海南汉族、黎族人葡萄糖-6-磷酸脱氢酶缺乏症的基因突变型分析及一种新的G6PD基因突变型的鉴定

目的 阐明海南汉族、黎族人群中葡萄糖－6－磷酸脱氢酶缺乏症的分子基础。方法 用聚合酶链反应、限制性内切酶消化筛查了1388G→A、1360C→T、1024C→T、517T→C、493A→G、487G→A、392G→T和95A→G突变。用单链构象多态性分析筛查其它突变;用核苷酸顺序分析鉴定具有SSCP异常区带样品的突变;。结果 在59例汉族G6PD缺乏症患者中,发现1388G→A14例（23．7％）、871G→A3例（5．1％）、835A→T1例（1．7％）517T→C1例（1．7％）、392G→T3例（5．1％）95A→G4例（6．8％）;在32例黎族G6PD缺乏症患者中,发现1388G→A6例（18．8％）、871G→A3例（9．4％）和95A→G2例（6．3％）;在1例汉族患者中发现了一种新的G6PD基因突变－835A→G突变,此突变导致第279位的苏氨酸被丙氨酸取代,将此突变型命名为G6PD－海口,其酶活性约是正常的10％,此835A→T突变的活性低,后者的酶活性约是正常的40％。分析人G6PD的三维结构模型表明,第279位苏氨酸残基的羟基是维持G6PD亚基相互作用的基团。结论 海南汉族、黎族人群中具有共同的常见G6PD基因突变型;与中国其它地区人群的G6PD基因突变谱比较,结果表明某些G6PD基因突变广泛分布于中国南方不同地区人群中;G6PD第279位苏氨酸残基的可能是维持G6PD亚基相互作用及酶活性的必需基团。     

Five novel mutations in the lysosomal sialidase gene (NEU1) in type II sialidosis patients and assessment of their impact on enzyme activity and intracellular targeting using adenovirus-mediated expression

Sialidosis is an autosomal recessive disease resulting from a deficiency of lysosomal sialidase. Type II sialidosis is a rare disease characterized clinically by hydrops fetalis, hepatosplenomegaly, and severe psychomotor retardation. Genomic DNA from four unrelated sialidosis patients was screened for mutations within the sialidase gene NEU1. Five novel mutations were identified. Four are missense and one is nonsense: c.674G>C (p.R225P), c.893C>T (p.A298V), c.3G>A (p.M1?), c.941C>G (p.R341G), and c.69G>A (p.W23X). We have used our findings and diagnostic tools to confirm the presence of a homozygous null allele in a neonate sibling. Recombinant adenoviruses expressing the mutant sialidase alleles in primary cell cultures were utilized to assess the impact of each mutation on enzyme activity and intracellular localization. None of the mutant alleles expressed significant enzymatic activity. The p.R341G mutation exerts its pathological effect by perturbing substrate binding, while the p.A298V and p.R225P mutations appear to impair the folding of the sialidase enzyme. Our findings point to mutation‐sensitive amino acids involved in catalytic function or structural stability and indicate the potential utility of these mutations for molecular diagnosis of this rare disease. Hum Mutat 23:32–39, 2004. © 2003 Wiley‐Liss, Inc.     

Characterization of six novel mutations in the methylenetetrahydrofolate reductase (MTHFR) gene in patients with homocystinuria

Severe deficiency of methylenetetrahydrofolate reductase (MTHFR) is the most common inborn error of folate metabolism. Patients are characterized by severe hyperhomocysteinemia, homocystinuria and a variety of neurological and vascular problems. Eighteen rare mutations have been reported in this group of patients. Two polymorphisms which cause mild enzyme deficiencies have been described (677C-->T and 1298A-->C). The first sequence change encodes a thermolabile enzyme and is associated with mild hyperhomocysteinemia. Six novel point mutations are described in patients with severe deficiency of MTHFR, along with their associated polymorphisms and clinical phenotypes. Of the two nonsense mutations (1762A-->T, 1134C-->G) and four missense mutations (1727C-->T, 1172G-->A, 1768G-->A, and 358G-->A), one was identified in the N-terminal catalytic domain, while the others were located in the regulatory C-terminal region. All four residues affected by missense mutations are conserved in one or more MTHFRs of other species. This report brings the total to 24 mutations identified in severe MTHFR deficiency, with two mutations identified in each of 22 patients.     

