The clinical presentation of ML is heterogeneous and ranges from hydrops fetalis to severe cardiac or respiratory disease, often fatal in the neonatal period, to mild cognitive impairment and near-to-normal life expectancy. Even within determined types of ML, the phenotypic spectrum is broad [1]:

• There are two subtypes of ML type 1. The more severe subtype may induce hydrops fetalis or manifest soon after birth. Affected individuals present dysmorphic facial features reminiscent of Hurler syndrome. They also suffer from severe skeletal dysplasia and learning impairment. Hepatosplenomegaly is frequently diagnosed. Death in the neonatal period or infancy is common. By contrast, prolonged survival is to be expected in those affected by cherry-red spot-myoclonus syndrome or normomorphic sialidosis. These patients present with myoclonus, ataxia, and cherry-red spots of the macula. Cognitive impairment is less severe and may not manifest until advanced age [2].
• Hurler-like, coarse facial features and severe skeletal dysplasia may also be observed in ML type 2 patients. These individuals are generally microcephalic, and hepatomegaly may be noted. Cardiorespiratory failure is a frequent cause of death and often occurs in or before childhood. Patients who survive for several years usually present learning difficulties.
• ML type 3 is an attenuated form of ML type 2, with symptom onset usually being delayed until childhood [3]. Skeletal dysplasia may be limited to the joints of shoulders, hips, and spine, and most of these patients are of short stature. They are predisposed to mild learning difficulties and valvular heart disease, but death from heart disease is rare.
• Motor developmental delays, ataxia, strabismus, and severe visual impairment due to corneal clouding, retinal degeneration, and optic atrophy are the clinical hallmarks of ML type 4, which is usually diagnosed in children aged 2-3 years [4] [5]. More or less severe cognitive impairment is common. The disease interferes with the secretion of gastric acid, leading to the formation of lysosomal inclusions in parietal cells and constitutive achlorhydria [4]. While laboratory analyses of blood samples reveal increased levels of gastrin, iron deficiency anemia may also be diagnosed.

• Developmental Delay

  Mucolipidosis II (MLII) is characterized by severe global developmental delay, coarse facial features, skeletal deformities, and other systemic involvement. It is caused by a deficiency in N-acetylglucosamine-1 phosphotransferase. [ncbi.nlm.nih.gov]

  Stiff skin Indurated skin An induration (hardening) of the skin Global developmental delay Psychomotor developmental delay, Psychomotor retardation, Developmental delay, Lack of psychomotor development, Cognitive delay, Delayed intellectual development [rarediseases.oscar.ncsu.edu]

  Symptoms Common signs and symptoms of Mucolipidosis IV include: Pronounced developmental delay in gross (e.g. sitting, standing, walking) and fine motor skills Marked cognitive development delay (e.g. speech) Visual problems including: Corneal clouding [jnetics.org]

  Developmental delays in their motor skills are generally more pronounced than delays in their cognitive abilities. Children with ML II eventually develop corneal opacity and due to the lack of growth, develop short trunk dwarfism. [tellmegen.com]

• Short Stature

  Abstract The musculoskeletal manifestations of mucolipidosis III include short stature, claw hands, carpal tunnel syndrome, and limited joint mobility. [ncbi.nlm.nih.gov]

  Kolodny Biology, Medicine Revista de neurologia 1998 INTRODUCTION The concept of the mucolipidoses developed as the result of encounters with patients whose clinical appearance suggested a mucopolysaccharidosis, i.e. coarse facies, short stature,… [semanticscholar.org]

  Individuals with mucolipidosis III gamma grow slowly and have short stature. They also have stiff joints and dysostosis multiplex, which refers to multiple skeletal abnormalities seen on x-ray. [snpedia.com]

• Episodic Pain

  A 5-year-old boy with mucolipidosis IV experienced recurrent episodes of severe ocular pain, tearing, and ipsilateral facial flushing. This was suggestive of reflex sympathetic dystrophy, a syndrome of pain and sympathetic hyperactivity. [ncbi.nlm.nih.gov]

• Splenomegaly

  In most ML II patients, the clinical conditions 'stand alone', 'walk without support' and 'speak single words' were impaired; however, the frequency of 'heart murmur', 'inguinal hernia' and 'hepatomegaly and/or splenomegaly' did not differ between ML [ncbi.nlm.nih.gov]

  […] cholesterol 272.7 dystopic (hereditary) 272.7 glycolipid 272.7 hereditary, dystopic 272.7 Lipoid - see also condition histiocytosis 272.7 Lipoidosis (see also Lipidosis) 272.7 Mucolipidosis I, II, III 272.7 Niemann-Pick disease (lipid histiocytosis) (splenomegaly [icd9data.com]

