CHAPTER TEN
INHERITED METABOLIC DISORDERS

METACHROMATIC LEUKODYSTROPHY

Metachromatic leukodystrophy (MLD) is an autosomal recessive deficiency of arylsulfatase A that results in accumulation of the myelin lipid sulfatide in oligodendrocytes and Schwann cells. In its most common variant, patients are normal up to age one or two years, and then develop progressive peripheral neuropathy, psychomotor retardation, and blindness. Signs of white matter involvement (spasticity, brisk tendon reflexes, extensor plantar responses) are prominent. Less severe variants cause adult onset dementia, psychiatric disorders, and neuropathy.

MLD:peripheral nerve	MLD:loss of myelin in the white matter	MLD: metachromatic deposits

Lysosomal storage of sulfatides kills oligodendrocytes and Schwann cells. Sulfatides discharged from dying cells are picked up byhistiocytes. The white matter shows diffuse loss of myelin which spares the subcortical fibers. Peripheral nerves show a demyelinative neuropathy. Sulfatides are also asymptomatically stored in a variety of somatic cells such as the epithelium of the gallbladder and renal tubules. Sulfatide deposits stain pink with H&E. With acid cresyl violet, they take on a brown color (brown metachromasia), hence the term metachromatic leukodystrophy.

The biochemical defect of MLD does not involve myelin synthesis, but rather the degradation and recycling of myelin lipids. Myelin is probably formed normally initially. Subsequently, dysfunction and loss of myelin-producing cells cause loss of myelin. Myelination of some tracts begins in utero. The bulk of myelin is produced in the first two years of life. This is when severe MLD becomes clinically manifest.

About one third of myelin lipid consists of galactocerebroside and its sulfated variant sulfatide. Deficiency of galactocerebrosidase causes a severe infantile leukodystrophy, Krabbe's disease.

GAUCHER DISEASE

Gaucher disease (GD) is due to deficiency of glucocerebrosidase (glucosylceramidase) and is characterized by storage of glucocerebroside (glucosylceramide) in monocyte-macrophage cells. Three clinical phenotypes are recognized. The most common is type 1 which is especially prevalent in Ashkenazi Jews. Type 1 GD presents from childhood to early adulthood and causes hepatosplenomegaly, bone disease (osteopenia, focal lytic or sclerotic lesions, osteonecrosis, pathologic fractures, chronic bone pain), anemia and thrombocytopenia due to hypersplenism, and pulmonary interstitial infiltrates. Spinal cord and root compression secondary to bone disease may also develop but there is no storage in the CNS. Type 2 (acute neuronopathic) GD patients have hepatosplenomegaly similar to type 1, but develop also neurological manifestations (stridor, strabismus and other oculomotor abnormalities, swallowing difficulty, opisthotonus, spasticity) which cause their death by 2 to 4 years of age. There is no special ethnic prevalence for type 2 GD. Type 3 (subacute neuronopathic) GD is frequent in Northern Sweden and has hematological and neurological manifestations similar to type 2 but milder and more slowly progressive. GD is the first LSD to be successfully managed by enzyme replacement.

Gaucher cells

GD is the prototype of storage histiocytosis. Lysosomal storage of glucocerebroside in cells of the monocyte-macrophage system leads to a characteristic cellular alteration of these cells. Gaucher cells (GC) have a large cytoplasmic mass with a striated appearance that has been likened to "wrinkled tissue paper" or "crumpled silk". GCs are present in the bone marrow, spleen, lymph nodes, hepatic sinusoids, and other organs and tissues in all forms of GD. An increased incidence of cancer including lymphoma, myeloma, and bone tumors has been reported in GD patients. There is no storage in neurons or glial cells. In type 2 and 3 GD, there are numerous GCs in perivascular CNS spaces and rare GCs in brain parenchyma. No part of the CNS is spared but the brainstem and deep nuclei are more severely affected than the cortex and account for most neurological deficits. Along with the presence of GCs, type 2 and 3 GD shows also neuronophagia, neuronal loss, and gliosis. No neuronal storage is seen. Neuronal degeneration and loss have been attributed to the neurotoxic action of glucosyl sphingosine, a by-product of glucocerebroside not normally present in the brain.

MUCOPOLYSACCHARIDOSES (MPS)

Mucopolysaccharides (now called Glycosaminoglycans-GAGs) are synthesized in the Golgi apparatus and secreted and assembled in the extracellular space. They are produced by all cells, and are especially abundant in connective tissues. They are an important component of the matrix of connective tissue, cartilage and bone. For recycling, GAGs are internalized and degraded in a stepwise fashion by lysosomal enzymes. Deficiency of these enzymes causes lysosomal storage of GAGs. There are six clinical groups of MPS caused by deficiencies of ten GAG-cleaving enzymes.