Several mutations including two novel mutations of the glucose-6-phosphate dehydrogenase gene in Polish G6PD deficient subjects with chronic nonspherocytic hemolytic anemia, acute hemolytic anemia, and favism

DNA sequencing revealed seven different glucose-6-phosphate dehydrogenase (G6PD) mutations in G6PD deficient subjects from 10 Polish families. Among them we found two novel mutations: 679C-->T (G6PD Radlowo, class 2) and a 1006A-->G (G6PD Torun, class 1). Variant G6PD Radlowo was characterized biochemically. Both novel mutations were analyzed using a model of the tertiary structure of the human enzyme. The main chain of G6PD Torun is different from the wild-type G6PD. The remaining mutations identified by us in deficient Polish patients were: 542A-->T (G6PD Malaga), 1160G-->A (G6PD Beverly Hills), 1178G-->A (G6PD Nashville), 1192G-->A (G6PD Puerto Limon), and 1246G-->A (G6PD Tokyo). Variant Tokyo was found in four families. In one of them favism was the first clinical sign of G6PD deficiency and chronic nonspherocytic hemolytic anemia (CNSHA) was diagnosed later. Variants G6PD Nashville and G6PD Puerto Limon were accompanied by the silent mutation 1311C-->T of the G6PD gene.     

Molecular pathology of NEU1 gene in sialidosis

Lysosomal sialidase (EC 3.2.1.18) has a dual physiological function; it participates in intralysosomal catabolism of sialylated glycoconjugates and is involved in cellular immune response. Mutations in the sialidase gene NEU1, located on chromosome 6p21.3, result in autosomal recessive disorder, sialidosis, which is characterized by the progressive lysosomal storage of sialylated glycopeptides and oligosaccharides. Sialidosis type I is a milder, late-onset, normosomatic form of the disorder. Type I patients develop visual defects, myoclonus syndrome, cherry-red macular spots, ataxia, hyperreflexia, and seizures. The severe early-onset form, sialidosis type II, is also associated with dysostosis multiplex, Hurler-like phenotype, mental retardation, and hepatosplenomegaly. We summarize information on the 34 unique mutations determined so far in the sialidase gene, including four novel missense and one novel nonsense mutations found in two Czech and two French sialidosis patients. The analysis of sialidase mutations in sialidosis revealed considerable molecular heterogeneity, reflecting the diversity of clinical phenotypes that make molecular diagnosis difficult. The majority of sialidosis patients have had missense mutations, many of which have been expressed; their effects on activity, stability, intracellular localization, and supramolecular organization of sialidase were studied. A structural model of sialidase allowed us to localize mutations in the sialidase molecule and to predict their impact on the tertiary structure and biochemical properties of the enzyme.     

广东客家人 G6PD基因突变型研究

目的　研究广东客家人葡萄糖 - 6 -磷酸脱氢酶缺乏症 (glucose- 6 - phosphate dehydrogenase,G6 PD)遗传多样性。方法　应用突变特异性扩增系统法 ,聚合酶链反应 -单链构象多态性并结合 DNA测序技术 ,确定客家人 G6 PD基因突变型的类型及频率。结果　在客家人中发现 G6 PD c DNA1388(G→ A)、1376 (G→ T)、95 (A→ G)、392 (G→ T)、10 2 4 (C→ T)及 1311(C→ T)复合 11内含子 93位 (T→ C)的突变。结论　 G6 PD基因 c DNA1388(G→A)、1376 (G→T)、95 (A→ G)、392 (G→ T)、10 2 4 (C→ T)、1311(C→T)复合 11内含子 93位 (T→C)突变是共同存在于中国人群中的 G6 PD基因突变型。c DNA1388(G→A)、c DNA 1376 (G→ T)是客家人群中最常见的 G6 PD基因突变型。在广东客家人群中未发现新的 G6 PD基因突变型。     

Eight novel mutations and consequences on mRNA and protein level in pyruvate kinase-deficient patients with nonspherocytic hemolytic anemia

