantile patients. On average 8% of samples showed activity below the reference range and were further assessed by another enzyme activity measurement or molecular genetic analysis. Pre-analytical conditions, like proper drying, greatly affect enzyme activity, and should be assessed with measurement of reference enzyme(s). In conclusion, at-risk testing can provide a good first step for the future introduction of newborn screening for Pompe disease. It yields immediate benefits for the patients regarding the availability and timeliness of the diagnosis. In addition, the laboratory can introduce the required methodology and gain insights in the evaluation of results in a lower throughput environment. Finally, awareness of such a rare condition is increased tremendously among local physicians which can aid in the introduction newborn screening.

α-Mannosidosis - An underdiagnosed lysosomal storage disease in individuals with an 'MPS-like' phenotype.

Individuals affected by alpha-Mannosidosis suffer from similar clinical symptoms such as respiratory infections, skeletal changes as patients with mucopolysaccharidoses (MPS). α-Mannosidosis is considered as an ultra-rare disorders and also diagnostic testing is often limited. With the availability of novel therapies and easy-to-access diagnostic tests (e.g. Tandem mass spectrometry) using dried blood spots for both enzymatic and genetic testing, the chance for the development of a better understanding of disease and awareness may be triggered. In a pilot study, we have investigated 1010 residual dried blood spot samples from individuals suspicious to MPS. In these study cohort, 158/1010 individuals were genetically confirmed for MPS. Additional biochemical and genetic confirmatory testing for α-mannosidases revealed four individuals with a final diagnosis of α-mannosidosis. This unexpected high number of individuals with α-mannosidosis demonstrated the urgent need of taking this rare disorder in clinical and diagnostic consideration particularly in patients suspicious to MPS.

Diagnostic strategy for females suspected of Fabry disease.

A total of 11 948 females suspicious of Fabry disease were tested by a combined biochemical and genetic approach. The enzyme activity, together with the concentration of lyso-GL-3 (lyso-Gb3) biomarker in dried blood spots (DBS), substantially improved the diagnostic detection of Fabry disease in females compared to the enzyme activity alone. Abnormal values for both were highly suspicious of Fabry disease (97% positive predictive value [PPV], similar to PPV in males). In cases with one abnormal biochemical value, elevated lyso-GL-3 is a far more important indicator than low enzyme activity (39% PPV vs 6% PPV). Cases with clearly negative results for both biochemical parameters are unlikely to have Fabry disease, even in clinically highly suspicious cases.

A click-flipped enzyme substrate boosts the performance of the diagnostic screening for Hunter syndrome.

We report on the unexpected finding that click modification of iduronyl azides results in a conformational flip of the pyranose ring, which led to the development of a new strategy for the design of superior enzyme substrates for the diagnostic assaying of iduronate-2-sulfatase (I2S), a lysosomal enzyme related to Hunter syndrome. Synthetic substrates are essential in testing newborns for metabolic disorders to enable early initiation of therapy. Our click-flipped iduronyl triazole showed a remarkably better performance with I2S than commonly used O-iduronates. We found that both O- and triazole-linked substrates are accepted by the enzyme, irrespective of their different conformations, but only the O-linked product inhibits the activity of I2S. Thus, in the long reaction times required for clinical assays, the triazole substrate substantially outperforms the O-iduronate. Applying our click-flipped substrate to assay I2S in dried blood spots sampled from affected patients and random newborns significantly increased the confidence in discriminating between these groups, clearly indicating the potential of the click-flip strategy to control the biomolecular function of carbohydrates.

Negative screening of Fabry disease in patients with conduction disorders requiring a pacemaker.

Identification of Fabry disease (FD) in cardiac patients has been restricted so far to patients with left ventricular hypertrophy. Conduction problems are frequent in FD and could precede other manifestations, offering a possible earlier diagnosis.We studied the prevalence of FD in 188 patients < 70 years with conduction problems requiring pacemaker implantation. Although classical manifestations of FD were not rare, no patient with FD was identified. Screening efforts should not be conducted in this population.

Vaginal Ureaplasma parvum serovars and spontaneous preterm birth.