  Clinical manifestations, such as ‘heart murmur’, ‘inguinal hernia’ and ‘hepatomegaly and/or splenomegaly’, were found in both ML II and III. [nature.com]

• Hepatosplenomegaly

  • The clinical characteristics of a 16-year-old white girl with mucolipidosis type III included early growth retardation, severe dysostosis multiplex, restricted joint motion, tight indurated skin, swollen eyelids, late-onset hepatosplenomegaly, umbilical [jamanetwork.com]

  […] features a slowly progressive loss of intellect, hepatosplenomegaly, ataxia, myoclonic seizures, and spasticity; the neuronopathic forms are characterized by neuronal loss with neuronophagia, and accumulation of glucocerebroside in neurons Caused by [icd9data.com]

  No patients had hepatosplenomegaly on ultrasound, 5/15 patients had heart valves disease. Enzyme assay showed α-Hexosaminidase of 1,885.98±338.7 nmoL/mg plasma/17 h, α-Iduronate sulfatase of 4,534.78±1,062.97 nmoL/mg plasma/4 h. [atm.amegroups.com]

  Type II has an earlier onset with coarse facial features, dysostosis multiplex, short stature, developmental delay, mental retardation and hepatosplenomegaly. [genedx.com]

• Hepatomegaly

  In most ML II patients, the clinical conditions 'stand alone', 'walk without support' and 'speak single words' were impaired; however, the frequency of 'heart murmur', 'inguinal hernia' and 'hepatomegaly and/or splenomegaly' did not differ between ML [ncbi.nlm.nih.gov]

  272.7 splenomegaly (cerebroside lipidosis) 272.7 Hepatomegaly (see also Hypertrophy, liver) 789.1 Gaucher's 272.7 Histiocytosis (acute) (chronic) (subacute) 277.89 lipid, lipoid (essential) 272.7 Hyperlipidosis 272.7 hereditary 272.7 Lipidosis 272.7 [icd9data.com]

  An abdominal examination revealed a soft hepatomegaly of 4 cm [Figure - 1]. [jpgmonline.com]

  Additional clinical symptoms of I-cell disease include hepatomegaly, cardiomegaly, umbilical hernias, and recurrent upper respiratory infections. The disease progresses rapidly with developmental delay and failure to thrive. [themedicalbiochemistrypage.org]

• Liver Dysfunction

  Abstract We describe a patient with failure to thrive, hepatomegaly, liver dysfunction, and elevation of multiple plasma lysosomal enzyme activities mimicking mucolipidosis II or III, in whom a diagnosis of hereditary fructose intolerance (HFI) was ultimately [ncbi.nlm.nih.gov]

• Joint Stiffness

  Carpal tunnel syndrome was treated surgically but joint stiffness and hip and knee contractures were managed by physiotherapy. [ncbi.nlm.nih.gov]

  Results and conclusions: Sixteen cases (seven girls and nine boys) onset at 5.93±4.28 years of age the onset age of 2.3±3.1 years (median 1.25) with the feature of joint stiffness and bone deformation. 100% cases admitted with the feature of joint stiffness [atm.amegroups.com]

• Back Pain

  pain >3 months Chronic back pain greater than 3 months duration Chronic coccyx pain >3 months Chronic pain in coccyx for more than 3 months (finding) Fabry disease Fabrys disease Fabry's disease Ganglioside sialidase deficiency Gaucher disease Gauchers [icd9data.com]

  Many forms of analgesia had been used in an attempt to decrease their hip and back pain. These had all been unsuccessful. [ismrd.org]

• Disproportionate Dwarfism

  They had clinical features commonly found in mucolipidosis II, including disproportionate dwarfism, coarse facial features and mental retardation. [ncbi.nlm.nih.gov]

  The patient had clinical features commonly found in mucolipidosis II, including disproportionate dwarfism, retarded psychomotor development, coarse facies features, gibbous and restricted joint mobility. [scielo.conicyt.cl]

• Hydrops Fetalis

  Type II patients may also present with a congenital-onset form associated with ascites and hydrops fetalis prenatally, an infantile-onset form with the absence of symptoms at birth or a juvenile form that has onset in late childhood and a relatively milder [genedx.com]

  The clinical presentation of ML is heterogeneous and ranges from hydrops fetalis to severe cardiac or respiratory disease, often fatal in the neonatal period, to mild cognitive impairment and near-to-normal life expectancy. [symptoma.com]