Pyruvate kinase (PK) deficiency (PKD) is an autosomal recessive disorder with the typical manifestation of nonspherocytic hemolytic anemia. We analyzed the mutant enzymes of 10 unrelated patients with PKD, whose symptoms ranged from a mild, chronic hemolytic anemia to a severe anemia, by sequence analysis for the presence of alterations in the PKLR gene. In all cases the patients were shown to be compound heterozygous. Eight novel mutations were identified: 458T-->C (Ile153Thr), 656T-->C (Ile219Thr), 877G-->A (Asp293Asn), 991G-->A (Asp331Asn), 1055C-->A (Ala352Asp), 1483G-->A (Ala495Thr), 1649A-->T (Asp550Val), and 183-184ins16bp. This 16 bp duplication produces a frameshift and subsequent stop codon resulting in a drastically reduced mRNA level, and probably in an unstable gene product. Surprisingly, the existence of M2-type PK could be demonstrated in the patient's red blood cells. The study of different polymorphic sites revealed, with one exception, a strict linkage of the 1705C, 1738T, IVS5(+51)T, T(10) polymorphisms and the presence of 14 ATT repeats in intron 11. Our analyses show the consequences of a distorted structure on enzyme function and we discuss the correlations between the mutations identified and the parameters indicative for enzyme function.     

Identification of 9 novel IDS gene mutations in 19 unrelated Hunter syndrome (mucopolysaccharidosis Type II) patients. Mutations in brief no. 202. Online

Hunter syndrome is an X-linked lysosomal storage disorder caused by a deficiency of the lysosomal enzyme iduronate-2-sulfatase (IDS). The IDS deficiency can be caused by several different types of mutations in the IDS gene. We have performed a molecular and mutation analysis of a total 19 unrelated MPS II patients of different ethnic origin and identified 19 different IDS mutations, 9 of which were novel and unique. SSCP analysis followed by DNA sequencing revealed four novel missense mutations: S143F, associated with the 562C-->T polymorphism, C184W, D269V and Y348H. Two novel nonsense mutations were found: Y103X (433C-->A) and Y234X (826C-->G). In two patients two novel minor insertions (42linsA and 499insA) were identified. In one patient a complete IDS deletion was found, extending from locus DXS1185 to locus DXS466).     

Molecular analysis of the hydroxymethylbilane synthase (HMBS) gene in Italian patients with acute intermittent porphyria: report of four novel mutations

Acute intermittent porphyria (AIP) is an autosomal disorder caused by molecular abnormalities in the hydroxymethylbilane synthase (HMBS) gene coding for the third enzyme in the heme biosynthetic pathway. So far, more than 160 different mutations responsible for AIP have been identified in this gene. We have now identified seven mutations in eight unrelated Italian patients with AIP: two splicing defects (IVS7+2T-->C, 612G-->T), three small deletions (308-309delTG, 730-731delCT, 182delA) and two missense mutations (134C-->A, 541C-->T). The splicing defects were responsible for activation of splicing cryptic sites respectively within intron 7 (15 bp insertion) and exon 10 (9 bp deletion). The small deletions resulted in frameshifts leading to the formation of premature stop codons. The 134C-->A and 541C-->T mutations caused the formation of stop codons likely to be responsible for drastic disruption of the HMBS structure (Ser45Ter, Gln181Ter). This is the first molecular study in AIP patients of Italian origin leading to the identification of four new mutations and three molecular defects that have already been described.     

Genetic mutation profile of isovaleric acidemia patients in Taiwan

Isovaleric acidemia (IVA), a rare recessive autosomal disorder, is caused by isovaleryl-CoA dehydrogenase (IVD) deficiency. IVA may present with symptoms during the acute stage of severe metabolic acidosis, ketosis, vomiting, and altered mental status. With the help of newborn screening (NBS) by tandem mass spectrometry (MS/MS), IVA can now be diagnosed presymptomatically. According to statistic data, the incidence of IVA in Taiwan was about 1/365,000. In this study, six IVA patients from five families were investigated and followed-up clinically. As for the timing, two patients were found before MS technique introduced to Taiwan, the others were identified after MS/MS applied to NBS. The blood level of C5-carnitine in our patients was 7.43-18.96 microM (with upper limit in our laboratory <0.51 microM) and all of their urines contained raised amounts of 3-hydroxyisovaleric acid and isovalerylglycine. Molecular analysis of their IVD gene revealed six mutation profiles, among which the 149G-->A (Arg21His) and 1174 C-->T (Arg363Cys) mutations have been reported previously, while the other four mutations, 386A-->G (His100Arg), 347C-->T (Ser87Phe), 1007G-->A (Cys307Tyr) and 1199A-->G (Tyr371Cys), were first reported. Specially, we found 1199A-->G (Tyr371Cys) mutated was a common recurring missense mutation in our population (4 in 10 mutant alleles).     

