Analysis of Mucolipidosis II/III GNPTAB Missense Mutations Identifies Domains of UDP-GlcNAc:lysosomal Enzyme GlcNAc-1-phosphotransferase Involved in Catalytic Function and Lysosomal Enzyme Recognition

UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase tags newly synthesized lysosomal enzymes with mannose 6-phosphate recognition markers, which are required for their targeting to the endolysosomal system. GNPTAB encodes the α and β subunits of GlcNAc-1-phosphotransferase, and mutations in this gene cause the lysosomal storage disorders mucolipidosis II and III αβ. Prior investigation of missense mutations in GNPTAB uncovered amino acids in the N-terminal region and within the DMAP domain involved in Golgi retention of GlcNAc-1-phosphotransferase and its ability to specifically recognize lysosomal hydrolases, respectively. Here, we undertook a comprehensive analysis of the remaining missense mutations in GNPTAB reported in mucolipidosis II and III αβ patients using cell- and zebrafish-based approaches. We show that the Stealth domain harbors the catalytic site, as some mutations in these regions greatly impaired the activity of the enzyme without affecting its Golgi localization and proteolytic processing. We also demonstrate a role for the Notch repeat 1 in lysosomal hydrolase recognition, as missense mutations in conserved cysteine residues in this domain do not affect the catalytic activity but impair mannose phosphorylation of certain lysosomal hydrolases. Rescue experiments using mRNA bearing Notch repeat 1 mutations in GNPTAB-deficient zebrafish revealed selective effects on hydrolase recognition that differ from the DMAP mutation. Finally, the mutant R587P, located in the spacer between Notch 2 and DMAP, was partially rescued by overexpression of the γ subunit, suggesting a role for this region in γ subunit binding. These studies provide new insight into the functions of the different domains of the α and β subunits.     

Post-translational modifications of the gamma-subunit affect intracellular trafficking and complex assembly of GlcNAc-1-phosphotransferase

GlcNAc-1-phosphotransferase plays a key role in the generation of mannose 6-phosphate, a recognition marker essential for efficient transport of lysosomal hydrolases to lysosomes. The enzyme complex is composed of six subunits (α(2)β(2)γ(2)). The α- and β-subunits are catalytically active, whereas the function of the γ-subunit is still unclear. We have investigated structural properties, localization, and intracellular transport of the human and mouse γ-subunits and the molecular requirements for the assembly of the phosphotransferase complex. The results showed that endogenous and overexpressed γ-subunits were localized in the cis-Golgi apparatus. Secreted forms of γ-subunits were detectable in media of cultured cells as well as in human serum. The γ-subunit contains two in vivo used N-glycosylation sites at positions 88 and 115, equipped with high mannose-type oligosaccharides. (35)S pulse-chase experiments and size exclusion chromatography revealed that the majority of non-glycosylated γ-subunit mutants were integrated in high molecular mass complexes, failed to exit the endoplasmic reticulum (ER), and were rapidly degraded. The substitution of cysteine 245 involved in dimerization of γ-subunits impaired neither ER exit nor trafficking through the secretory pathway. Monomeric γ-subunits failed, however, to associate with other GlcNAc-1-phosphotransferase subunits. The data provide evidence that assembly of the GlcNAc-1-phosphotransferase complex takes place in the ER and requires dimerization of the γ-subunits.     

Three novel homozygous mutations in the GNPTG gene that cause mucolipidosis type III gamma

Background

Mucolipidosis type III gamma (MLIII gamma) is an autosomal recessive disease caused by a mutation in the GNPTG gene, which encodes the γ subunit of the N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase). This protein plays a key role in the transport of lysosomal hydrolases to the lysosome.

Methods

Three Chinese children with typical skeletal abnormalities of MLIII were identified, who were from unrelated consanguineous families. After obtaining informed consent, genomic DNA was isolated from the patients and their parents. Direct sequencing of the GNPTG and GNPTAB genes was performed using standard PCR reactions.

