Introduction

Mucolipidosis IV MLIV (OMIM 252650) was first described by Berman in 1974 in an infant with eye abnormalities [1]. Already in the first patient pathological examination of various tissues revealed intracellular accumulation of large vacuoles, which seemed similar to those discovered in mucopolysaccharidosis and sphingolipidosis, leading to the categorization of the disease as mucolipidosis. Initial studies focused on identifying a distinct deficiency in lysosomal function [2–7] in various tissues including brain biopsy [8]. However, most of the lysosomal enzyme activities appeared normal, whereas a wide range of metabolites seemed to accumulate in patient’s cells. Because the first patients were discovered in Ashkenazi families, the disease was considered to be restricted to the Ashkenazi Jewish population [9].

Cultured cells obtained from MLIV patients revealed autofluorescence [10], a feature that is commonly seen in neuronal ceroid lipofuscinosis, a family of neurodegenerative genetic diseases caused by mutations in lysosomal peptidases and related proteins. Subsequently, several non-Jewish patients with MLIV were diagnosed [11], creating a large enough cohort to perform a linkage analysis study on MLIV that eventually led to the discovery of the mutant gene [12–15].

The natural history of MLIV was investigated at the National Institutes of Health. This study confirmed the developmental aspects of the disease [16,17]. In addition, it demonstrated a severe lack of myelin in the brains of the patients, resulting in a distinct loss of the corpus callosum and a small cerebellum [16,18].

A seminal discovery was initiated by the observation that MLIV patients have severe iron deficiency anemia secondary to hypochlorhydria, that is, they do not secrete acid in their stomach [19,20]. Importantly, the lack of gastric acid also causes a dramatic increase in the level of blood gastrin, a hormone that stimulates gastric acid secretion. This observation has provided a simple laboratory tool to diagnose the disease [16]. High serum gastrin remains the only biochemical abnormality specific to MLIV that can be utilized for drug discovery investigation.

The discovery of the MCOLN1 gene has attracted investigators from the channel biology research field to join the investigation of MLIV. In addition, this gene was ablated in several species, including mice [21–25], C. elegans [23,24], drosophila [25], and Dictyostelium [26]. These model systems shed new light on the function of the gene whose defect is responsible for MLIV.

Several studies have addressed the cellular localization of mucolipin-1 and other proteins that interact with it. Other research efforts, however, focused on the metabolites that accumulate in the large vesicles, although these entities are not specific to MLIV nor do they seem to affect cellular viability or specific cellular functions.

Clinical Presentation

MLIV typically presents during the first 6 months of life with slow motor development and eye abnormalities [1,16,17,27]. However, the diagnosis is often missed (in many cases, MLIV is misdiagnosed as cerebral palsy) until the eye doctor recognizes the loss of retinal function. Skin and conjunctiva biopsies reveal large vacuoles that assist in making the correct diagnosis [28]. High levels of blood gastrin, followed by detection of mutations in MCOLN1, confirm the clinical diagnosis [28,29]. Most patients are small for their age and seem to have distinct facial features (although those are not too abnormal). Head MRI reveals an almost complete absence of corpus callosum and a small cerebellum [16,18]. The amount of gray matter in the brain is, however, not reduced; importantly, the gray matter is not lost with age either, an important feature that questions the possibility of progressive neurodegeneration [30].

MLIV seems to have variable clinical penetrance. The abnormalities in the brain correlate with the severity of the disease and are absent in mildly forms [31–34]. They also correlate with the predicted severity of the mutations in MCOLN1 [16].

Eye abnormalities are common to all the patients diagnosed so far [35]. The epithelial layers of cells in the cornea are opaque to some degree, presumably due to accumulation of lysosomal vacuoles [36–38]. Patients also suffer problems with eye movement. Retinal degeneration starts in the first 5 years and progresses more rapidly with age [27,39]. The central vision is affected first, and patients can rely on peripheral vision to some extent. The clinical presentation of the vision loss is, however, variable: For example, one patient with a mild form of MLIV had a ring pattern of blindness sparing the central and the most peripheral vision [34].

The majority of the patients cannot speak and many use sign language to communicate [27]. This creates a huge problem when they lose their vision and sign language is not an option anymore. Hearing is not affected in MLIV, indicating that inability to speak is not caused by a hearing defect, and the sensory function loss is restricted to vision.

The typical patient cannot walk independently but can use a walker to some extent. Patients also suffer from spasticity and other movement abnormalities, mostly in the area of fine motor skills. Muscles cells contain abnormal vacuoles; based on these findings, it was initially thought that the movement deficits are caused by a muscle disease [40].

About half of the patients show evidence of epilepsy detected by electroencephalogram (EEG) [41]. Many MLIV patients also suffer from episodic pain in half of their face [42]. Combined, these observations imply a defect in the activity of motor afferents in the central nervous system.

Cellular Abnormalities in MLIV Patients

MLIV patients do not secrete acid in their stomach [19,20]. The defect is in the parietal cells of the stomach that are responsible for acid secretion. Upon stimulation, parietal cells normally undergo morphological changes to form long intracellular canaliculi (ducts). Hydrochloric acid is produced in these canaliculi by an interaction of the K⁺/H⁺ pump (that secretes hydronium ions) with the chloride channels. In MLIV parietal cells, the morphological change is not complete, indicating that the signal for acid secretion is impaired (Figure 24.1). The pump components are there but they do not seem to be organized around the canaliculi. There is also an abundance of vacuoles in the cells, some of them multilamellar and some seem to contain microvilli that were supposed to line the canaliculi (Figures 24.2 and 24.3). Pancreatic acinar cells, which secret digestion enzymes to the duodenum, are also highly vacuolated in MLIV. It is not known, however, whether or not these enzymes are secreted at a normal rate [43,44]. Digestion in MLIV patients is not optimal, but this seems to be a result of reduced intestinal motility. Iron absorption is deficient due to the lack of stomach acid, and the patients need iron supplementation [16,27].

Recently, it was discovered that mitochondria are fragmented in MLIV cells [45,46]. This may explain some of the clinical features of the disease. Many of the cells that require high oxidative metabolism are affected in patients, including brain cells, retinal cells, parietal cells of the stomach, and muscle cells. It is possible that dysfunctional mitochondria are directly responsible for the functional deficits in those cell types.

Defects in autophagy are present in MLIV cells just like in any other disease associated with lysosomal dysfunction [47–51]. Slower rate of metabolism of certain components will slow the uptake of new material into the system and autophagy is delayed.

MCOLN1 and Mucolipin-1

MCOLN1 is a 12,000 bp gene with 14 exons that is located in human chromosome 19p13. The gene product, mucolipin-1, is a protein with six transmembrane domains, an outside loop between the first and the second transmembrane domain, and a cation channel pore between the fifth and the sixth domains (Figure 24.4). The three mammalian mucolipins form the subfamily of TRPML channels and are close relatives of PKD2. They have the same distance between the different transmembrane domains and are probably of the same phylogenetic origin [15]. Invertebrates, unicellular organisms, and plants there have only one mucolipin, and it has a high homology with mucolipin-3.