Quaternary diagnostics scheme for mucolipidosis II and detection of novel mutation in GNPTAB gene

Background

Mucolipidosis II (ML II α/β) is an inherited lysosomal storage disorder caused by deficiency of GlcNAc-phosphotransferase enzyme and results in mis-targeting of multiple lysosomal enzymes. Affected patients are characterized by skeletal deformities and developmental delay. Homozygous or compound heterozygous mutations in GNPTAB gene are associated with the clinical presentation. This is the first study to characterize the underlying genetics of ML among a cohort of Egyptian patients. ML II diagnosis established by clinical assessment, biochemical evaluation of enzymes, electron microscopy examination of gingival inclusion bodies, and molecular study of GNPTAB gene using targeted next-generation sequencing panel in 8 patients form 8 unrelated Egyptian families.

Results

Sequencing revealed 3 mutations in GNPTAB gene; 1 novel frame-shift mutation in exon 19 (c.3488_3488delC) and 2 previously reported mutations (c.1759C>T in exon 13 and c.3503_3504delTC in exon 19). All patients were homozygous for their corresponding mutations and the parents were consanguineous.

Conclusions

According to the established quaternary diagnostic scheme, ML II was the final diagnosis in eight patients. The most common mutation was the frame shift c.3503_3504delTC mutation, found in 5 patients and associated with a severe phenotype.

Hip disease in Mucopolysaccharidoses and Mucolipidoses: A review of mechanisms, interventions and future perspectives

The hips are frequently involved in inheritable diseases which affect the bones. The clinical and radiological presentation of these diseases may be very similar to common hip disorders as developmental dysplasia of the hip, osteoarthritis and avascular necrosis, so the diagnosis may be easily overlooked and treatment may be suboptimal. Mucopolysaccharidosis (MPS) and Mucolipidosis (ML II and III) are lysosomal storage disorders with multisystemic involvement. Characteristic skeletal abnormalities, known as dysostosis multiplex, are common in MPS and ML and originate from intra-lysosomal storage of glycosaminoglycans in cells of the cartilage, bones and ligaments. The hip joint is severely affected in MPS and ML. Hip pathology results in limitations in mobility and pain from young age, and negatively affects quality of life. In order to better understand the underlying process that causes hip disease in MPS and ML, this review first describes the normal physiological (embryonic) hip joint development, including the interplay between the acetabulum and the femoral head. In the second part the factors contributing to altered hip morphology and function in MPS and ML are discussed, such as abnormal development of the pelvic- and femoral bones (which results in altered biomechanical forces) and inflammation. In the last part of this review therapeutic options and future perspectives are addressed.

Novel compound heterozygous MCOLN1 mutations identified in a Japanese girl with severe developmental delay and thin corpus callosum

Mucolipidosis type IV (MLIV) is a rare lysosomal storage disorder causing severe psychomotor developmental delay and progressive visual impairment. MLIV is an autosomal recessive disease caused by mutations in MCOLN1, which encodes for mucolipin-1. Here, we report a case of a 4-year-old Japanese girl with severe intellectual disability and motor deficits. Brain magnetic resonance imaging showed signal abnormalities in the white matter and thinning of the corpus callosum. Whole-exome sequencing was performed on the proband and her parents, and novel compound heterozygous mutations at c.936_938del (p.Phe313del) and c.1503dupC (p.Ile502Hisfs*106) in MCOLN1 (NM_020533.2) were identified in the proband. Additional biochemical examinations revealed elevated serum gastrin level and iron deficiency anemia, leading to the diagnosis of MLIV. More reports of such pathogenic mutations are expected to broaden the understanding of the channel function of mucolipin-1 and genotype-phenotype correlations.

Structural insights into group II TRP channels

The seven members of the TRP channel superfamily are divided into two main groups with five members comprising group I (TRPC/V/M/N/A) and TRPML (TRP MucoLipin) and TRPP (TRP Polycystin) making up group II. Group II channels share a high sequence homology on their transmembrane domains and are distinct from group I members as they contain a large luminal/extracellular domain between transmembrane helix 1 (S1) and S2. Since 2016, there are more than ten research papers reporting various structures of group II channels by either cryo-EM or X-ray crystallography. These studies along with recent functional analysis by the other groups have considerably strengthened our knowledge on TRPML and TRPP channels. In this review, we summarize and discuss these reports providing molecular insights into the group II TRP channel family.

Craniosynostosis and metabolic bone disorder. A review

INTRODUCTION

Some metabolic bone disorders may result in the premature closure of one or more calvarial sutures during childhood, potentially leading to a cranioencephalic disproportion. The aim of this paper is to review the characteristics and consequences of craniosynostosis associated with metabolic disorder.

MATERIAL AND METHODS

A review of the literature on metabolic forms of craniosynostosis was performed.

