A novel homozygous MCOLN1 double mutant allele leading to TRP channel domain ablation underlies Mucolipidosis IV in an Italian Child

A Novel Homozygous Variant in the MCOLN1 Gene Associated With Severe Oromandibular Dystonia and Parkinsonism

BACKGROUND

Mucolipidosis type IV (MLIV) is a rare, progressive lysosomal storage disorder characterized by severe intellectual disability, delayed motor milestones and ophthalmologic abnormalities. MLIV is an autosomal recessive disease caused by mutations in the MCOLN1 gene, encoding mucolipin-1 which is responsible for maintaining lysosomal function.

OBJECTIVES AND METHODS

Here, we report a family of four Iranian siblings with cognitive decline, progressive visual and pyramidal disturbances, and abnormal movements manifested by severe oromandibular dystonia and parkinsonism. MRI scans of the brain demonstrated signal abnormalities in the white matter and thinning of the corpus callosum.

RESULTS AND CONCLUSIONS

Whole-exome sequencing identified a novel homozygous variant, c.362C > T:p. Thr121Met in the MCOLN1 gene consistent with a diagnosis of MLIV. The presentation of MLIV may overlap with a variety of other neurological diseases, and genetic analysis is an important strategy to clarify the diagnosis. This is an important point that clinicians should be familiar with. The novel variant c.362C > T:p. Thr121Met herein described may be related to a comparatively older age at onset. Our study also expands the clinical spectrum of MLIV associated with the MCOLN1 variants and introduces a novel likely pathogenic variant for testing in MLIV cases that remain unresolved.

Characterization of Endo-Lysosomal Cation Channels Using Calcium Imaging

Endo-lysosomes are membrane-bound acidic organelles that are involved in endocytosis, recycling, and degradation of extracellular and intracellular material. The membranes of endo-lysosomes express several Ca²⁺-permeable cation ion channels, including two-pore channels (TPC1-3) and transient receptor potential mucolipin channels (TRPML1-3). In this chapter, we will describe four different state-of-the-art Ca²⁺ imaging approaches, which are well-suited to investigate the function of endo-lysosomal cation channels. These techniques include (1) global cytosolic Ca²⁺ measurements, (2) peri-endo-lysosomal Ca²⁺ imaging using genetically encoded Ca²⁺ sensors that are directed to the cytosolic endo-lysosomal membrane surface, (3) Ca²⁺ imaging of endo-lysosomal cation channels, which are engineered in order to redirect them to the plasma membrane in combination with approaches 1 and 2, and (4) Ca²⁺ imaging by directing Ca²⁺ indicators to the endo-lysosomal lumen. Moreover, we will review useful small molecules, which can be used as valuable tools for endo-lysosomal Ca²⁺ imaging. Rather than providing complete protocols, we will discuss specific methodological issues related to endo-lysosomal Ca²⁺ imaging.

The High Association of Ophthalmic Manifestations in Individuals With Mucolipidosis Type IV

PURPOSE

To present a case report of mucolipidosis type IV (ML4) and review the literature for all of the ophthalmic abnormalities associated with this disease.

METHODS

A systematic review of the literature using PubMed/Medline was conducted, and with the addition of the current case report, the eye and ocular adnexa findings of 93 patients with ML4 are summarized.

RESULTS

The most common ophthalmic findings reported among the 93 patients included corneal clouding (90.3%), strabismus (58.1%), optic nerve pallor (52.2%), retinal dystrophy/pigmentary changes (50.5%), and retinal vascular attenuation (38.9%). Other less commonly reported findings included nystagmus, photophobia, ocular pain, excessive lacrimation, ptosis, and cataracts.

CONCLUSIONS

The ophthalmic findings discussed in the current case report and literature review serve as indicators for ML4. Early diagnosis of ML4 is important in forming a multidisciplinary management plan, genetic counseling strategy, and maximizing the visual development of affected individuals. [J Pediatr Ophthalmol Strabimus. 2022;59(5):332-337.].

Mucolipidosis Type IV in Omani Families with a Novel MCOLN1 Mutation: Search for Evidence of Founder Effect

Mucolipidosis Type IV (MLIV) is caused by a deficiency of the mucolipin cation channel encoded by Mucolipin TRP Cation Channel 1 gene (MCOLN1). It is a slowly progressive neurodevelopmental and neurodegenerative disorder causing severe psychomotor developmental delay and progressive visual impairment, which is often misdiagnosed as cerebral palsy. We describe six patients with MLIV from two Omani families with a novel c.237+5G>A mutation in the MCOLN1 gene predicted to affect mRNA splicing. Mutation screening with a high-resolution melting (HRM) assay in a large population sample did not detect this mutation in control subjects. This report highlights the importance of considering MLIV in the differential diagnosis of patients in a pediatric age group with cerebral palsy-like presentation. Although the same rare mutation was seen in two apparently unrelated families, this was not seen in the sample screened from the general population. The HRM assay provides a cost-effective assay for population screening for the c.237+5G>A mutation.

