Mucolipidosis type IV (ML IV, ganglioside sialidase deficiency, or ML4) is an

autosomal An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosomal (sex chromosome) pairs, which may have different structures. The DNA in autosom ...

recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

lysosomal storage disorder. Individuals with the disorder have many symptoms including delayed psychomotor development and various ocular aberrations. The disorder is caused by mutations in the MCOLN1 gene, which encodes a non-selective cation channel, mucolipin1. These mutations disrupt cellular functions and lead to a neurodevelopmental disorder through an unknown mechanism. Researchers dispute the physiological role of the protein product and which ion it transports.

Signs and symptoms

Most patients with ML IV show psychomotor retardation (i.e., delayed development of movement and coordination),

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...

l opacity,

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

l degeneration and other

ophthalmological Ophthalmology ( ) is a surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a medic ...

abnormalities. Other symptoms include agenesis of the corpus callosum,

iron deficiency Iron deficiency, or sideropenia, is the state in which a body lacks enough iron to supply its needs. Iron is present in all cells in the human body and has several vital functions, such as carrying oxygen to the tissues from the lungs as a key ...

resulting from an absence of acid secretion in the stomach,

achlorhydria Achlorhydria and hypochlorhydria refer to states where the production of hydrochloric acid in gastric secretions of the stomach and other digestive organs is absent or low, respectively. It is associated with various other medical problems. Sig ...

. Achlorhydria in these patients results in an increase in blood gastrin levels. These symptoms typically manifest early in life (within the first year). After disease onset there occurs a period of stability, typically lasting two to three decades during which very little disease progression occurs.

Pathophysiology

Mucolipin1 is thought to be localized in

endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane can ...

s. An important property of mucolipin1 is that decreasing pH (acidification) results in deactivation of the protein, likely through an assembly defect. There are at least 29 known mutations in MCOLN1, located throughout the gene. Many of the known mutations result in no expression of mucolipin1. Milder mutations, such as ΔF408 and V446L, produce a dysfunctioning form of the cation channel. Mutations that alter only the

C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...

of the protein also result in a mild phenotype of the disorder, usually sparing the brain. ML IV causes affected cells to accumulate auto-fluorescent vacuoles considered to be aberrant lysosomes. Several evidences exist for a defect in both exocytosis and endocytosis. There are conflicting indications of abnormal lysosomal pH in MLIV. It is not yet clear why these abnormalities will cause incomplete development of the brain, achlorhydria, and failure in the maintenance of retinal tissue.

Diagnosis

Diagnosis includes genetic testing and Gastrin blood test to check for low iron in the blood.

Treatment

There is no specific treatment to this disorder. However, several symptoms may be alleviated. For instance,

anemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, t ...

is treated by

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

supplements. Some of the movement deficiencies may be corrected with orthopedic intervention. The corneal clouding can be, at least, temporarily corrected by corneal transplantation. ''See the equivalent section in the main mucolipidosis article.

Epidemiology

Mucolipidosis type IV is severely under-diagnosed. It is often misdiagnosed as

cerebral palsy Cerebral palsy (CP) is a group of movement disorders that appear in early childhood. Signs and symptoms vary among people and over time, but include poor coordination, stiff muscles, weak muscles, and tremors. There may be problems with sens ...

. In the Ashkenazi Jewish population there are two severe mutations with a higher carrier frequency of 1:90 to 1:100.

References

External links

* {{DEFAULTSORT:Mucolipidosis Type Iv Lysosomal storage diseases Autosomal recessive disorders Channelopathies Ashkenazi Jews topics