  The more severe congenital form of type II sialidosis has onset in utero and results in hydrops fetalis, hepatomegaly, and either still birth or death within a period of months. [emedicine.medscape.com]

  fetalis to almost unaffected Oligosaccharidoses Mannosidosis features similar to Hurler, but variable severity hearing loss Fucosidosis wide picture severe neurological picture, mild skeletal changes, hepatomegaly angiokeratoma of the skin is suggestive [emilytam.com]

• Visual Impairment

  Mucolipidosis IV (ML4) is a neurodegenerative condition that is characterised by significant psychomotor and cognitive development delay, visual impairment, and poor muscle tone. [jnetics.org]

  This condition interferes with mental and physical development and predisposes to developmental delays, skeletal dysplasia and dysmorphic features, heart disease, and visual impairment. [symptoma.com]

  Mucolipidosis type IV (MLIV) is a neurodevelopmental as well as neurodegenerative disorder with severe psychomotor developmental delay, progressive visual impairment, and achlorydria. [ncbi.nlm.nih.gov]

  Background information for Mucolipidosis, Type IV ( MCOLN1), 2 Variants: Characteristics: Mucolipidosis type IV is characterized by early onset of severe psychomotor delay and progressive visual impairment due to corneal clouding and retinal degeneration [ltd.aruplab.com]

• Strabismus

  Definition An autosomal recessive lysosomal storage disorder characterized by severe psychomotor retardation and ophthalmologic abnormalities, including corneal opacity, retinal degeneration and strabismus. [uniprot.org]

  Surgical correction of strabismus. Oral iron to prevent iron deficiency anaemia due to poor absorption of dietary iron (unknown efficacy on the speed of resolution and recovery). [jnetics.org]

  These included strabismus, white matter signal abnormalities, and a hypoplastic corpus callosum at 2years of age. After a molecular diagnosis, a markedly elevated serum gastrin level (which is also common in MLIV) was confirmed. [ncbi.nlm.nih.gov]

  Related Pages Requisition Clinical Significance Mucolipidosis IV (MLP4) is a neurodegenerative lysosomal storage disorder characterized by severe psychomotor delay, ophthalmological abnormalities including corneal clouding, retinal degeneration and strabismus [sickkids.ca]

• Corneal Opacity

  This presentation provides guidelines for the clinical diagnosis of mucolipidosis type IV. lysosomal storage disorder mucolipidosis IV corneal opacity Received June 30, 1986. [pediatrics.aappublications.org]

  Abstract Mucolipidosis type IV is a rare neurodegenerative lysosomal storage disorder that usually presents during the first year of life with severe mental retardation, delayed motor milestones and corneal opacities. [ncbi.nlm.nih.gov]

  opacities, and only slightly impaired mental and neurological development. [jamanetwork.com]

• Retinal Pigmentation

  Our data show that RPE1 (retinal pigmented epithelial 1) cells develop a punctate mitochondrial phenotype within 48 h of siRNA-induced TRPML1-KD (knockdown). [ncbi.nlm.nih.gov]

• Ataxia

  followed by myoclonus, mental decline mild skeletal features Salla disease mental retardation, ataxia, progressive psychomotor decline ISSD (infantile sialic storage disease) more severe than Salla disease, although same mutation Mucolipidoses Sialidosis [emilytam.com]

  About 5% of patients have a milder, atypical form characterized by progressive ataxia and eye abnormalities, and they may learn to walk. Most patients live into adulthood, although life expectancy is shorter than normal. [sema4.com]

  Ataxia and seizures have also been reported in type I patients. Type II has an earlier onset with coarse facial features, dysostosis multiplex, short stature, developmental delay, mental retardation and hepatosplenomegaly. [genedx.com]

  Bipyramidal signs, ataxia, elbow contractures and multifocal myoclonus were noted. There was no organomegaly. Hematology and routine biochemistry was normal. [neurologyindia.com]

  Other symptoms include ataxia (lack of coordinated muscle movement), tremors, vision disturbances, seizures, enlarged liver and spleen, lack of muscle tone and mental retardation. Babies/children with ML pick up recurrent respiratory infections. [medindia.net]

• Myoclonus

  Syndrome — Myoclonus-Cherry Red Spot Syndrome — Mucolipidosis Type I — Mucolipidosis I — Cherry Red Spot Myoclonus Syndrome — Myoclonus Cherry Red Spot Syndrome — Mucolipidosis Type 1 — Type III Mucolipidosis — Mucolipidoses, Type III — Mucolipidosis [mesh.kib.ki.se]

  Mucolipidosis and progressive myoclonus epilepsy: A distinctive phenotype. [neurologyindia.com]

