Lipid Storage Disorders 

Lipid storage disorders are a family of diverse diseases related by their molecular pathology. In each disorder, a deficiency of a lysosomal hydrolase is inherited, which leads to lysosomal accumulation of the enzyme's specific sphingolipid substrate.^{[1, 2]} Lipid substrates share a common structure, including a ceramide backbone (2-N -acyl-sphingosine), in which various sphingolipids are derived by substitution of hexoses, phosphorylcholine, or one or more sialic acid residues on terminal hydroxyl groups of the ceramide molecule.

Pathways of glycosphingolipid metabolism in both nervous tissue and visceral organs are elucidated, and for each catabolic step, a genetically determined metabolic derangement is identified. Disorders include GM1 gangliosidoses,^[3] GM2 gangliosidoses,^[3] Gaucher disease, Niemann-Pick disease (NPD),^[4] Fabry disease, fucosidosis, Schindler disease, metachromatic leukodystrophy (MLD), Krabbe disease, multiple sulfatase deficiency, Farber disease, Wolman disease, and cholesterol ester storage disease (CESD).

The biochemical basis of lipid storage disorders is well characterized and includes determining properties of enzymatic activities and various storage products. Research has led to development of diagnostic assays for identification of affected individuals, which usually rely on measurement of specific enzymatic activity in isolated leukocytes or cultured fibroblasts. For most disorders, carrier identification and prenatal diagnosis are available as well. Making a specific diagnosis in an affected individual is essential in order to provide accurate genetic counseling.

More recently, investigators have focused efforts on determining the molecular basis of each of these disorders. These studies have resulted in identifying specific disease-causing mutations and have led to improved clinical and laboratory diagnosis, prenatal diagnosis, and carrier identification. In addition, for some disorders (eg, Gaucher disease), making genotype-phenotype correlations that predict disease severity and allow more accurate genetic risk counseling is possible. Advances in understanding the molecular and biochemical basis include cloning and characterization of most genes that encode specific enzymes required for sphingolipid metabolism. These investigations permit development of improved therapeutic options, such as recombinant enzyme replacement therapy. Other therapeutic options, such as gene therapy and bone marrow transplantation, for selected lipidoses may also result in improved prognosis.

Because glycosphingolipids are essential components of all cell membranes, inability to degrade these substances and their subsequent accumulation results in physiologic and morphologic alterations of specific tissues and organs that lead to characteristic clinical manifestations. In particular, progressive lysosomal accumulation of glycosphingolipids in the central nervous system can lead to a neurodegenerative course; whereas, storage in visceral cells can lead to organomegaly, skeletal abnormalities, bone marrow dysfunction, pulmonary infiltration, and other manifestations. In general, storage of any particular substrate in a specific tissue is dependent on the normal distribution of the compound in the body. Thus, various disorders of lipid metabolism have characteristic patterns of organ involvement and clinical history, depending on the particular substrate that is stored.

Frequency

United States

Lipid storage disorders are rare disorders, although some have an ethnic predilection with more appreciable frequency.

International

Frequency is similar to that in the United States.

Mortality/Morbidity

Infantile forms are usually fatal. Juvenile-onset and adult-onset disorders have variable survival rates that depend on particular manifestations.

Race

Most lipid storage disorders are panethnic; however, an ethnic predilection has been noted for Tay-Sachs disease, type 1 Gaucher disease, and NPD type A, which all occur at increased frequency in Ashkenazi Jews. Guidelines for carrier screening for genetic disorders in individuals of Ashkenazi Jewish descent have been established.^[5]

Other important ethnic predilections include the following:

• NPD type C1 has a high incidence in Acadians from Nova Scotia, individuals of Hispanic descent in parts of the southwestern United States, and a Bedouin group in Israel.
• Late-onset form of Fabry disease is found in increased incidence in Italy
• Gaucher disease type 3 is more common in the Norrbottnian region of Sweden
• Tay-Sachs disease has an increased incidence in French Canadians, Cajuns from Louisiana, and Old Order Amish in Pennsylvania.
• Metachromatic leukodystrophy has an increased incidence in the Habbanite Jewish in Israel, Isaraeli and Christian Israeli Arabs, and the western portion of the Navajo nation in the United States.

