Identification and expression of a missense mutation (Y446C) in the acid sphingomyelinase gene from a Japanese patient with type A Niemann-Pick disease

Niemann-Pick disease A or B in four pediatric patients and SMPD1 mutation carrier frequency in the Mexican population

INTRODUCTION AND OBJECTIVES

Niemann-Pick disease type A (NPD-A) and B (NPD-B) are lysosomal storage diseases with a birth prevalence of 0.4-0.6/100,000. They are caused by a deficiency in acid sphingomyelinase, an enzyme encoded by SMPD1. We analyzed the phenotype and genotype of four unrelated Mexican patients, one with NPD-A and three with NPD-B.

PATIENTS AND METHODS

Four female patients between 1 and 7 years of age were diagnosed with NPD-A or NPD-B by hepatosplenomegaly, among other clinical characteristics, and by determining the level of acid sphingomyelinase enzymatic activity and sequencing of the SMPD1 gene. Additionally, a 775bp amplicon of SMPD1 (from 11:6393835_6394609, including exons 5 and 6) was analyzed by capillary sequencing in a control group of 50 unrelated healthy Mexican Mestizos.

RESULTS

An infrequent variant (c.1343A>G p.Tyr448Cys) was observed in two patients. One is the first NPD-A homozygous patient reported with this variant and the other a compound heterozygous NPD-B patient with the c.1829_1831delGCC p.Arg610del variant. Another compound heterozygous patient had the c.1547A>G p.His516Arg variant (not previously described in affected individuals) along with the c.1805G>A p.Arg602His variant. A new c.1263+8C>T pathogenic variant was encountered in a homozygous state in a NPD-B patient. Among the healthy control individuals there was a heterozygous carrier for the c.1550A>T (rs142787001) pathogenic variant, but none with the known pathogenic variants in the 11:6393835_6394609 region of SMPD1.

CONCLUSIONS

The present study provides further NPD-A or B phenotype-genotype correlations. We detected a heterozygous carrier with a pathogenic variant in 1/50 healthy Mexican mestizos.

SMPD1 Mutation Update: Database and Comprehensive Analysis of Published and Novel Variants

Niemann-Pick Types A and B (NPA/B) diseases are autosomal recessive lysosomal storage disorders caused by the deficient activity of acid sphingomyelinase (ASM) because of the mutations in the SMPD1 gene. Here, we provide a comprehensive updated review of already reported and newly identified SMPD1 variants. Among them, 185 have been found in NPA/B patients. Disease-causing variants are equally distributed along the SMPD1 gene; most of them are missense (65.4%) or frameshift (19%) mutations. The most frequently reported mutation worldwide is the p.R610del, clearly associated with an attenuated NP disease type B phenotype. The available information about the impact of 52 SMPD1 variants on ASM mRNA and/or enzymatic activity has been collected and whenever possible, phenotype/genotype correlations were established. In addition, we created a locus-specific database easily accessible at http://www.inpdr.org/genes that catalogs the 417 SMPD1 variants reported to date and provides data on their in silico predicted effects on ASM protein function or mRNA splicing. The information reviewed in this article, providing new insights into the genotype/phenotype correlation, is extremely valuable to facilitate diagnosis and genetic counseling of families affected by NPA/B.

Evidence for the association of ultraviolet-C and H(2)O(2)-induced apoptosis with acid sphingomyelinase activation

Ceramide appears to be a potent second messenger implicated in the regulation of diverse cellular processes such as cell growth and differentiation, gene transcription, ligand binding, and cell death. Environmental stress-induced apoptosis is believed to be associated with the sphingomyelin degradation pathway, which generates ceramide as a second messenger in initiating the apoptosis response. To date, two distinct sphingomyelinases, a lysosomal acid sphingomyelinase (ASM), which is deficient in patients affected with types A and B Niemann-Pick disease (NPD), and a neutral, magnesium-dependent sphingomyelinase (NSM), are candidate enzymes which respond to apoptotic stimulations and cause sphingomyelin hydrolysis and subsequent ceramide generation. Using Epstein-Barr virus (EBV)-transformed lymphoblast cells from type A NPD patient which have defined splicing site mutation in the ASM gene, we showed that ASM-deficient cells were defective in ultraviolet-C (UV-C) and hydrogen peroxide (H(2)O(2)) induced apoptosis. As another induction of apoptosis, we exposed this cell line to serum starvation which influences to p53 expression and leads to apoptosis. There were no differences by the degree of apoptosis between ASM-deficient lymphoblast cells and normal lymphoblast cells. These results are evidence that ASM plays one of the important roles in apoptosis induction by UV-C and H(2)O(2).

Niemann-Pick disease: mutation update, genotype/phenotype correlations, and prospects for genetic testing

Niemann-Pick Disease (NPD) is an autosomal recessive lysosomal storage disorder caused by a deficiency of acid sphingomyelinase (ASM). NPD occurs in two forms, neuronopathic Type A and nonneuronopathic Type B. The incidence of Type A NPD is highest among Ashkenazi Jews. Type B NPD is more common in non-Jews but has been reported in Ashkenazi Jews. Different mutations in ASM are presumed to be responsible for the different NPD phenotypes. Three mutations are predicted to account for > 95% of all Type A NPD chromosomes among Ashkenazi Jews (L302P, R496L, fsP330). Based on limited screens for these mutations among Ashkenazi Jews, a carrier frequency for Type A NPD of 1:90 is reported for this population. Less is known about mutations responsible for Type B NPD, although one mutation (delta R608) has been identified in both Ashkenazi Jews and non-Jews. Screening of the Ashkenazi Jewish population to detect > 95% of NPD carriers can be accomplished with a four-mutation panel that includes L302P, R496L, fsP330, and delta R608, the three predominant Type A mutations and one recurrent Type B mutation.

Heterogeneity of liver disorder in type B Niemann-Pick disease

Patients with type B Niemann-Pick disease (NPD) are known to be complicated with varying degrees of prognosis-determining liver dysfunction. To see heterogeneity of the dysfunction histologically, we performed liver biopsies on three NPD patients from three different families, who were diagnosed by enzyme assay of acid sphingomyelinase (ASM) and analysis of the ASM gene. In a severe case, of a female patient in her childhood, the liver showed definite fibrosis despite her age. In contrast, in a very mild case, of an adult male patient, the liver showed little fibrosis, though the ballooning of hepatocytes and infiltration of foamy histiocytes were observed in the tissue. Three homo-allelic mutations (S436R, A599T, and S231P) were identified in the patients. Thus, various hepatic phenotypes in type B NPD were shown to be caused by the heterogeneity of liver lesions originating from different ASM gene mutations.