  Seizures, myoclonus and renal involvement have also been documented in type II cases. The frequency of diagnosed sialidosis in the general population is estimated at approximately one in four million live births. [genedx.com]

  The myoclonus is aggravated by smoking and menses, among other factors, and may become debilitating. Myoclonic seizures are poorly controlled by the standard antiepileptics. [emedicine.medscape.com]

• Learning Difficulties

  They are predisposed to mild learning difficulties and valvular heart disease, but death from heart disease is rare. [symptoma.com]

The diagnosis of ML is based on clinical suspicion and confirmative studies:

• First, analyses of blood and urine samples should be carried out to detect an increased excretion of sialic acid (ML type 1), high concentrations of urinary oligosaccharides with a terminal mannose, sialic acid, or galactose (ML types 2 and 3), or elevated serum levels of gastrin (ML type 4) [1] [6] [7].
• If results are obtained that support a tentative diagnosis of ML, enzyme activity measurements should be realized. Deficiency of neuraminidase or N-acetylglucosamine-1-phosphotransferase (GlcNAc-PT), as observed in those affected by ML type 1, or ML types 2 or 3, respectively, may be determined in cultured fibroblasts [1]. Because the lack of GlcNAc-PT triggers the release of lysosomal enzymes to the extracellular space, analyses of plasma samples and other body fluids from these patients yield enhanced concentrations of most lysosomal hydrolases. Acid sphingomyelinase, arylsulfatase A, iduronate 2-sulfatase, and N-sulfamidase shall be mentioned as examples [8].
• Histological examinations have long since been an essential part of the diagnosis of ML type 4. Lysosomal inclusions are found in most tissues, with skin biopsy specimens being most frequently assessed [5]. Of note, enlarged lysosomes filled with undigested macromolecules may also be observed in tissue samples obtained from those suffering from other types of ML [9].
• Finally, molecular biological studies should be conducted to identify the underlying mutation in genes NEU1, GNPTAB, GNPTG, or MCOLN1. In patients of Ashkenazi Jewish decent, a straightforward approach may be chosen to determine the presence of MCOLN1 mutations.

To date, all types of ML remain incurable. A multidisciplinary approach is required to maintain the patients' quality of life. In this context, symptomatic treatment and supportive care should be provided, and they may comprise physical and occupational therapy, surgery, visual aids, dietary supplementation, and psychological support [5] [10].

Hematopoietic stem cell transplantation has been realized in few individuals diagnosed with ML type 2, but the neurological outcome was poor, with few patients ever achieving maturity for education. Most of them also required tracheostomy for mechanical ventilation and gastrostomy tubes for nutrition. Disease progression couldn't be halted and eventually lead to death in the majority of cases [11] [12]. Beyond that, enzyme replacement therapy has been considered in patients diagnosed with ML types 1, 2, or 3. Although no such therapy has yet been approved, research is conducted to this end [2] [13].

ML are progressive disorders, but there are slowly progressive forms of the disease and those leading to death in the neonatal period or infancy. The entire spectrum of disease severity may be observed in those suffering from ML type 1. In case of ML types 2 and 3, residual enzyme activity is a favorable prognostic marker. Older age at symptom onset, a lower degree of severity, a more slowly progressive course, and higher life expectancy are to be expected in those with partial deficiencies of GlcNAc-PT. ML type 4 is one of the slowly progressive forms of ML. After symptom onset in early childhood, there is little to no deterioration in the clinical picture for several decades [4]. Data regarding the long-term outcome have yet to be provided.

ML are genetic disorders. They are all inherited in an autosomal recessive manner. Both homozygosity and compound heterozygosity may trigger the disease. The underlying mutations differ between distinct types of ML:

• ML type 1 is caused by neuraminidase deficiency. Due to pathogenic mutations of the NEU1 gene, the activity of this enzyme is severely decreased. Neuraminidase catalyzes the removal of sialic acid residues from lysosomal-membrane associated protein 1 (LAMP1). In the absence of neuraminidase, oversialylated LAMP1 accumulates in the plasma membrane and mediates the exocytosis of lysosomal hydrolases [14].
• ML types 2 and 3 are generally triggered by mutations of the GNPTAB gene, which is located at 12q23.2 and encodes for the α and β subunits of the membrane-bound GlcNAc-PT. Thus, ML types 2 and 3 are allelic disorders. They are usually defined as two variants of a single disease rather than as distinct entities. For the sake of completeness, it shall be mentioned that N-GlcNAc-PT contains a third subunit, the so-called γ subunit, which is encoded by the GNPTG gene. Mutations of this gene have also been related to ML type 3 but have never been identified in ML type 2 patients [3]. Both GNPTAB and GNPTG mutations impede the formation of a functional heterohexameric enzyme complex, which is required for the synthesis of mannose 6-phosphate recognition markers, that, in turn, attach to acid hydrolases and target them for transport to the lysosomes. Disease severity depends on the degree of enzyme deficiency: Complete loss of GlcNAc-PT activity causes ML type 2, which is the most severe variant of the disease, while ML type 3 is associated with residual enzyme activity [3].
• ML type 4 results from mutations of the MCOLN1 gene. This gene, to be found at 19p13.2, encodes for mucolipin 1, a transient receptor potential locating to the lysosomal and late endosomal membrane [15].