BACKGROUND: Ureaplasma species (spp) are the bacteria most often isolated from the amniotic cavity of women with preterm labor or preterm premature rupture of membranes; thus, the link between intrauterine Ureaplasma spp infection and adverse pregnancy outcome clearly is established. However, because vaginal Ureaplasma spp colonization is very common in pregnant women, the reason that these microorganisms cause ascending infections in some cases but remain asymptomatic in most pregnancies is not clear. Previous studies suggested an association between vaginal colonization with Ureaplasma parvum as opposed to U urealyticum and preterm delivery. However, because of the high frequency of vaginal Ureaplasma spp colonization during pregnancy, additional risk factors are needed to select a group of women who might benefit from treatment. OBJECTIVE: To further identify pregnant women who are at increased risk for preterm delivery, the aim of the present study was to investigate U parvum serovar-specific pathogenicity in a large clinical cohort. STUDY DESIGN: We serotyped 1316 samples that were positive for U parvum using a high-resolution melt polymerase chain reaction assay, and results were correlated with pregnancy outcome. RESULTS: Within U parvum positive samples, serovar 3 was the most common isolate (43.3%), followed by serovar 6 (31.4%) and serovar 1 (25.2%). There was a significantly increased risk for spontaneous preterm birth at very low (<32 weeks gestation; P<.005) and extremely low (<28 weeks gestation; P<.005) gestational age in the group with vaginal U parvum serovar 3 colonization compared with the control group of pregnant women who tested negative for vaginal Ureaplasma spp colonization. This association was found for neither serovar 1 nor serovar 6. The combination of vaginal U parvum serovar 3 colonization and diagnosis of bacterial vaginosis in early pregnancy or a history of preterm birth further increased the risk for adverse pregnancy outcome. CONCLUSION: Colonization with U parvum serovar 3, but not serovar 1 or serovar 6, in early pregnancy is associated with preterm delivery at very and extremely low gestational age. The combination of U parvum serovar 3 colonization and a history of preterm birth or bacterial vaginosis further increases the risk for spontaneous preterm birth at low gestational age and may define a target group for therapeutic intervention studies.

Low Prevalence Estimates of Late-Onset Glycogen Storage Disease Type II in French-Speaking Belgium are not Due to Missed Diagnoses.

BACKGROUND: Late-onset glycogen storage disease type II is associated with variable muscle phenotypes. Epidemiological data suggest that its prevalence is lower in Belgium than in bordering countries like The Netherlands. OBJECTIVE: We investigated whether such low estimated prevalence is due to missed diagnoses. METHODS: We screened 100 patients with muscle phenotypes of undetermined origin using a dried blood spot test for alpha-acid glucosidase (GAA) activity. Patients with low activity at screening were re-tested by the same method and, if low activity was confirmed, GAA gene analysis was performed. RESULTS: The screening test revealed lower than normal GAA activity in 15 patients, but in only two of them it was low enough to be considered in the disease range. Retesting confirmed lower than normal GAA activity in five patients, but in all of them it was above the disease range. A single patient carried a heterozygous known pathogenic GAA mutation, whose significance in this case remains undetermined. CONCLUSIONS: We conclude that reported low prevalence estimates in Belgium are not likely to be due to an underdiagnosis bias. Lower prevalence compared to neighbouring The Netherlands may be due to different ethnic stratification of our patients. Diagnostic strategies should keep into account the expected prevalence of a disease in specific populations.

Pulmonary involvement in Fabry disease: effect of plasma globotriaosylsphingosine and time to initiation of enzyme replacement therapy.

INTRODUCTION: Anderson-Fabry disease (AFD) is an X-linked lysosomal storage disorder caused by mutations of GLA gene leading to reduced α-galactosidase activity and resulting in a progressive accumulation of globotriaosylceramide (Gb3) and its deacylated derivative, globotriaosyl-sphingosine (Lyso-Gb3). Plasma Lyso-Gb3 levels serve as a disease severity and treatment monitoring marker during enzyme replacement therapy (ERT). METHODS: Adult patients with AFD who had yearly pulmonary function tests between 1999 and 2015 were eligible for this observational study. Primary outcome measures were the change in z-score of forced expiratory volume in the first second (FEV1) and FEV1/FVC over time. Plasma Lyso-Gb3 levels and the age of ERT initiation were investigated for their association with lung function decline. RESULTS: Fifty-three patients (42% male, median (range) age at diagnosis of AFD 34 (6-61) years in men, 34 (13-67) in women) were included. The greatest decrease of FEV1/FVC z-scores was observed in Classic men (-0.048 per year, 95% CI -0.081 to -0.014), compared with the Later-Onset men (+0.013,95% CI -0.055 to 0.082), Classic women (-0.008, 95% CI -0.035 to +0.020) and Later-Onset women (-0.013, 95% CI -0.084 to +0.058). Cigarette smoking (P=0.022) and late ERT initiation (P=0.041) were independently associated with faster FEV1 decline. FEV1/FVC z-score decrease was significantly reduced after initiation of ERT initiation (-0.045 compared with -0.015, P=0.014). Furthermore, there was a trend towards a relevant influence of Lyso-Gb3 (P=0.098) on airflow limitation with age. CONCLUSION: Early ERT initiation seems to preserve pulmonary function. Plasma Lyso-Gb3 is maybe a useful predictor for airflow limitation. Classic men need a closer monitoring of the lung function.

