Mucolipidosis type IV: accumulation of phospholipids and gangliosides in cultured amniotic cells. A tool for prenatal diagnosis

Neuropathophysiology, Genetic Profile, and Clinical Manifestation of Mucolipidosis IV-A Review and Case Series

Mucolipidosis type IV (MLIV) is an ultra-rare lysosomal storage disorder caused by biallelic mutations in MCOLN1 gene encoding the transient receptor potential channel mucolipin-1. So far, 35 pathogenic or likely pathogenic MLIV-related variants have been described. Clinical manifestations include severe intellectual disability, speech deficit, progressive visual impairment leading to blindness, and myopathy. The severity of the condition may vary, including less severe psychomotor delay and/or ocular findings. As no striking recognizable facial dysmorphism, skeletal anomalies, organomegaly, or lysosomal enzyme abnormalities in serum are common features of MLIV, the clinical diagnosis may be significantly improved because of characteristic ophthalmological anomalies. This review aims to outline the pathophysiology and genetic defects of this condition with a focus on the genotype–phenotype correlation amongst cases published in the literature. The authors will present their own clinical observations and long-term outcomes in adult MLIV cases.

TRPML1: The Ca(2+)retaker of the lysosome

Efficient functioning of lysosome is necessary to ensure the correct performance of a variety of intracellular processes such as degradation of cargoes coming from the endocytic and autophagic pathways, recycling of organelles, and signaling mechanisms involved in cellular adaptation to nutrient availability. Mutations in lysosomal genes lead to more than 50 lysosomal storage disorders (LSDs). Among them, mutations in the gene encoding TRPML1 (MCOLN1) cause Mucolipidosis type IV (MLIV), a recessive LSD characterized by neurodegeneration, psychomotor retardation, ophthalmologic defects and achlorhydria. At the cellular level, MLIV patient fibroblasts show enlargement and engulfment of the late endo-lysosomal compartment, autophagy impairment, and accumulation of lipids and glycosaminoglycans. TRPML1 is the most extensively studied member of a small family of genes that also includes TRPML2 and TRPML3, and it has been found to participate in vesicular trafficking, lipid and ion homeostasis, and autophagy. In this review we will provide an update on the latest and more novel findings related to the functions of TRPMLs, with particular focus on the emerging role of TRPML1 and lysosomal calcium signaling in autophagy. Moreover, we will also discuss new potential therapeutic approaches for MLIV and LSDs based on the modulation of TRPML1-mediated signaling.

Systematic screens for proteins that interact with the mucolipidosis type IV protein TRPML1

Mucolipidosis type IV is a lysosomal storage disorder resulting from mutations in the MCOLN1 gene, which encodes the endosomal/lysosomal Transient Receptor Potential channel protein mucolipin-1/TRPML1. Cells isolated from Mucolipidosis type IV patients and grown in vitro and in in vivo models of this disease both show several lysosome-associated defects. However, it is still unclear how TRPML1 regulates the transport steps implicated by these defects. Identifying proteins that associate with TRPML1 will facilitate the elucidation of its cellular and biochemical functions. We report here two saturation screens for proteins that interact with TRPML1: one that is based on immunoprecipitation/mass spectrometry and the other using a genetic yeast two-hybrid approach. From these screens, we identified largely non-overlapping proteins, which represent potential TRPML1-interactors., Using additional interaction assays on some of the potential interactors from each screen, we validated some proteins as candidate TRPML1 interactors In addition, our analysis indicates that each of the two screens not only identified some false-positive interactors, as expected from any screen, but also failed to uncover potential TRPML1 interactors. Future studies on the true interactors, first identified in these screens, will help elucidate the structure and function of protein complexes containing TRPML1.

Lysosomal Ca(2+) homeostasis: role in pathogenesis of lysosomal storage diseases

Disrupted cellular Ca(2+) signaling is believed to play a role in a number of human diseases including lysosomal storage diseases (LSD). LSDs are a group of ∼50 diseases caused predominantly by mutations in lysosomal proteins that result in accumulation of macromolecules within the lysosome. We recently reported that Niemann-Pick type C (NPC) is the first human disease to be associated with defective lysosomal Ca(2+) uptake and defective NAADP-mediated lysosomal Ca(2+) release. These defects in NPC cells leads to the disruption in endocytosis and subsequent lipid storage that is a feature of this disease. In contrast, Chediak-Higashi Syndrome cells have been reported to have enhanced lysosomal Ca(2+) uptake whilst the TRPML1 protein defective in mucolipidosis type IV is believed to function as a Ca(2+) channel. In this review we provide a summary of the current knowledge on the role of lysosomal Ca(2+) signaling in the pathogenesis of this group of diseases.