In general, ML are rare disorders. The overall incidence of ML is <1 per 100,000 life births. The incidence of distinct types of ML has been estimated as follows: 0.05, 0.016, and 0.08 for ML types 1, 2, and 3, respectively, per 100,000 life births [16]. Similar to the aforementioned types of ML, ML type 4 is a panethnic disease. However, >80% of affected individuals are of Ashkenazi Jewish descent. In this population, the incidence of the disease is 1 per 40,000 life births. About 1% of Ashkenazi Jews are carriers of pathogenic mutations of the MCOLN1 gene [5].

The pathogenesis of ML type 1 is incompletely understood. The accumulation of oversialylated LAMP1 is assumed to be the cause of excessive lysosomal exocytosis, which, in turn, induces alterations of the characteristics and composition of the plasma membrane and extracellular matrix. This results in an overall loss of tissue homeostasis, but further research is required to shed more light on the pathophysiological events leading to developmental delays, skeletal dysplasia, and ophthalmological complications [2] [14].

In ML types 2 and 3, the insufficient synthesis of mannose 6-phosphate recognition markers hinders the transport of distinct enzymes to the lysosomal compartment. Thus, enzymes required for lysosomal functions are released to the extracellular space and may be detected in body fluids. Here they are, however, unable to fulfill their metabolic functions [1]. What's more, they may mediate pathological processes such as the loss of retention of hematopoietic progenitor cells [14].

The role of mucolipin 1 for lysosomal function is less clear. It has been implicated in chaperone-mediated autophagy [15]. A broad spectrum of acid glycosaminoglycans, gangliosides, phospholipids, and neutral lipids has been identified as the storage substances, suggesting the deficiency of diverse lysosomal enzymes, similar to what is known for the other types of ML [4]. However, concentrations of the aforementioned carbohydrates and lipids as well as levels of lysosomal hydrolases in blood samples are normal [5].

Affected families may benefit from genetic counseling and targeted prenatal diagnosis. Precise knowledge regarding the disease' molecular background and metabolic consequences largely facilitates the identification of affected fetuses [1]. Beyond that, newborn screening programs for lysosomal storage diseases have been implemented in some countries. Depending on the method of testing, these screening programs may or may not cover all types of ML: The assessment of concentrations of lysosomal hydrolases, for instance, will identify only those suffering from ML types 2 and 3 [17]. For the Ashkenazi Jewish population, screenings for common mutations of the MCOLN1 gene have been proposed [18].

ML refers to the progressive accumulation of carbohydrates and lipids within the lysosomes of cells and is the result of enzyme deficiencies. There are four types of ML:

• ML type 1, which is also referred to as sialidosis
• ML type 2, otherwise known as I cell disease
• ML type 3, or pseudo-Hurler polydystrophy, is allelic to ML type 2
• ML type 4

These types of ML differ largely with regards to their clinical presentation, to hematological and biochemical parameters. Accordingly, distinct studies are required to confirm a tentative diagnosis of either type of ML. Curative treatment is available for neither of them.

Mucolipidosis (ML) is a general term referring to a heterogeneous group of hereditary metabolic disorders. There are four types of ML, namely ML type 1 or sialidosis, ML type 2 or I cell disease, ML type 3 or pseudo-Hurler polydystrophy, and ML type 4. They are all caused by rare mutations of genes that interfere with the function of lysosomes. Lysosomes are cell organelles whose task is to break down and recycle a myriad of substances produced by cells of all tissues. In those affected by ML, these substances are inadequately digested and accumulate within the lysosomes. This may lead to severe delays in physical and mental development, dysmorphic features, skeletal dysplasia, visual impairment, heart disease, and recurrent respiratory infections. The severity of ML varies largely. While the more severe forms of the disease may be fatal in the neonatal period or infancy, mild ML may be associated with mild learning difficulties and a near-to-normal life expectancy. Unfortunately, there is no curative treatment. Patients are provided symptomatic therapy and supportive care, but, according to current knowledge, disease progression can hardly be halted.

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