Mucolipidosis Type IV

Abstract

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.