Intracellular storage of GAGs in hepatocytes and other cells causes hepatomegaly, cellular dysfunction, and cell death. The most severe somatic changes in the MPS are due to accumulation of GAGs in matrix due to impaired recycling and to discharge of GAGs from dying mesenchymal cells. Because they are negatively charged, GAGs attract a lot of water that causes their molecules to swell to tremendous volumes. High GAG content of connective tissues affects collagen synthesis and causes increased collagen deposition.

MPS	MPS: thickened cardiac valves	MPS-coronary artery: intimal thickening

The skin, connective tissues, and cartilage become swollen and distorted. The connective tissue and cutaneous changes cause facial deformity and macroglossia which gave rise to the insensitive term gargoylism. Cardiac valves and chordae tendineae become thickened and stiff. Endocardial and interstitial myocardial fibrosis develops. The intima of coronary arteries may be thickened to the point of occlusion and the aorta develops fibrous intimal plaques without lipid deposition. These changes cause a fatal cardiomyopathy and ischemic heart disease. GAG storage causes joint stiffening and swelling and complex skeletal deformities known as dysostosis multiplex. Storage in corneal fibroblasts causes corneal clouding.

MPS: Hydrocephalus	MPS: "zebra bodies"

GAG deposition in connective tissues of the brain and spinal cord causes thickening of the dura which along with distortion of vertebraeresults in compression myelopathy. Thickening of the arachnoid membrane impairs CSF flow, causing communicating hydrocephalus. But the most devastating neurological effects of MPS are due to neuronal storage of gangliosides. The mechanism of this storage is poorly understood. It is probably due to inhibition of neuraminidase and other lysosomal enzymes induced by the storage of GAGs. Thus, in addition to the skeletal, cardiovascular and other lesions, many MPS also cause neuronal lipidosis. Gangliosides stored in nerve cells take the form of concentric membranes (membranous cytoplasmic bodies) or stacks of membranes (zebra bodies).

NEURONAL CEROID LIPOFUSCINOSES

NCL: curvilinear inclusions	NCL: cerebral and cerebellar atrophy

Lipofuscin and ceroid are undegradable materials formed in lysosomes by peroxidation of membrane lipids. They take the form of golden-brown, autofluorescent, PAS-positive, and acid-fast granules and are especially prominent in neurons and cardiac myocytes. The neuronal ceroid lipofuscinoses (NCLs) are a group ofautosomal recessive disorders in which similar products accumulate in nerve cells, histiocytes, and many other cells and tissues. They are divided into four main groups, infantile, late infantile, juvenile, and adult NCL and four other less frequent entities. The term Batten disease is used for juvenile NCL, first described about 100 years ago, and is also applied to the entire group of NCLs. The NCLs are the most common causes of neuronal storage disease and, according to some authors, the most common neurodegenerative diseases in children. Pathologically, they show neuronal ballooning due to lysosomal storage of a variety of granular, lamellar, curvilinear, and other products. This storage causes neuronal loss, cortical atrophy, and cerebellar and retinal degeneration resulting in seizures, myoclonus, ataxia, and blindness. The NCLs are inexorably progressive diseases, and most are fatal.

The storage material contains subunit C of mitochondrial ATP synthase and components of sphingolipid associated proteins (saposins). The NCLs are linked to six known CLN genes. Two of these (CLN1 and CLN2) encode lysosomal enzymes, Palmitoyl Protein Thioesterase (PPT) and Tripeptidyl Peptidase 1 respectively, which are involved in the lysosomal degradation of proteins. The products of the other CLN genes are lysosomal and endosomal membrane proteins whose function is unkown. The pathogenesis of NCLs has been unclear. For a long time, no enzyme deficiency could be demonstrated, but now it appears that they belong in the LSDs after all. They are probably caused by deficiencies of lysosomal proteases. Histiocytic storage of ceroid and lipofuscin (sea blue histiocytes) develops also in neoplastic and non-neoplastic blood disorders with accelerated turnover of hematopoietic cells.

NIEMANN-PICK DISEASE TYPE C

Type A and B Niemann-Pick disease are neurovisceral storage diseases caused by deficiency of sphingomyelinase. Niemann-Pick type C (NPC) is an LSD with protean clinical manifestations including neonatal hydrops, neonatal hepatitis, storage histiocytosis and neuronal lipidosis. The material that is stored in lysosomes in NPC is not sphingomyelin but cholesterol. Patients with NPC can import LDL cholesterol into lysosomes and remove the cholesteryl ester generating free cholesterol, but they cannot move free cholesterol to its normal cellular destinations. Thus, cholesterol accumulates in lysosomes. The mutant gene is located on 18q and its product, the NPC1 protein, is a transmembrane protein which acts as "gatekeeper" in the transport of lysosomal cholesterol to its other cellular targets. The "filipin test", which is used for diagnosis of NPC, consists of feeding cultured fibroblasts with LDL cholesterol tagged with the fluorescent dye filipin. The fibroblasts show bright fluorescence due to accumulation of cholesterol. NPC is rare but its study has produced some important insights into intracellular cholesterol homeostasis and trafficking.