应用变性高效液相色谱技术筛查广东汉族及壮族G6PD基因变异型

目的建立变性高效液相色谱筛查G6PD基因变异的技术。观察广东汉族和壮族G6PD基因变异型的分布。方法应用变性高效液相色谱技术筛查广东汉族和壮族G6PD基因变异型。与ARMS及DNA测序比较，考察其准确性、灵敏度、效率及耗资。结果在73例G6PD缺陷者中发现G6PD1388G→A20例、1376G→T24例、95A→G6例、1311C→T8例、1024C→T3例、392G→T2例、871G→A1例、1376G→A／95A→G4例、1388pA／95A→G3例、1311C→T／95A→G2例，与DNA测序结果吻合；在酶学正常对照组中发现男1例女2例1388突变。结论变性高效液相色谱技术对G6PD基因变异型的筛查具有准确、高效、半自动化及经济等优点。尤其对无症状女性杂合子及临床Ⅳ型个体的筛查具有重要意义。     

Investigation of citrullinemia type I variants by in vitro expression studies

Mild citrullinemia is an allelic variant of classical citrullinemia type I also caused by deficiency of the urea cycle enzyme argininosuccinate synthetase (ASS). Affected patients comprise a biochemical but no clinical phenotype. However, there is no reliable parameter allowing conclusions regarding the course of the disorder or its type of manifestation. The aim of this study was to test the importance of varying levels of ASS residual activities for the severity at diagnosis. Bacterial in vitro expression studies allowed the enzymatic analysis of purified wild-type and the mutant ASS proteins p.Ala118Thr (c.352G>A), p.Trp179Arg (c.535T>C), p.Val263Met (c.787G>A), p.Arg265Cys (c.793C>T), p.Met302Val (c.904A>G), p.Gly324Ser (c.970G>A), p.Gly362Val (c.1085G>T), and p.Gly390Arg (c.1168G>A). In the chosen system, classical mutations do not show any significant enzymatic activity, whereas mutations associated with a mild course yield significant ASS activity levels. The mutation p.Ala118Thr (c.352G>A) impresses by a high residual activity (62%) but a severe reduction of affinity toward the substrates citrulline and aspartate. This mutation was identified in a hitherto healthy female adult with no history of known citrullinemia who had died during the postpartum period from hyperammonemic coma. The results of this study suggest that even a high level of residual ASS activity is not a reliable prognostic marker for an uneventful clinical course. Determination of ASS residual activities, therefore, cannot help in anticipating the risk of metabolic derangement. This study should guide clinicians as well as patients with mild citrullinemia toward a lifelong awareness of the disorder.     

Lysosomal multienzymatic complex-related diseases: a genetic study among Portuguese patients

The functional activity of lysosomal enzymes sialidase, β-galactosidase and N-acetylaminogalacto-6-sulfate-sulfatase in the cell depends on their association in a multienzyme complex with cathepsin A. Mutations in any of the components of this complex result in functional deficiency thereby causing severe lysosomal storage disorders. Here, we report the molecular defects underlying sialidosis (mutations in sialidase; gene NEU1), galactosialidosis (mutations in cathepsin A; gene PPGB) and GM1 gangliosidosis (mutations in β-galactosidase; gene GLB1) in Portuguese patients. We performed molecular studies of the PPGB, NEU1 and GLB1 genes in biochemically diagnosed Portuguese patients. Gene expression was determined and the effect of each mutation predicted at protein levels. In the NEU1 gene, we found three novel missense mutations (p.P200L, p.D234N and p.Q282H) and one nonsense mutation (p.R341X). In the PPGB gene, we identified two missense mutations, one novel (p.G86V) and one already described (p.V104M), as well as two new deletions (c.230delC and c.991-992delT) that give rise to non-functional proteins. We also present the first molecular evidence of a causal missense mutation localized to the cathepsin A active site. Finally, in the GLB1 gene, we found six different mutations, all of them previously described (p.R59H, p.R201H, p.H281Y, p.W527X, c.1572-1577InsG and c.845-846delC). Seven novel mutations are reported here, contributing to our knowledge of the mutational spectrum of these diseases and to a better understanding of the genetics of the lysosomal multienzymatic complex. The results of this study will allow carrier detection in affected families and prenatal molecular diagnosis, leading to the improvement of genetic counseling.     