White matter abnormalities and iron deposition in prenatal mucolipidosis IV- fetal imaging and pathology

Mucolipidosis type IV (MLIV; OMIM 252,650) is an autosomal recessive lysosomal disorder caused by mutations in MCOLN1. MLIV causes psychomotor impairment and progressive vision loss. The major hallmarks of postnatal brain MRI are hypomyelination and thin corpus callosum. Human brain pathology data is scarce and demonstrates storage of various inclusion bodies in all neuronal cell types. The current study describes novel fetal brain MRI and neuropathology findings in a fetus with MLIV. Fetal MRI was performed at 32 and 35 weeks of gestation due to an older sibling with spastic quadriparesis, visual impairment and hypomyelination. Following abnormal fetal MRI results, the parents requested termination of pregnancy according to Israeli regulations. Fetal autopsy was performed after approval of the high committee for pregnancy termination. A genetic diagnosis of MLIV was established in the fetus and sibling. Sequential fetal brain MRI showed progressive curvilinear hypointensities on T2-weighted images in the frontal deep white matter and a thin corpus callosum. Fetal brain pathology exhibited a thin corpus callosum and hypercellular white matter composed of reactive astrocytes and microglia, multifocal white matter abnormalities with mineralized deposits, and numerous aggregates of microglia with focal intracellular iron accumulation most prominent in the frontal lobes. This is the first description in the literature of brain MRI and neuropathology in a fetus with MLIV. The findings demonstrate prenatal white matter involvement with significant activation of microglia and astrocytes and impaired iron metabolism.

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.

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.

Current concepts in the neuropathogenesis of mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive, lysosomal storage disorder causing progressively severe intellectual disability, motor and speech deficits, retinal degeneration often culminating in blindness, and systemic disease causing a shortened lifespan. MLIV results from mutations in the gene MCOLN1 encoding the transient receptor potential channel mucolipin-1. It is an ultra-rare disease and is currently known to affect just over 100 diagnosed individuals. The last decade has provided a wealth of research focused on understanding the role of the enigmatic mucolipin-1 protein in cell and brain function and how its absence causes disease. This review explores our current understanding of the mucolipin-1 protein in relation to neuropathogenesis in MLIV and describes recent findings implicating mucolipin-1's important role in mechanistic target of rapamycin and TFEB (transcription factor EB) signaling feedback loops as well as in the function of the greater endosomal/lysosomal system. In addition to addressing the vital role of mucolipin-1 in the brain, we also report new data on the question of whether haploinsufficiency as would be anticipated in MCOLN1 heterozygotes is associated with any evidence of neuron dysfunction or disease. Greater insights into the role of mucolipin-1 in the nervous system can be expected to shed light not only on MLIV disease but also on numerous processes governing normal brain function. This article is part of the Special Issue "Lysosomal Storage Disorders".

Impaired myelination and reduced brain ferric iron in the mouse model of mucolipidosis IV

Mucolipidosis type IV (MLIV) is a lysosomal storage disease caused by mutations in the MCOLN1 gene, which encodes the lysosomal transient receptor potential ion channel mucolipin-1 (TRPML1). MLIV causes impaired motor and cognitive development, progressive loss of vision and gastric achlorhydria. How loss of TRPML1 leads to severe psychomotor retardation is currently unknown, and there is no therapy for MLIV. White matter abnormalities and a hypoplastic corpus callosum are the major hallmarks of MLIV brain pathology. Here, we report that loss of TRPML1 in mice results in developmental aberrations of brain myelination as a result of deficient maturation and loss of oligodendrocytes. Defective myelination is evident in Mcoln1(-/-) mice at postnatal day 10, an active stage of postnatal myelination in the mouse brain. Expression of mature oligodendrocyte markers is reduced in Mcoln1(-/-) mice at postnatal day 10 and remains lower throughout the course of the disease. We observed reduced Perls' staining in Mcoln1(-/-) brain, indicating lower levels of ferric iron. Total iron content in unperfused brain is not significantly different between Mcoln1(-/-) and wild-type littermate mice, suggesting that the observed maturation delay or loss of oligodendrocytes might be caused by impaired iron handling, rather than by global iron deficiency. Overall, these data emphasize a developmental rather than a degenerative disease course in MLIV, and suggest that there should be a stronger focus on oligodendrocyte maturation and survival to better understand MLIV pathogenesis and aid treatment development.