Stratification of patients with lysosomal acid lipase deficiency by enzyme activity in dried blood spots.

Background: Lysosomal acid lipase deficiency (LAL-D) is a phenotypic continuum between the severe Wolman disease and the attenuated cholesteryl ester storage disease (CESD). Objective: To study if the amount of residual LAL enzymatic activity in dried blood spots (DBS) correlates with the LAL-D disease severity. Methods: DBS from Wolman and CESD patients, LAL-D carriers, and presumably unaffected random newborns were acquired. LAL enzymatic activity in DBS were measured using a novel, highly specific LAL substrate. Results: Patients with Wolman disease displayed significantly lower LAL enzymatic activity compared to CESD patients. This was not observed with the traditional assay in which a non-specific substrate was used together with an LAL-specific inhibitor. Conclusion: The new LAL enzymatic activity assay using the specific substrate offers an improved biochemical genetics method for the diagnosis of LAL-D in symptomatic patients and more importantly, for the prognosis of asymptomatic patients who test positive in population-wide LAL-D newborn screening.

LC-MS/MS-based enzyme assay for lysosomal acid lipase using dried blood spots.

Lysosomal acid lipase deficiency (LAL-D) (OMIM: 278000) is a lysosomal storage disorder with two distinct disease phenotypes such as Wolman disease and cholesteryl ester storage disorder (CESD), characterized by an accumulation of endocytosed cholesterol in the body. Due to the presence of multiple lipases in DBS, previous studies measured LAL enzyme activity in the presence of Lalistat-2, an established LAL-specific inhibitor (Hamilton J et al Chim Clin Acta (2012) 413:1207-1210). Alternatively, a novel substrate specific for LAL has been reported very recently (Masi S. et al Clin Chem (2018) 64:690-696). In this study, we examined the LAL enzyme activity of a Japanese population with the LAL-specific substrate using liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based enzyme assay whether an affected individual can be identified among this population. To achieve this, we first performed assay validation using LC-MS/MS. Under our experimental setting, typically we obtained LAL enzyme activity for QC High (100% enzyme activity) as 261.9 ± 3.2 µmol/h/L (n = 5) and for QC Low as (5% enzyme activity) as 14.7 ± 0.5 µmol/h/L (n = 5). The percentage of coefficient of variation for interday assay for QC High was 9.6% (n = 4) and for QC Low was 7.9% (n = 4), respectively. Based on these results, we further examined the LAL enzyme activity of control Japanese population and that of affected individuals with Wolman disease and CESD. The averaged enzyme activity for control newborns, Wolman, and CESD was 123.9 ± 53.9 µmol/h/L (n = 131), 6.6 ± 0.9 µmol/h/L (n = 3), and 4.8 ± 0.3 µmol/h/L (n = 3), respectively. These results suggest that an LAL-D-affected individual can be readily identified by enzyme activity using LC-MS/MS-based technique.

Best practice in the measurement and interpretation of lysosomal acid lipase in dried blood spots using the inhibitor Lalistat 2.

Lysosomal acid lipase deficiency (LAL-D) is an inherited, autosomal recessive lysosomal storage disorder characterized by progressive damage in multiple organ systems. Diagnosis is especially important in infants, in whom the course of disease is rapidly lethal without treatment. The recent regulatory approval of recombinant human lysosomal acid lipase (LAL), sebelipase alfa, merits rapid diagnosis in clinical routine, particularly in infants. A method for measuring LAL activity in dried blood spot (DBS) samples using the highly specific LAL inhibitor Lalistat 2 is available. This method is shown to effectively discriminate between individuals with LAL-D and unaffected controls. With the increase in DBS LAL testing since the original publication of this method, a need to optimise assay performance has been identified. Here, we describe refinements to the DBS assay, including technical modifications, quality control measures and best-practice guidance for interpreting and reporting results. Particular attention is paid to alternatives to the use of mercuric chloride as the stop reagent and the choice of excitation wavelength for 4-methylumbelliferone palmitate under assay conditions at pH4.0. In addition, a simpler method of reporting results is proposed using cutoffs based on percentage mean normal enzyme activity.