The thermolabile variant 677C-->T can further reduce activity when expressed in cis with severe mutations for human methylenetetrahydrofolate reductase

Methylenetetrahydrofolate reductase (MTHFR) catalyses the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, a carbon donor for homocysteine remethylation to methionine. Severe MTHFR deficiency is associated with hyperhomocysteinemia and homocystinuria. These patients show a wide variety of neurological and vascular symptoms, with variable age of onset. Residual enzyme activity is usually less than 20% of control values, and correlates reasonably well with age of onset of symptoms. A milder deficiency of MTHFR, with 30%-50% residual enzyme activity and increased enzyme thermolability, has been described as a risk factor for vascular disease and for neural tube defects. In earlier work, we isolated the human cDNA for MTHFR, and reported 14 mutations in severe MTHFR deficiency, as well as a common 677C-->T missense mutation (Ala-->Val) that encodes the thermolabile MTHFR. This variant has also been observed in some patients with severe MTHFR deficiency, in cis with their severe mutations. We report here the in vitro expression of seven severe MTHFR mutations in a bacterial expression system; six of these were expressed in cis with the Val allele to mimic the situation in the patients. We show that three of these constructs have significantly reduced enzyme activity (<10% of control); the presence of the thermolabile variant in these patients in cis is unlikely to affect enzyme function since activity is already low. One mutation causes a dramatic increase in activity when it is expressed in cis with the Ala allele, but is associated with extreme lability when in cis with the Val allele. Three mutations cause moderate decreases in enzyme activity, with a further decrease in activity when they are in cis with the Val allele. We hypothesize that deleterious mutations which alter stability may be compromised to a greater degree when the thermolabile variant is present on the same allele.     

Transport, enzymatic activity, and stability of mutant sulfamidase (SGSH) identified in patients with mucopolysaccharidosis type III A

Mucopolysaccharidosis type IIIA (MPSIIIA) is an autosomal recessive lysosomal storage disease caused by mutations in the N-sulfoglucosamine sulfohydrolase gene (SGSH; encoding sulfamidase, also sulphamidase) leading to the lysosomal accumulation and urinary excretion of heparan sulfate. Considerable variation in the onset and severity of the clinical phenotype is observed. We report here on expression studies of four novel mutations: c.318C>A (p.Ser106Arg), c.488T>C (p.Leu163Pro), c.571G>A (p.Gly191Arg), and c.1207_1209delTAC (p.Tyr403del), and five previously known mutations: c.220C>T (p.Arg74Cys), c.697C>T (p.Arg233X), c.1297C>T (p.Arg433Trp), c.1026dupC (p.Leu343fsX158), and c.1135delG (p.Val379fsX33) identified in MPSIIIA patients. Transient expression of mutant sulfamidases in BHK or CHO cells revealed that all the mutants were enzymatically inactive with the exception of c.318C>A (p.Ser106Arg), which showed 3.3% activity of the expressed wild-type enzyme. Western blot analysis demonstrated that the amounts of expressed mutant sulfamidases were significantly reduced compared with cells expressing wild type. No polypeptides were immunodetectable in extracts of cells transfected with the cDNA carrying the c.697C>T (p.Arg233X) nonsense mutation. In vitro translation and pulse-chase experiments showed that rapid degradation rather than a decrease in synthesis is responsible for the low, steady-state level of the mutant proteins in cells. The amounts of secreted mutant precursor forms, the cellular stability, the proteolytic processing, and data from double-label immunofluorescence microscopy suggest that the degradation of the majority of newly synthesized c.220C>T (p.Arg74Cys), c.571G>A (p.Gly191Arg), c.1297C>T (p.Arg433Trp), c.1026dupC (p.Leu343fsX158), and c.1135delG (p.Val379fsX33) mutant proteins probably occurs in the ER, whereas c.488T>C (p.Leu163Pro) mutant protein showed instability in the lysosomes.     

Clinical spectrum and molecular basis of recessive congenital methemoglobinemia in India

We report the clinical features and molecular characterization of 23 patients with cyanosis due to NADH‐cytochrome b5 reductase (NADH‐CYB5R) deficiency from India. The patients with type I recessive congenital methemoglobinemia (RCM) presented with mild to severe cyanosis only whereas patients with type II RCM had cyanosis associated with severe neurological impairment. Thirteen mutations were identified which included 11 missense mutations causing single amino acid changes (p.Arg49Trp, p.Arg58Gln, p.Pro145Ser, p.Gly155Glu, p.Arg160Pro, p.Met177Ile, p.Met177Val, p.Ile178Thr, p.Ala179Thr, p.Thr238Met, and p.Val253Met), one stop codon mutation (p.Trp236X) and one splice‐site mutation (p.Gly76Ser). Seven of these mutations (p.Arg50Trp, p.Gly155Glu, p.Arg160Pro, p.Met177Ile, p.Met177Val, p.Ile178Thr, and p.Thr238Met) were novel. Two mutations (p.Gly76Ser and p.Trp236X) were identified for the first time in the homozygous state globally causing type II RCM. We used the three‐dimensional (3D) structure of human erythrocyte NADH‐CYB5R to evaluate the protein structural context of the affected residues. Our data provides a rationale for the observed enzyme deficiency and contributes to a better understanding of the genotype–phenotype correlation in NADH‐CYB5R deficiency.     