TRP-ML1 regulates lysosomal pH and acidic lysosomal lipid hydrolytic activity

Mucolipidosis type IV (MLIV) is caused by mutations in the ion channel mucolipin 1 (TRP-ML1). MLIV is typified by accumulation of lipids and membranous materials in intracellular organelles, which was hypothesized to be caused by the altered membrane fusion and fission events. How mutations in TRP-ML1 lead to aberrant lipolysis is not known. Here we present evidence that MLIV is a metabolic disorder that is not associated with aberrant membrane fusion/fission events. Thus, measurement of lysosomal pH revealed that the lysosomes in TRP-ML1(-/-) cells obtained from the patients with MLIV are over-acidified. TRP-ML1 can function as a H(+) channel, and the increased lysosomal acidification in TRP-ML1(-/-) cells is likely caused by the loss of TRP-ML1-mediated H(+) leak. Measurement of lipase activity using several substrates revealed a marked reduction in lipid hydrolysis in TRP-ML1(-/-) cells, which was rescued by the expression of TRP-ML1. Cell fractionation indicated specific loss of acidic lipase activity in TRP-ML1(-/-) cells. Furthermore, dissipation of the acidic lysosomal pH of TRP-ML1(-/-) cells by nigericin or chloroquine reversed the lysosomal storage disease phenotype. These findings provide a new mechanism to account for the pathogenesis of MLIV.

Caenorhabditis elegans functional orthologue of human protein h-mucolipin-1 is required for lysosome biogenesis

Mucolipidosis type IV (MLIV) is an autosomal recessive lysosomal storage disease characterized by severe psychomotor retardation, achlorhydria, and ophthalmological abnormalities. Cells from several tissues in MLIV patients accumulate large vacuoles that are presumed to be lysosomes, but whose exact nature remains to be determined. Other defects include the deterioration of neuronal integrity in the retina and the cerebellum. MCOLN1, the gene mutated in MLIV patients, encodes a protein called h-mucolipin-1 that has six predicted transmembrane domains and functions as a Ca(2+)-permeable channel that is modulated by changes in Ca2+ concentration. CUP-5 is the Caenorhabditis elegans functional orthologue of h-mucolipin-1. Mutations in cup-5 result in the accumulation of large vacuoles in several cells, in increased cell death, and in embryonic lethality. We demonstrate here that CUP-5 functions in the biogenesis of lysosomes originating from hybrid organelles. We also show that at least two h-mucolipin family members rescue cup-5 mutant endocytic defects, indicating that there may be functional redundancy among the human proteins. Finally, we propose a model that relates the lysosome biogenesis defect in the absence of CUP-5/h-mucolipin-1 to cellular phenotypes in worms and in humans.

Rapid detection of the two common mutations in Ashkenazi Jewish patients with mucolipidosis type IV

Among Ashkenazi Jewish individuals with mucolipidosis IV (ML IV), two mutations in the ML IV gene, IVS3-1A --> G and delEX1-EX7, account for more than 95% of disease alleles. The reported method of genotyping for the delEX1-EX7 mutation involves a cumbersome multistep procedure. In the present study, a new simplified one-step procedure is described that detects this mutation in both patients and carriers. An improved procedure is also described for detection of the IVS3-1A --> G mutation. Using these improved procedures, we have characterized the ML IV mutant alleles in 27 patients and 95 of their relatives from 22 families, and in 123 unrelated and unaffected Ashkenazi Jewish controls. Of the 27 ML IV patients, 16 patients (59.3%) were found to be homozygous for the IVS3-1A --> G mutation and 1 patient (3.7%) homozygous for the delEX1-EX7 mutation. Additionally, 9 patients (33.3%) were compound heterozygotes for IVS3-1A --> G/delEX1-EX7. Among the 123 Ashkenazi Jewish controls, two individuals were identified as heteroallelic with one IVS3-1A --> G mutation (carrier frequency: approximately 1 in 61); none showed the delEX1-EX7 mutation. The modifications described here provide a more facile means of genotyping patients and carriers and expand the possibilities for screening at-risk populations.