RESULTS

The most common forms of craniosynostosis associated with metabolic bone disorder were isolated sagittal suture fusion with or without scaphocephaly, and sagittal suture fusion associated with coronal suture fusion (oxycephaly) or also with lambdoid suture fusion (pansynostosis). Synostosis may be well-tolerated, but in some subjects results in neurodevelopmental and functional impairment that is sometimes severe.

CONCLUSION

The impact of metabolic synostosis is very variable, depending on the specific underlying metabolic disease, with a large spectrum of morphological and functional consequences. Diagnosis should be early and management should be carried out by a multidisciplinary team with expertise in both rare skeletal disorders and craniosynostosis. The impact of emergent medical therapies recently developed for some of these diseases will be assessed by systematic coherent follow-up of international registries.

Clinical, biochemical and molecular characterization of Korean patients with mucolipidosis II/III and successful prenatal diagnosis

BACKGROUND

Mucolipidosis types II and III (ML II/III) are autosomal recessive disorders caused by a deficiency in the lysosomal enzyme N-acetylglucosamine-1-phosphotransferase. We investigated the molecular genetic characteristics of the GNPTAB gene, which codes for the alpha/beta subunits of a phosphotransferase, in Korean ML II/III patients. We included prenatal tests and evaluated the spectrum of mutations in East Asian populations with ML II/III through a literature review.

METHODS

Seven patients from six families were enrolled in the study including two prenatal tests using chorionic villi samples. A diagnosis of ML II/III was made based on clinical findings and increases in serum lysosomal enzyme levels. PCR and direct sequencing were performed to identify GNPTAB mutations.

RESULTS

We found 14 mutant alleles including seven known mutations of c.2189delT (p.Leu730fs*7), c.1090C > T (p.Arg364*), c.2681G > A (p.Trp894*), c.3565C > T (p.Arg1189*), c.310C > T (p.Gln104*), c.1071G > A (p.Trp357*) and c.2574_2575delGA (p.Asn859Glnfs*2). Four were novel variants of unknown significance: c.992A > G (p.Tyr331Cys), c.2666 T > A (p.Leu889*), c.637-6 T > G (p.Thr213Phefs*11), and c.471_472delTT (p.Tyr158Serfs*8). Family studies revealed the probands to be compound heterozygotes. The fetuses carried the same GNPTAB mutations as the mucolipidosis II/III probands in the prenatal diagnosis.

CONCLUSIONS

We identified GNPTAB mutations in all patients with ML II/III, but did not identify a hot spot in Korean patients. We successfully performed prenatal diagnosis using molecular investigation.

Pitfalls in the prenatal diagnosis of mucolipidosis II alpha/beta: A case report

Mucolipidosis II alpha/beta is an autosomal recessive disorder caused by deficient activity of GlcNAc-1-phosphotransferase. We report the prenatal diagnosis of a fetus who was found to exhibit normal levels of lysosomal enzymes in the amniotic fluid but low levels in amniocytes, and who was found to be heterozygous for the most common GNPTAB mutation. As in some carriers of Mucolipidosis II biochemical abnormalities may hinder prenatal diagnosis, we suggest DNA analysis should be performed whenever possible.

Quantitative neuroimaging in mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive disorder resulting from mutations in the MCOLN1 gene. This gene encodes the endosomal/lysosomal transient receptor potential channel protein mucolipin-1 (TRPML1). Affected patients suffer from neurodevelopmental abnormalities and progressive retinal dystrophy. In a prospective natural history study we hypothesized the presence of an additional slow cerebral neurodegenerative process. We have recruited 5 patients, tested their neurodevelopmental status, and measured cerebral regional volumes and white matter integrity using MRI yearly. Over a period of up to 3 years, MLIV patients remained neurologically stable. There was a trend for increased cortical and subcortical gray matter volumes and increased ventricular size, while white matter and cerebellar volumes decreased. Mean diffusivity (MD) was increased and fractional anisotropy (FA) values were below normal in all analyzed brain regions. There was a positive correlation between motor scores of the Vineland Scale and the FA values in the corticospinal tract (corr coef 0.39), and a negative correlation with the MD values (corr coef -0.50) in the same brain region. We conclude from these initial findings that deficiency in mucolipin-1 affects the entire brain but that there might be a selective regional cerebral neurodegenerative process in MLIV. In addition, these data suggest that diffusion-weighted imaging might be a good biomarker for following patients with MLIV. Therefore, our findings may be helpful for designing future clinical trials.

