Mucolipidosis II alpha/beta (also known as I-cell disease) is a progressively debilitating disorder that affects many parts of the body. Most affected individuals do not survive past early childhood.

At birth, children with mucolipidosis II alpha/beta are small and have weak muscle tone (hypotonia) and a weak cry. Affected individuals grow slowly after birth and usually stop growing during the second year of life. Development is delayed, particularly the development of speech and motor skills such as sitting and standing.

Children with mucolipidosis II alpha/beta typically have several bone abnormalities, many of which are present at birth. Affected individuals may have an abnormally rounded upper back (kyphosis), feet that are abnormally rotated (clubfeet), dislocated hips, unusually shaped long bones, and short hands and fingers. People with this condition also have joint deformities (contractures) that significantly affect mobility. Most children with mucolipidosis II alpha/beta do not develop the ability to walk independently. Affected individuals have dysostosis multiplex, which refers to multiple skeletal abnormalities seen on x-ray.

Other features of mucolipidosis II alpha/beta include a soft out-pouching around the belly-button (umbilical hernia) or lower abdomen (inguinal hernia), heart valve abnormalities, distinctive-looking facial features that are described as "coarse," and overgrowth of the gums (gingival hypertrophy). Vocal cords can stiffen, resulting in a hoarse voice. The airway is narrow, which can contribute to prolonged or recurrent respiratory infections. Affected individuals may also have recurrent ear infections, which can lead to hearing loss.

Mutations in the GNPTAB gene cause mucolipidosis II alpha/beta. This gene provides instructions for making part of an enzyme called GlcNAc-1-phosphotransferase. This enzyme helps prepare certain newly made enzymes for transport to lysosomes. Lysosomes are compartments within the cell that use digestive enzymes to break down large molecules into smaller ones that can be reused by cells. GlcNAc-1-phosphotransferase is involved in the process of attaching a molecule called mannose-6-phosphate (M6P) to specific digestive enzymes. Just as luggage is tagged at the airport to direct it to the correct destination, enzymes are often "tagged" after they are made so they get to where they are needed in the cell. M6P acts as a tag that indicates a digestive enzyme should be transported to the lysosome.

Mutations in the GNPTAB gene that cause mucolipidosis II alpha/beta prevent the production of any functional GlcNAc-1-phosphotransferase. Without this enzyme, digestive enzymes cannot be tagged with M6P and transported to lysosomes. Instead, they end up outside the cell and have increased digestive activity. The lack of digestive enzymes within lysosomes causes large molecules to accumulate there. Conditions that cause molecules to build up inside lysosomes, including mucolipidosis II alpha/beta, are called lysosomal storage disorders. The signs and symptoms of mucolipidosis II alpha/beta are most likely caused by the lack of digestive enzymes within lysosomes and the effects these enzymes have outside the cell.

Mutations in the GNPTAB gene can also cause a similar but milder disorder called mucolipidosis III alpha/beta. Instead of preventing the production of any enzyme, these mutations reduce the activity of GlcNAc-1-phosphotransferase. Mucolipidosis III alpha/beta and mucolipidosis II alpha/beta represent two ends of a spectrum of disease severity.

Normal Function

The GNPTAB gene provides instructions for making two different parts, the alpha and beta subunits, of an enzyme called GlcNAc-1-phosphotransferase. This enzyme is made up of two alpha (α), two beta (β), and two gamma (γ) subunits. The gamma subunit is produced from a different gene, called GNPTG. GlcNAc-1-phosphotransferase helps prepare certain newly made enzymes for transport to lysosomes. Lysosomes are compartments within the cell that use digestive enzymes called hydrolases to break down large molecules into smaller ones that can be reused by cells.

GlcNAc-1-phosphotransferase is involved in the first step of making a molecule called mannose-6-phosphate (M6P). M6P acts as a tag that indicates a hydrolase should be transported to the lysosome. Specifically, GlcNAc-1-phosphotransferase transfers a molecule called GlcNAc-1-phosphate to a newly produced hydrolase. In the next step, a molecule is removed to reveal an M6P attached to the hydrolase. Once a hydrolase has an M6P tag, it can be transported to a lysosome.

Health Conditions Related to Genetic Changes

Mucolipidosis II alpha/beta

Around 50 mutations in the GNPTAB gene have been found to cause mucolipidosis II alpha/beta. These mutations prevent the production of any functional GlcNAc-1-phosphotransferase. Without this enzyme, hydrolases cannot be tagged with M6P and transported to lysosomes. Instead, hydrolases end up outside the cell and have increased digestive activity. The lack of hydrolases within lysosomes causes large molecules to accumulate there. Conditions that cause molecules to build up inside lysosomes, including mucolipidosis II alpha/beta, are called lysosomal storage disorders. The signs and symptoms of mucolipidosis II alpha/beta are most likely caused by the lack of hydrolases within lysosomes and the effects these enzymes have outside the cell.

Mucolipidosis III alpha/beta

Mutations in the GNPTAB gene have also been found to cause mucolipidosis III alpha/beta. Affected individuals have mutations that result in reduced activity of GlcNAc-1-phosphotransferase, which disrupts tagging of hydrolases with M6P. Digestive enzymes that do not receive the M6P tag end up outside the cell, where they have increased activity. The shortage of these digestive enzymes within lysosomes causes large molecules to accumulate there. Mucolipidosis III alpha/beta is also considered to be a lysosomal storage disorder. The signs and symptoms of mucolipidosis III alpha/beta are most likely due to the shortage of hydrolases inside lysosomes and the effects these enzymes have outside the cell.

Other Names for This Gene

• alpha-beta GlcNAc-1-phosphotransferase
• DKFZp762B226
• GlcNAc phosphotransferase
• GlcNAc-1-phosphotransferase
• GNPTA
• GNPTA_HUMAN
• KIAA1208
• MGC4170
• N-acetylglucosamine-1-phosphate transferase
• N-acetylglucosamine-1-phosphate transferase alpha and beta subunits
• N-acetylglucosamine-1-phosphate transferase, alpha and beta subunits
• UDP-N-acetylglucosamine-lysosomal-enzyme N-acetylglucosamine
• uridine 5'-diphosphate-N-acetylglucosamine: lysosomal hydrolase N-acetyl-1-phosphotransferase

Genomic Location

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Mucolipidosis II alpha/beta is a rare disorder, although its exact prevalence is unknown. It is estimated to occur in about 1 in 100,000 to 400,000 individuals worldwide.