Results

The three probands showed clinical features typical of MLIII gamma, such as joint stiffness and vertebral scoliosis without coarsened facial features. Mutation analysis of the GNPTG gene showed that three novel mutations were identified, two in exon seven [c.425G>A (p.Cys142Val)] and [c.515dupC (p.His172Profs27X)], and one in exon eight [c.609+1G>C]. Their parents were determined to be heterozygous carriers when compared to the reference sequence in GenBank on NCBI.

Conclusions

Mutation of the GNPTG gene is the cause of MLIII gamma in our patients. Our findings expand the mutation spectrum of the GNPTG gene and extend the knowledge of the phenotype–genotype correlation of the disease.     

Down-regulation of pyrophosphate: fructose 6-phosphate 1-phosphotransferase (PFP) activity in sugarcane enhances sucrose accumulation in immature internodes

Pyrophosphate: fructose 6-phosphate 1-phosphotransferase (PFP) activity was successfully down-regulated in sugarcane using constitutively expressed antisense and untranslatable forms of the sugarcane PFP-β gene. In young internodal tissue activity was reduced by up to 70% while no residual activity could be detected in mature tissues. The transgenic plants showed no visible phenotype or significant differences in growth and development under greenhouse and field conditions. Sucrose concentrations were significantly increased in the immature internodes of the transgenic plants but not in the mature internodes. This contributed to an increase in the purity of the immature tissues, resembling an early ripening phenotype. Both the immature and mature internodes of the transgenic plants had significantly higher fibre contents. These findings suggest that PFP influences the ability of young, biosynthetically active sugarcane culm tissue to accumulate sucrose but that the equilibrium of the glycolytic intermediates, including the stored sucrose, is restored when ATP-dependent phosphofructokinase and the residual PFP activity is sufficient to sustain the required glycolytic flux as the tissue matures. Moreover, it suggests a role for PFP in glycolytic carbon flow, which could be rate limiting under conditions of high metabolic activity.     

Identification of two novel missense WFS1 mutations, H696Y and R703H,in patients with non-syndromic low-frequency sensorineural hearing loss

Non-syndromic low-frequency sensorineural hearing loss (LFSNHL) is an unusual type of hearing loss in which frequencies ≤2000 Hz predominantly are affected. To date, different mutations in two genes, DIAPH1 and WFSI, have been found to be associated with LFSNHL.Here, we report a five-generation Chinese family with postlingual and progressive LFSNHL. We mapped the disease locus to a 2.5 Mb region on chromosome 4p16 between markers SNP_A-2167174 and D4S431, overlapping with the DFNA6/14/38 locus. Sequencing of candidate gene revealed a heterozygous c.2086C＞T substitution in exon 8 of WFS1, leading to p. H696Y substitution at the C-terminus of Wolframin (WFS1).In addition, we performed mutational screening of WFS1 in 37 sporadic patients, 7-50 years of age, with LFSNHL. We detected a heterozygous c.2108G＞A substitution in exon 8 of WFS1, leading to p. R703H substitution in a patient. The H696 and R703 in WFS1 are highly conserved across species, including human, orangutan, rat, mouse, and frog (Xenopus). Sequence analysis demonstrated the absence of c.2086C＞T or c.2108G＞A substitutions in the WFS1 genes among 200 unrelated control subjects of Chinese background, supporting the hypothesis that they represent causative mutations, and not rare polymorphisms. Our data provide additional molecular and clinical information for establishing a better genotype-phenotype correlation for LFSNHL.     

Identification of two novel missense WFS1 mutations,H696Y and R703H,in patients with non-syndromic low-frequency sensorineural hearing loss

Non-syndromic low-frequency sensorineural hearing loss(LFSNHL) is an unusual type of hearing loss in which frequencies≤2000 Hz predominantly are affected.To date,different mutations in two genes,DIAPH1 and WFS1,have been found to be associated with LFSNHL. Here,we report a five-generation Chinese family with postlingual and progressive LFSNHL.We mapped the disease locus to a 2.5 Mb region on chromosome 4p16 between markers SNP_A-2167174 and D4S431,overlapping with the DFNA6/14/38 locus.Sequencing of cand...