Noninvasive diagnosis and ophthalmic features of mucolipidosis type IV

OBJECTIVE

To comprehensively describe the ophthalmic characteristics of patients with mucolipidosis type IV.

DESIGN

Prospective natural history study.

PARTICIPANTS

Twenty-two patients with confirmed mucolipidosis type IV. METHODS OR TESTING: External and slit-lamp examination with dilated funduscopy, photography of corneal and retinal lesions, and exfoliative conjunctival cytology were performed.

MAIN OUTCOME MEASURES

Grading of corneal, optic nerve, retinal vasculature, and pigmentary abnormalities.

RESULTS

All patients exhibited some degree of corneal epithelial haze, optic nerve pallor, retinal vascular attenuation, and retinal pigment epithelial changes. The associated ocular findings observed in decreasing order of frequency were strabismus, corneal erosion, cataract, corneal abnormalities, fundus abnormalities, and ptosis. The older patients were significantly more likely to demonstrate severe optic nerve pallor, retinal vascular attenuation, and corneal epithelial haze. Conjunctival cytologic studies showed characteristic lysosomal inclusions on light and electron microscopy.

CONCLUSIONS

Patients with mucolipidosis type IV have characteristic ophthalmic features, most of which have a progressive course. Conjunctival cytologic studies help confirm the diagnosis of this disorder.

Delayed lysosomal metabolism of lipids in mucolipidosis type IV fibroblasts after LDL-receptor-mediated endocytosis

We specifically probed the low-density lipoprotein-receptor-dependent endosomal/lysosomal pathway of lipid degradation in control and mucolipidosis type IV fibroblasts using either [choline-methyl-14C]sphingomyelin in complex with apolipoprotein E, or cholesteryl [14C]oleate-labelled low-density lipoprotein as a substrate. Mucolipidosis type IV fibroblasts metabolized [14C]sphingomyelin and cholesteryl [14C]oleate significantly more slowly than controls and fibroblasts from patients with Hurler disease or Niemann-Pick disease type C. So far, no lysosomal enzyme deficiency has been reported for mucolipidosis type IV. Rather, the defect in mucolipidosis type IV cells has recently been suggested to be related to intracellular trafficking. Our results suggest that the defect in mucolipidosis type IV also affects the low-density lipoprotein-receptor-mediated endocytosis pathway.

Mucolipidosis type IV

Mucolipidosis type IV (MLIV) is a neurodegenerative lysosomal storage disorder characterized by psychomotor retardation and ophthalmological abnormalities, including corneal opacities, retinal degeneration, and strabismus. Severely affected as well as milder patients have been described. Over 80% of the MLIV patients are Ashkenazi Jews; the estimated heterozygote frequency in this population is 1/100. The disease is classified as a mucolipidosis due to the simultaneous lysosomal storage of lipids together with water-soluble substances. A broad spectrum of lipids and acid mucopolysaccharides were identified as the storage substances. Kinetic studies demonstrated that this heterogeneous storage stems from an abnormal endocytosis process in cells from MLIV patients of membrane components from late endosomes to the lysosomes and/or delayed efflux to the Golgi apparatus. The MLIV gene was mapped to chromosome 19p13.2--13.3 where a novel gene, MCOLN1, with MLIV-causing mutations, was identified. Two mutations were found among 95% of the Ashkenazi MLIV alleles, including an intronic acceptor splice-site mutation in 72% of the alleles and a partial gene deletion in 23%. Each of these mutations was associated with a defined haplotype in this chromosomal region. Other mutations were mostly identified in single, Ashkenazi and non-Ashkanazi patients, including missense, nonsense nucleotide deletions, and insertions. All mutations but one were identified in patients exhibiting the severe phenotype, an in-frame amino acid deletion was identified in a mild patient. MCOLN1 encodes a 580 aa protein, mucolipin 1, which is a member of a new protein family of unknown function at present, the mucolipins. Mucolipin 1 is a membrane protein with 6 transmembrane domains, a serine lipase, and nuclear localization signal motives. The protein shows homology to a group of calcium channels of the TRP/TRPL family. The involvement of this protein in the endocytosis process of membrane components is currently studied. A population screening operation among the Ashkenazi population for the detection of heterozygotes has been started in Israel as a prevention program.