Sex

Each disorder is transmitted as an autosomal recessive trait, except Fabry disease, which is an X-linked recessive trait.

Age

Congenital presentation

The perinatal lethal form of Gaucher disease is associated with nonimmune hydrops fetalis, arthrogryposis, ichthyosiform or collodion skin abnormalities, hepatosplenomegaly, and pancytopenia.

Perinatal forms of GM1 gangliosidosis, NPD type C, Wolman disease, and Farber disease are associated with nonimmune hydrops fetalis.

Presentation in infancy

• GM1 gangliosidosis type 1 and NPD type A usually appear in early infancy. GM2 gangliosidoses, which include Tay-Sachs disease and Sandhoff disease, have infantile forms.
• The clinical phenotypes for MLD widely vary. Patients who are severely affected usually present in the first year of life with developmental delay and somatic features, similar to those of mucopolysaccharidoses. Late infantile forms of MLD, which is most common, usually present in infants aged 12-18 months with irritability, inability to walk, and hyperextension of the knee, causing genu-recurvatum.
• Infantile forms of Krabbe disease are rapidly progressive and present early in infancy with irritability, seizures, and hypertonia. Optic atrophy is evident in the first year of life and mental development is severely impaired. A second, late infantile form of Krabbe disease is also observed and presents in children older than 2 years. Affected individuals have a disease course similar to early infantile form.
• Wolman disease is a fatal disorder of infancy. Clinical features become apparent in the first week of life and include failure to thrive, relentless vomiting, abdominal distention, and hepatosplenomegaly.
• Multiple sulfatase deficiency is typically diagnosed in infancy and childhood. Affected patients have ichthyosis, dysostosis multiplex, and symptoms of MLD due the deficient activity of several sulfatases.
• The few reported cases of Farber disease describe the presence of irritability, hoarse cry, and nodular, erythematous swelling of the wrists during the first few weeks of life, with severe motor and mental retardation and death by 2 years of age.
• Patients with Schindler disease type 1 have infantile onset of neuroaxonal dystrophy, developmental delay, and rapidly progressive psychomotor deterioration without organomegaly.
• NPD type A is a fatal disorder of infancy. Hepatosplenomegaly develops by 6 months of age and development does not progress beyond 12 months. A relentless neurodegenerative course then follows with death by 21 months of age.

Presentation in childhood

• GM1 and GM2 gangliosidoses type 2 are juvenile-onset forms.
• NPD type B has a variable age of presentation but frequently appears early in childhood when hepatosplenomegaly is detected.
• Angiokeratomas that appear in Fabry disease usually occur in childhood and can lead to early diagnosis.
• Juvenile forms of MLD have more indolent courses and onset can occur in persons as old as 20 years. This form presents with gait disturbances, mental deterioration, urinary incontinence, and emotional difficulties.
• Gaucher disease types 2 and 3 (neuronopathic) and more severe cases of type 1 (non-neuronopathic) present during childhood.
• Cholesterol ester storage disease (CESD) is the milder form of Wolman disease, with later onset in childhood, less severe symptoms, and lifespan into adulthood.
• Schindler disease type III has milder neurologic manifestations and later onset in childhood.
• Patients with classic NPD type C develop normally for the first 2 years of life, followed by the development of ataxia, grand mal seizures, loss of speech, and other neurologic manifestations leading to death in mid to late childhood.
• Patients with NPD type B primarily have visceral involvement, sometimes massive, without neurologic symptoms and often survive into adulthood.

Presentation in adulthood

Adult forms of MLD, which present after the second decade of life, are similar to juvenile forms in clinical manifestations, although emotional difficulties and psychosis are more prominent features. Late-onset and variant forms with onset or diagnosis in adulthood due to milder symptoms include Krabbe, NPD type C, Gaucher disease type 1, and Schindler disease type II (Kanzaki disease).