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Lysosomal acid lipase mutations that determine phenotype in Wolman and cholesterol ester storage disease
R A Anderson1, G M Bryson, J S Parks
1Department of Internal Medicine, Wake Forest University School of Medicine, Winston-Salem, North Carolina, 27157, USA. ande489@ibm.net
Insights
Genetic defects in human lysosomal acid lipase (hLAL) cause Wolman disease and cholesterol ester storage disease. CESD patients often retain some hLAL function due to specific mutations, distinguishing them from WD patients.
Area of Science:
- Biochemistry
- Genetics
- Molecular Biology
Background:
- Lysosomal acid lipase (hLAL) deficiency leads to Wolman disease (WD) and cholesterol ester storage disease (CESD), presenting distinct clinical phenotypes.
- Investigating the molecular mechanisms underlying these divergent phenotypes is crucial for understanding hLAL function and disease pathology.
Observation:
- HLAL activity was severely reduced (<2%) in both WD and CESD patients, with no detectable hLAL protein in fibroblasts from any patient.
- Mutational analysis revealed distinct genetic defects: WD patients had severe mutations, while four CESD patients possessed a specific exon 8 splice junction mutation (c.894 G>A) encoding a truncated hLAL mRNA.
Findings:
- The exon 8 splice mutation in CESD allows for a shortened, potentially functional hLAL mRNA, unlike the null mutations typically found in WD.
- One CESD case with a different mutation (S289C) exhibited measurable residual acid esterase activity, suggesting partial enzyme function.
- While overall hLAL activity is low in both, CESD is characterized by at least one allele capable of producing residual enzymatic function, distinguishing it from WD.
Implications:
- This study clarifies the molecular basis for the phenotypic divergence between WD and CESD, highlighting the importance of residual hLAL activity in ameliorating disease severity.
- Identifying the specific splicing mutation in CESD offers a potential diagnostic marker and suggests therapeutic strategies targeting residual enzyme function.
- Understanding these genotype-phenotype correlations is vital for accurate diagnosis, prognosis, and the development of targeted treatments for hLAL-deficient lysosomal storage diseases.
Abstract:
Mechanisms producing the divergent phenotypes, Wolman disease (WD) and cholesterol ester storage disease (CESD), associated with the genetic deficiency of human lysosomal acid lipase/cholesterol ester hydrolase (hLAL) function were investigated with the determination of HLAL activity levels, mRNA and protein expression, and defects in structural gene sequences in cells from three WD and five CESD patients. Measured with natural substrates, HLAL activities were all below 2% of normal, regardless of phenotype. Immunoblotting showed a lack of detectable hLAL protein in all mutant fibroblasts. Four CESD, but no WD genomes contained at least one allele with a specific exon 8 splice junction mutation, c.894 G>A, that encodes a shortened form of hLAL mRNA. Other CESD mutations were identical in type to the WD defects: nucleotide deletions (positions 397, 684, 980), insertions (594), or substitutions (193, 347) that result in premature terminations precluding any function. The only exception was a substitution at nucleotide 866 in the CESD case without an exon 8 splicing mutation; expression of the predicted S289C change in a transfection assay produced a low, but clearly measurable, level of acid esterase activity. Although it is not easily demonstrated in conventional assays, CESD is distinct from WD in that at least one mutant allele has the potential to produce enough residual enzymatic function to ameliorate the phenotype; in the majority of CESD cases this may come from a single, easily detected, splicing mutation in one allele.