Regulation of endocytosis by CUP-5, the Caenorhabditis elegans mucolipin-1 homolog

Loss of the human mucolipin-1 gene underlies mucolipidosis type IV (MLIV), a lysosomal storage disease that results in severe developmental neuropathology. Unlike other lysosomal storage diseases, MLIV is not associated with a lack of lysosomal hydrolases; instead, MLIV cells display abnormal endocytosis of lipids and accumulate large vesicles, indicating that a defect in endocytosis may underlie the disease. Here we report the identification of a loss-of-function mutation in the Caenorhabditis elegans mucolipin-1 homolog, cup-5, and show that this mutation results in an enhanced rate of uptake of fluid-phase markers, decreased degradation of endocytosed protein and accumulation of large vacuoles. Overexpression of cup-5(+) causes the opposite phenotype, indicating that cup-5 activity controls aspects of endocytosis. Studies in model organisms such as C. elegans have helped illuminate fundamental mechanisms involved in normal cellular function and human disease; thus the C. elegans cup-5 mutant may be a useful model for studying conserved aspects of mucolipin-1 structure and function and for assessing the effects of potential therapeutic compounds.

G(M2)-ganglioside metabolism in situ in mucolipidosis IV fibroblasts

Mucolipidosis IV (ML IV) is an inherited lysosomal disorder for which the primary biochemical defect has not been identified. In order to detect any defect in glycosphingolipid metabolism, we have examined the metabolism of sphingosine-labeled (3H)G(M2) in situ in fibroblasts from patients diagnosed with ML IV. Fibroblasts were exposed for 10 days in medium containing (3H)G(M2) (15 uM; Sp. Act. 35000 cpm/nmole), washed, harvested and analyzed for radioactivity in extracted lipids. Control cells metabolized about half of the internalized ganglioside, mostly to ceramide. In ML IV fibroblasts, 70-80% of the cellular radioactivity was present as G(M2) indicating reduced degradation. This is not as severe as in G(M2) gangliosidosis as a small amount of G(M2) was metabolized in ML IV cells to ceramide. Since there is no defect in the lysosomal enzyme profile in these cells, it is possible that an abnormality in the translocation of membrane constituents to the lysosomes may explain the slower ganglioside metabolism.

Electroencephalographic findings in patients with mucolipidosis type IV

OBJECTIVE

To describe the electroencephalographic findings in mucolipidosis type IV (ML IV), a lysosomal storage disease of unknown etiology characterized clinically by corneal clouding, retinal degeneration and severe psychomotor retardation. Most patients are of Ashkenazi-Jewish ancestry. The EEG findings in this syndrome have not been characterized.

METHODS

We analyzed the EEGs of 10 patients with the diagnosis of ML IV, aged between of 2.5 and 21 years. All patients had 21 channel recordings in the international 10/20 system without sedation.

RESULTS

Six of the 10 patients had slowing of the background in the theta frequency range, and 4 had excessive beta frequency activity without the administration of medications. Two patients were able to reach stage 2 sleep, and were noted to have both synchronous and asynchronous spindles and vertex waves. Of the 10 patients, 6 had epileptiform spikes, all of which were noted frequently. The location of the spikes varied, from the frontal and temporal regions to the central regions, although location was consistent in each patient. Only one patient with epileptiform spikes had a history of clinical seizures. None of the other patients had a history of seizures.

CONCLUSIONS

These findings imply that epileptiform discharges are common in patients with ML IV, but are infrequently associated with clinical seizures.