Targeted Metabolomic Profiling of Peritoneal Dialysis Effluents Shows Anti-oxidative Capacity of Alanyl-Glutamine.

Readily available peritoneal dialysis (PD) effluents from PD patients in the course of renal replacement therapy are a potentially rich source for molecular markers for predicting clinical outcome, monitoring the therapy, and therapeutic interventions. The complex clinical phenotype of PD patients might be reflected in the PD effluent metabolome. Metabolomic analysis of PD effluent might allow quantitative detection and assessment of candidate PD biomarkers for prognostication and therapeutic monitoring. We therefore subjected peritoneal equilibration test effluents from 20 stable PD patients, obtained in a randomized controlled trial (RCT) to evaluate cytoprotective effects of standard PD solution (3.86% glucose) supplemented with 8 mM alanyl-glutamine (AlaGln) to targeted metabolomics analysis. One hundred eighty eight pre-defined metabolites, including free amino acids, acylcarnitines, and glycerophospholipids, as well as custom metabolic indicators calculated from these metabolites were surveyed in a high-throughput assay requiring only 10 µl of PD effluent. Metabolite profiles of effluents from the cross-over trial were analyzed with respect to AlaGln status and clinical parameters such as duration of PD therapy and history of previous episodes of peritonitis. This targeted approach detected and quantified 184 small molecules in PD effluent, a larger number of detected metabolites than in all previous metabolomic studies in PD effluent combined. Metabolites were clustered within substance classes regarding concentrations after a 4-h dwell. PD effluent metabolic profiles were differentiated according to PD patient sub-populations, revealing novel changes in small molecule abundance during PD therapy. AlaGln supplementation of PD fluid altered levels of specific metabolites, including increases in alanine and glutamine but not glutamate, and reduced levels of small molecule indicators of oxidative stress, such as methionine sulfoxide. Our study represents the first application of targeted metabolomics to PD effluents. The observed metabolomic changes in PD effluent associated with AlaGln-supplementation during therapy suggested an anti-oxidant effect, and were consistent with the restoration of important stress and immune processes previously noted in the RCT. High-throughput detection of PD effluent metabolomic signatures and their alterations by therapeutic interventions offers new opportunities for metabolome-clinical correlation in PD and for prescription of personalized PD therapy.

Rapid and Modular Assembly of Click Substrates To Assay Enzyme Activity in the Newborn Screening of Lysosomal Storage Disorders.

Synthetic substrates play a pivotal role in the development of enzyme assays for medical diagnostics. However, the preparation of these chemical tools often requires multistep synthetic procedures complicating structural optimization and limiting versatility. In particular, substrates for enzyme assays based on tandem mass spectrometry need to be designed and optimized to fulfill the requirements to finally enable the development of robust diagnostic assays. In addition, isotope-labeled standards need to be prepared to facilitate accurate quantification of enzyme assay products. Here we report the development of a building block strategy for rapid and modular assembly of enzyme substrates using click chemistry as a key step. These click substrates are made up of a sugar moiety as enzyme responsive unit, a linker that can easily be isotope-labeled for the synthesis of internal standards, and a modifier compound that can readily be exchanged for structural optimization and analytical/diagnostic tuning. Moreover, the building block assembly eliminates the need for extensive optimization of different glycosylation reactions as it enables the divergent synthesis of substrates using a clickable enzyme responsive unit. The outlined strategy has been applied to obtain a series of synthetic α-l-iduronates and sulfated ß-d-galactosides as substrates for assaying α-l-iduronidase and N-acetylgalactosamine-6-sulfate sulfatase, enzymes related to the lysosomal storage disorders mucopolysaccharidosis type I and type IVa, respectively. Selected click substrates were finally shown to be suitable to assay enzyme activities in dried blood spot samples from affected patients and random newborns.

First Trimester Vaginal Ureaplasma Biovar Colonization and Preterm Birth: Results of a Prospective Multicenter Study.