Drosophila TRPML forms PI(3,5)P2-activated cation channels in both endolysosomes and plasma membrane

Transient Receptor Potential mucolipin (TRPML) channels are implicated in endolysosomal trafficking, lysosomal Ca(2+) and Fe(2+) release, lysosomal biogenesis, and autophagy. Mutations in human TRPML1 cause the lysosome storage disease, mucolipidosis type IV (MLIV). Unlike vertebrates, which express three TRPML genes, TRPML1-3, the Drosophila genome encodes a single trpml gene. Although the trpml-deficient flies exhibit cellular defects similar to those in mammalian TRPML1 mutants, the biophysical properties of Drosophila TRPML channel remained uncharacterized. Here, we show that transgenic expression of human TRPML1 in the neurons of Drosophila trpml mutants partially suppressed the pupal lethality phenotype. When expressed in HEK293 cells, Drosophila TRPML was localized in both endolysosomes and plasma membrane and was activated by phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) applied to the cytoplasmic side in whole lysosomes and inside-out patches excised from plasma membrane. The PI(3,5)P2-evoked currents were blocked by phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), but not other phosphoinositides. Using TRPML A487P, which mimics the varitint-waddler (Va) mutant of mouse TRPML3 with constitutive whole-cell currents, we show that TRPML is biphasically regulated by extracytosolic pH, with an optimal pH about 0.6 pH unit higher than that of human TRPML1. In addition to monovalent cations, TRPML exhibits high permeability to Ca(2+), Mn(2+), and Fe(2+), but not Fe(3+). The TRPML currents were inhibited by trivalent cations Fe(3+), La(3+), and Gd(3+). These features resemble more closely to mammalian TRPML1 than TRPML2 and TRPML3, but with some obvious differences. Together, our data support the use of Drosophila for assessing functional significance of TRPML1 in cell physiology.

Two homozygous nonsense mutations of GNPTAB gene in two Chinese families with mucolipidosis II alpha/beta using targeted next-generation sequencing

Mucolipidosis II alpha/beta (ML II alpha/beta; I-cell disease) is a rare, inherited, metabolic disease and has often been clinically misdiagnosed. ML II alpha/beta results from a deficiency of the enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-PT), which causes the lysosomal enzymes to accumulate in plasma. We identified two new Chinese patients with ML II alpha/beta by lysosomal enzyme assay. Using targeted next-generation sequencing genetic analysis, we located two homozygous nonsense mutations in the GNPTAB gene, c.1071G>A (p.W357X) and c.1090C>T (p.R364X). These results were confirmed by Sanger sequencing. To our knowledge, the c.1071G>A mutation has not been previously reported. Our findings add to the number of reported cases of this rare illness and to the GNPTAB pathogenic mutation database. This work also demonstrates the application of lysosomal enzyme assay and targeted next-generation sequencing for the genetic screening analysis and diagnosis of ML II alpha/beta.

A case of mucolipidosis II presenting with prenatal skeletal dysplasia and severe secondary hyperparathyroidism at birth

Mucolipidosis II (ML II) or inclusion cell disease (I-cell disease) is a rarely occurring autosomal recessive lysosomal enzyme-targeting disease. This disease is usually found to occur in individuals aged between 6 and 12 months, with a clinical phenotype resembling that of Hurler syndrome and radiological findings resembling those of dysostosis multiplex. However, we encountered a rare case of an infant with ML II who presented with prenatal skeletal dysplasia and typical clinical features of severe secondary hyperparathyroidism at birth. A female infant was born at 37⁺¹ weeks of gestation with a birth weight of 1,690 g (<3rd percentile). Prenatal ultrasonographic findings revealed intrauterine growth retardation and skeletal dysplasia. At birth, the patient had characteristic features of ML II, and skeletal radiographs revealed dysostosis multiplex, similar to rickets. In addition, the patient had high levels of alkaline phosphatase and parathyroid hormone, consistent with severe secondary neonatal hyperparathyroidism. The activities of β-D-hexosaminidase and α-N-acetylglucosaminidase were moderately decreased in the leukocytes but were 5- to 10-fold higher in the plasma. Examination of a placental biopsy specimen showed foamy vacuolar changes in trophoblasts and syncytiotrophoblasts. The diagnosis of ML II was confirmed via GNPTAB genetic testing, which revealed compound heterozygosity of c.3091C>T (p.Arg1031X) and c.3456_3459dupCAAC (p.Ile1154GlnfsX3), the latter being a novel mutation. The infant was treated with vitamin D supplements but expired because of asphyxia at the age of 2 months.

Severe dilated cardiomyopathy as an unusual finding in a young infant with mucolipidosis type 2

A neonate presented with mucopolysaccharidosis-like phenotypic expression and typical signs of dysostosis multiplex but without urinary excretion of glycosaminoglycans. Investigations of lysosomal enzymes in cultured fibroblasts revealed a mucolipidosis type 2, known as I-cell disease. We describe the fatal course of the patient due to complications of an uncommon dilated cardiomyopathy in this rare disease and discuss the pathogenesis.