Prenatal diagnosis of lysosomal storage diseases

The prenatal diagnosis of lysosomal storage disorders can be achieved, once the diagnosis is confirmed in the index case, by a variety of techniques including analysis of amniotic fluid, asay of enzymic activity in cultured amniotic fluid cells, cultured chorionic villus cells and by direct assay of activity in chorionic villus samples. These studies can be accompanied by ultrastructural observations which give an independent means of diagnosis. In some instances molecular genetic studies for mutation detection or linkage analysis are appropriate for prenatal diagnosis. Pseudodeficiencies of some of the lysosomal enzymes, which cause no clinical problems, can complicate the initial diagnosis particularly in metachromatic leucodystrophy where the pseudodeficiency is more common than the disease itself. Mutation analysis as well as enzyme assay is necessary not only in the index case but also in the parents before the same techniques are applied to a sample for prenatal diagnosis. A large number of lysosomal storage disorders may present as fetal hydrops and the diagnosis can be established at this late stage by fetal blood sampling and examination by microscopy as well as by biochemical assay of the appropriate enzyme or metabolite in amniotic fluid. All prenatal diagnoses in which an affected fetus is indicated should have confirmation of the diagnosis as soon as possible to reassure anxious parents, and to act as audit of the laboratory's competence to undertake prenatal diagnosis. A combined approach to prenatal diagnosis involving biochemical, molecular genetic and morphological studies is recommended.

Mucolipidosis type IV: abnormal transport of lipids to lysosomes

Mucolipidosis type IV (MLIV) is a lysosomal storage disorder in which various phospholipids and gangliosides accumulate. The cause of this storage has not yet been identified. Loading experiments in cultured fibroblasts with radioactive phosphatidylcholine ([14C]PC) labelled either in the acyl groups or in the choline residue, indicated increased retention of this lipid in the lysosomes of these patients. Chase experiments in intact fibroblasts, on the other hand, indicated normal degradation and discharge of the radioactive PC in MLIV lysosomes. This was further supported by measurements of the degradation of [14C]PC by isolated lysosomes which indicated normal breakdown of PC in MLIV. Cultured fibroblasts from Hunter (MPSII) patients, which contain enlarged and numerous lysosomes, did not store [14C]PC when compared to normal controls, indicating that the storage of this lipid in MLIV is not a secondary phenomenon caused by the presence of enlarged and numerous lysosomes in these cells. Incubation of [14C]PC at 18 degrees C limits the endocytosis process only up to early endosomes. This temperature did not yield increased retention of [14C]PC in MLIV, indicating that accumulation occurs only after the PC reached late endosomes or the lysosomes. The data indicate that PC as well as other phospholipids and gangliosides accumulate in MLIV apparently owing to a defect in the endocytosis process of membranous components. This defect apparently leads to excessive transportation of these macromolecules into lysosomes rather than their recycling to the plasma membrane. The endocytosis of membrane components is different from receptor-mediated endocytosis which is not affected in MLIV. Once the membrane macromolecules reach the lysosomes in MLIV they are normally catabolized and normally discharged. This may explain the heterogeneity of the stored materials in MLIV. The normal catabolism of macromolecules in the lysosomes is reflected in the minor deterioration in the clinical manifestations of these patients.

Mucolipidosis IV fibroblasts synthesize normal amounts of hyaluronic acid

Mucolipidosis IV (ML IV) (McKusick 252650) is an autosomal recessive metabolic disorder that displays signs of both lipid and mucopolysaccharide (glycosaminoglycan) storage. It has been reported that fibroblasts from ML IV patients exhibit abnormally high synthesis of hyaluronic acid in culture. In our search for a biochemical marker that will enable positive identification of ML IV, we studied glycosaminoglycan synthesis in fibroblast cultures from patients with this disease. ML IV and normal control fibroblasts were incubated with [3H]glucosamine and [35S]sulphate. Labelled glycosaminoglycans were extracted from the cell layer and medium. Chondroitin sulphate and hyaluronic acid were determined by analysis of disaccharides after digestion with chondroitinase ABC. Synthesis of neither of these two glycosaminoglycans differed significantly between control and ML IV fibroblasts. Synthesis of hyaluronic acid was nearly linear for 24 h, with mean calculated values of 11.7 +/- 1.4 and 14.4 +/- 1.6 pg/cell per 24 h in control and ML IV cultures respectively. The variability within the two groups is attributed primarily to population variability and possibly to culture density. These experiments exclude the possibility that a general metabolic defect in hyaluronic acid synthesis is responsible for the ML IV phenotype, nor can such a defect be used as a diagnostic tool for the disease.