Patched homologue 1 mutations in four Japanese families with basal cell nevus syndrome

AIM: To search for patched homologue 1 (PTCH1) mutations in four families with basal cell nevus syndrome (BCNS). METHODS: Mutation analysis of PTCH1 in unrelated Japanese families affected with BCNS was carried out by direct sequencing. RESULTS: Six novel PTCH1 mutations, 833G-->A in exon 6, 1415C-->A and 1451G-->T in exon 10, 2798delC in exon 17, 2918-2925dupAGTTCCCT in exon 18 and 3956C-->A in exon 23, were identified. CONCLUSIONS: Among the six PTCH1 mutations, two frameshift mutations (2798delC and 2918-2925dupAGTTCCCT) and one nonsense mutation (833G-->A) are predicted to lead to premature termination of PTCH1 protein translation. Three simultaneous mutations, 1415C-->A (A472D) and 1451G-->T (G484V) in exon 10, and 3956G-->A (R1319H) in exon 23, were found on one allele in only affected members in one family and none of them were found among 90 unrelated healthy Japanese. The three mutations on one chromosome may have resulted from errors in the recombinational repair process and this is the first report on the PTCH1 mutations due to such a mechanism.     

The major allele of the alanine:glyoxylate aminotransferase gene: seven novel mutations causing primary hyperoxaluria type 1

We describe 7 novel mutations occurring on the major allele of the human AGT gene in patients with primary hyperoxaluria type 1, an autosomal recessive disease resulting from a deficiency of the liver peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT; EC 2.6.1.44). These mutations include 3 small deletions, 570delG, 744delC, and 983_988del, two splice junction mutations, IVS7-1G-->C and IVS8+1G-->T, and two nonsense mutations, R111X and W251X. We have also identified recurrences of previously identified reported mutations, 679-(IVS6+2)delAAgt, IVS8-3C-->G and 33insC. Deletion mutation 679-(IVS6+2)delAAgt has now been identified in a second Chinese patient and may be specific to that population. In contrast, 33insC has been found in patients of varying ethnic and racial backgrounds; a single vs multiple origin for this mutation is thus an intriguing question. It also appears to occur at a high frequency on the major allele. Five of the novel mutations were detected in patients who were compound heterozygotes for one of the common mis-targeting mutation, G170R or F152I, while the other two mutations occurred in the same patient.     

A point mutation in the neu-1 locus causes the neuraminidase defect in the SM/J mouse

Lysosomal neuraminidase (sialidase) occurs in a high molecular weight complex with the glycosidase beta-galactosidase and the serine carboxypeptidase protective protein/cathepsin A (PPCA). Association of the enzyme with PPCA is crucial for its correct targeting and lysosomal activation. In man two genetically distinct storage disorders are associated with either a primary or a secondary deficiency of lysosomal neuraminidase: sialidosis and galactosialidosis. In the mouse the naturally occurring inbred strain SM/J presents with a number of phenotypic abnormalities that have been attributed to reduced neuraminidase activity. SM/J mice were originally characterized by their altered sialylation of several lysosomal glycoproteins. This defect was linked to a single gene, neu-1 , on chromosome 17, which was mapped by linkage analysis to the H-2 locus. In addition, these mice have an altered immune response that has also been coupled to a deficiency of the Neu-1 neuraminidase. Here we report the identification in SM/J mice of a single amino acid substitution (L209I) in the Neu-1 protein which is responsible for the partial deficiency of lysosomal neuraminidase. We propose that the reduced activity is caused by the enzyme's altered affinity for its substrate, rather than a change in substrate specificity or turnover rate. The mutant enzyme is correctly compartmentalized in lysosomes and maintains the ability to associate with its activating protein, PPCA. We propose that it is this mutation that is responsible for the SM/J phenotype.