Polymorphic variation in the 11beta-hydroxylase gene associates with reduced 11-hydroxylase efficiency.

The -344 C/T and intron 2 conversion variants in the CYP11B2 gene, encoding aldosterone synthase, have been associated with markers of impaired 11beta-hydroxylase activity. We hypothesize that this association is because of variations in the adjacent 11beta-hydroxylase gene (CYP11B1) and arises through linkage disequilibrium between CYP11B1 and CYP11B2. The pattern of variation across the entire CYP11B locus was determined by sequencing 26 normotensive subjects stratified by and homozygous for the -344 and intron conversion variants. Eighty-three variants associated with -344 and intron conversion were identified. Haplotype analysis revealed 4 common haplotypes, accounting for 68% of chromosomes, confirming strong linkage disequilibrium across the region. Two novel CYP11B1 polymorphisms upstream of the coding region (-1889 G/T and -1859 A/G) were identified as contributing to the common haplotypes. Given the potential for such mutations to affect transcriptional regulation of CYP11B1, these were analyzed further. A total of 512 hypertensive subjects from the British Genetics of Hypertension Study population were genotyped for these polymorphisms. A significant association was identified between the -1889 polymorphism and urinary tetrahydrodeoxycortisol/total cortisol metabolite ratio, indicating reduced 11beta-hydroxylase efficiency. A similar pattern was observed for the -1859 polymorphism, but this did not achieve statistical significance. Functional studies in vitro using luciferase reporter gene constructs show that these polymorphisms significantly alter the transcriptional response of CYP11B1 to stimulation by adrenocorticotropic hormone or forskolin. This study strongly suggests that the impaired 11beta-hydroxylase efficiency associated previously with the CYP11B2 -344 and intron conversion variants is because of linkage with these newly identified polymorphisms in CYP11B1.

Functional effects of genetic variants in the 11beta-hydroxylase (CYP11B1) gene.

OBJECTIVE: We previously described an association between the -344C/T 5'-untranslated region (UTR) polymorphism in the CYP11B2 (aldosterone synthase) gene and hypertension with a raised aldosterone to renin ratio (ARR); the same genetic variant is also associated with impaired adrenal 11beta-hydroxylase efficiency. The -344 polymorphism does not seem to be functional, so is likely to be in linkage with variants in CYP11B1 that determine the associated variation in 11beta-hydroxylase efficiency. We therefore aimed to determine whether there is an association between CYP11B1 variants and hypertension and/or an altered ARR. DESIGN AND MEASUREMENTS: We screened 160 subjects divided into four groups, normotensive controls, unselected hypertensive subjects, and hypertensive subjects with either a high (> or = 750) or low ARR (< or = 200), for variants in the coding region of CYP11B1 by single-stranded conformation polymorphism (SSCP) and direct sequencing. The effects of these variants on enzyme function were assessed by conversion of 11-deoxycortisol to cortisol and 11-deoxycorticosterone (DOC) to corticosterone. RESULTS: Eight novel missense mutations were identified in the CYP11B1 gene that alter the encoded amino acids: R43Q, L83S, H125R, P135S, F139L, L158P, L186V and T196A. In each case they were heterozygous changes. However, no mutations were identified that could account for hypertension and/or a raised ARR. The variants L158P and L83S severely impaired enzyme function while R43Q, F139L, P135S and T196A enzymes resulted in product levels that were approximately 30-50% that of wild-type levels. The variant enzymes H125R and L186V resulted in substrate-specific alterations in enzyme function. H125R decreased conversion of 11-deoxycortisol to cortisol and L186V increased 11-deoxycortisol conversion. Neither had an effect on the conversion of DOC to corticosterone. CONCLUSION: No variants were identified in the coding region of CYP11B1 that could account for hypertension and/or a raised ARR. However, this in vitro study identifies the importance of these affected residues to enzyme function and will inform subsequent studies of structure-function relationships.