BACKGROUND: While there is a proven association of upper genital tract Ureaplasma infection during pregnancy with adverse pregnancy outcome, the effect of vaginal Ureaplasma colonization on preterm delivery has been controversially debated. OBJECTIVES: We hypothesized that women with isolation of vaginal U. parvum but not U. urealyticum are at increased risk for spontaneous preterm birth (SPB) compared to women with negative results. METHODS: A vaginal swab taken between 12 and 14 weeks of gestation was analyzed for the presence of Ureaplasma biovars by PCR in 4,330 pregnant women. RESULTS: Of the study cohort, 37% were positive for U. parvum, 5.9% for U. urealyticum, and 3.1% for both. The rates of SPB were 10.4% (OR 1.7, 95% CI 1.3, 2.2, p < 0.001) and 8.9% (OR 1.4, 95% CI 0.9, 2.3, p = 0.193) in the groups with isolation of U. parvum and U. urealyticum, respectively, compared to 6.4% in the group with negative PCR results. Multiple logistic regression and interaction analyses showed that vaginal colonization with U. parvum but not U. urealyticum was a statistically significant risk factor for SPB (adjusted OR 1.6, 95% CI 1.2, 2.1, p < 0.001), independent of other risk factors such as bacterial vaginosis and history of SPB. CONCLUSION: Our study demonstrates a statistically significant and independent association between first-trimester vaginal colonization with U. parvum and subsequent SPB.

Genotype, phenotype and disease severity reflected by serum LysoGb3 levels in patients with Fabry disease.

BACKGROUND: Fabry disease (FD) is a rare X-linked lysosomal storage disease caused by mutations in the α-galactosidase A (GLA) gene causing deficiency of α-galactosidase A which results in progressive glycosphingolipid accumulation, especially globotriaosylceramide (Gb3), in body liquids and lysosomes. In a large cohort of FD patients, we aimed to establish genotype/phenotype relations as indicated by serum LysoGb3 (deacylated Gb3). METHODS: In 69 consecutive adult FD patients (males: n=28 (41%)) with a GLA-mutation confirmed diagnosis, we conducted a multidisciplinary clinical characterization during their routine annual examinations, and measured serum LysoGb3 levels by high-sensitive electrospray ionization liquid chromatography tandem mass spectrometry. RESULTS: Serum levels of LysoGb3 were significantly higher in Classic compared with Later-Onset phenotype and higher in the latter compared with controls, both in males (52 [40-83] vs 9.5 [4.5-20] vs 0.47 [0.41-0.61] ng/ml, P<0.001) and in females (9.9 [7.9-14] vs 4.9 [1.6-4.9] vs 0.41 [0.33-0.48] ng/ml, P<0.001), respectively. Multivariate linear regression analysis showed that LysoGb3 levels were independently associated with, serum creatinine (ß=0.09, 95%CI 0.04-0.13, P<0.001) and the presence of cardiomyopathy (ß=25, 95%CI 9.8-41, P=0.002). LysoGb3 levels were higher in males with frame-shift and nonsense mutations than in males with missense mutations (84 [72-109] vs 41 [37-52] ng/ml, P=0.002). CONCLUSION: LysoGb3 relates to disease severity, enzyme replacement response, and to the genotype severity in males. LysoGb3 supports identifying patients at risk who require intensive monitoring and treatment. LysoGb3 appears to be one marker of metabolic phenotyping of FD.

Correlation of Lyso-Gb3 levels in dried blood spots and sera from patients with classic and Later-Onset Fabry disease.

BACKGROUND: Fabry disease (FD), an X-linked lysosomal storage disorder, results from the deficient activity of α-galactosidase A (α-Gal A) and the accumulation of its substrates, globotriaosylceramide (Gb3) and its deacylated derivative, globotriaosyl-sphingosine (Lyso-Gb3). Here, we compared the levels of Lyso-Gb3 in dried blood spots (DBS) and sera in affected males and heterozygotes with the "Classic" and "Later-Onset" phenotypes. METHODS: The Lyso-Gb3 concentrations in DBS and sera from 56 FD patients were determined by highly sensitive electrospray ionization liquid chromatography tandem mass spectrometry. RESULTS: The serum Lyso-Gb3 levels in 18 and 5 affected males with the Classic and Later-Onset phenotypes, were 61±38 and 14±12ng/mL, respectively. Lyso-Gb3 levels in 30 females from Classic families and three females from Later-Onset families were 10±5.4 and 2.4±1.0ng/mL, respectively. The linear regression model with serum Lyso-Gb3 as the dependent variable and DBS Lyso-Gb3 an independent variable was described by the function y=-1.83+1.68∗x and showed a high coefficient of determination, R2=0.976. The overall correlation between the Lyso-Gb3 levels in DBS and sera was high (R=0.99; p<0.001). CONCLUSION: DBS provides a convenient, sensitive, and reproducible source to measure Lyso-Gb3 levels for diagnosis, initial phenotypic assignment, and therapeutic monitoring in patients with Fabry disease.