Pelizaeus–Merzbacher Disease and X-linked Spastic Paraplegia Type 2

(1)

34.1 Clinical Features

and Laboratory Investigations Pelizaeus–Merzbacher disease (PMD) is a rare neurological disorder affecting the myelination of the CNS.

The disease has an X-linked recessive mode of inheritance and is usually subdivided into three types: the classical type (type I), the connatal type (type II), and the transitional type (type III).

The classical type has its onset in the first year of life. Those affected are almost exclusively boys. The disease manifests initially by irregular nystagmoid eye movements referred to as “dancing,” “trembling,”

or “roving.” In some children stridor occurs, caused by either laryngeal abductor paralysis or laryngoma- lacia. There is variable but always marked to severe developmental delay with only very slow developmental progress. Growth is retarded and the head size is small, in the low normal or microcephalic range.

Seizures occur early in the course of the disease.

Characteristically there is a tremor or bobbing, nodding, or shaking movement of the head. Signs of tetraspasticity, cerebellar ataxia, and extrapyramidal movement disturbances with hyperkinesia, dystonia, and choreoathetosis become manifest as the patient becomes older. Most patients never walk. The sensory system is usually well preserved. Optic atrophy with visual failure is common. The nystagmus disappears in the course of a few years. Social interactions are often relatively well preserved and the intellectual capacities are higher than the motor capacities. Skeletal abnormalities, such as osteoporosis and kyphoscolio- sis, result from the chronic motor disease and occur after the disease has been manifest for many years.

The course of the disease is chronic. Patients may improve in performance up to the age of 10–12 years.

From that age onwards, a very slow progression of the neurological signs and a decline of mental level is usually noted. Death occurs in most patients in early or mid adulthood and is usually due to intercurrent illnesses.

Some patients have a milder variant. They may achieve aided or unaided walking and demonstrate slow neurological deterioration. They may have signs of polyneuropathy.

The connatal type, also called Seitelberger type, is a more rare and severe form of PMD. The disease is already manifest in the neonatal or early infantile period. The neonatal period may be characterized by

hypotonia, feeding problems, absent primitive reflex- es, and sometimes stridor. Abnormal, nystagmoid eye movements and extrapyramidal hyperkinesia occur early, followed by the development of epilepsy, spasticity, cerebellar ataxia, and optic atrophy. Titubation is often present. From birth onwards there is a complete failure of psychomotor development or an early loss of attained milestones. Microcephaly and growth retardation develop in the subsequent years. Progres- sion is rapid, with death occurring in the first decade, usually in early childhood.

The transitional form between the classical and connatal types has its onset in the neonatal or early infantile period, but its course is less rapid. The dis- tinction between the classical, transitional, and connatal forms of PMD is ill-defined. The age of onset does not appear to be the most valuable discriminat- ing factor between the types of PMD. The rate of progression is the most useful and earliest reliable means of differentiating the different types.

The nosology of PMD is a matter of debate. Some include all patients with evidence of severe hypomyelination on MRI and a Pelizaeus–Merzbacher- like clinical picture under the heading of PMD, even in sporadic cases, in female patients, or in the case of autosomal recessive inheritance. In particular with connatal onset, female patients can be found.An adult variant of PMD has been suggested: the Löwen- berg–Hill type. We prefer to include under the heading of PMD only patients in whom a proteolipid protein (PLP) gene mutation has been found or in whom the family history indicates an X-linked recessive inheritance. Another basic defect should be sought for the remaining patients.

Mothers of boys suffering from classical PMD sometimes develop neurological problems including spasticity, bladder dysfunction, personality changes, dementia, and polyneuropathy. Incidentally female patients in sibships with classical PMD show identical clinical and postmortem neuropathological findings.

X-linked spastic paraplegia type 2 (SPG2) and PMD are allelic conditions. The pure form of SPG2 leads to spasticity only. The legs are more severely involved than the arms. An intention tremor may be present. There are no other neurological deficits and intelligence is normal. The disease is slowly progressive and the life span is nearly normal. The age of onset varies from a few years of life to the teenage period. The complicated form of SPG2 also leads to slow-

Pelizaeus–Merzbacher Disease

and X-linked Spastic Paraplegia Type 2

Chapter 34

(2)

ly progressive spasticity, but there are additional signs in common with PMD, which may include nystagmus, optic atrophy, cerebellar ataxia, sensory disturbances, and dysarthria. Walking may be delayed but all patients achieve unaided walking. Mild mental retardation is present in some of the patients. Analy- sis of SPG2 and PMD families shows a great overlap of clinical findings, both within and between families.

Therefore, mutations in the PLP gene lead to a spec- trum of disorders ranging from connatal PMD through the various manifestations of the classical PMD disease to pure SPG2.

Routine and metabolic laboratory investigations are of no help in establishing the diagnosis. Nerve conduction velocities and electromyography are usually normal, but in some patients mild to moderate slowing of nerve conduction velocities and neuro- genic changes in electromyography are found. Evoked potential studies are of some help in PMD. In BAEP studies usually only wave I or waves I and II are present and later components are absent, indicative of abnormalities at brain stem level. ERG is normal.VEP and SSEP are absent or abnormal with increased la- tency, abnormal shape, and decreased amplitude.

Definite diagnosis in PMD and SPG2 is DNA-based.

Prenatal diagnosis and carrier detection are possible using DNA techniques.

34.2 Pathology

In PMD, the brain is too light for the patient’s age and shows signs of diffuse atrophy, involving cerebral hemispheres and in particular brain stem and cerebellum. On sectioning the white matter appears reduced in volume to a variable extent and the corpus callosum is markedly reduced in width. Microscopic examination shows lack of myelin in all parts of the CNS. The pathological picture is basically the same in all PMD patients irrespective of subtype, but with variable severity.

In connatal PMD, the pathological picture varies from a marked lack of myelin to a complete absence of myelin in all parts of the brain and spinal cord. The myelin present is usually found in the spinal cord and the deeper parts of the brain: the diencephalon (globus pallidus, posterior limb of the internal capsule, thalamus), the brain stem (tegmentum of pons and mesencephalon, mesencephalic pyramidal tracts), and the central part of the cerebellum. The myelin is usually present in perivascular islets. Also, residual myelin islets are sometimes present in the subcortical white matter, especially in the pre- and postcentral gyri. There are no signs of active demyelination.

There are no or little sudanophilic breakdown products in the white matter. Oligodendrocytes are reduced in number or completely absent. The axons are

relatively well preserved. Some axonal loss may be seen in completely demyelinated areas. The severity of the concomitant fibrillary gliosis varies from slight to dense. The gray matter is also affected; the intra- cortical myelin is completely or almost completely absent, but the normal cytoarchitecture is preserved.

The cerebellar cortex shows loss of Purkinje cells and granular cells. Myelin is also deficient in the optic nerves and chiasm.

The abnormalities in classical PMD are less pro- nounced and myelin deficiency is less severe. There is a patchy absence of myelin with preservation of numerous myelin islets giving the white matter a so-called tigroid pattern. Most of the myelin islets surround small blood vessels. The myelin sheaths in these islets are thin and composed of only a few myelin lamellae. Microscopically some remaining myelin sheaths are also seen in the areas, which are otherwise devoid of myelin.At most small amounts of sudanophilic lipid products are found. Oligodendro- cytes are numerically reduced, especially in the areas lacking myelin.All parts of the CNS are affected in the same way, but the spinal cord, brain stem, cerebellum, diencephalic structures, and subcortical white matter show a relatively good state of myelin preservation. In all areas of the CNS axons are relatively well preserved. Where myelin is absent, the remaining white matter is mainly composed of naked axons. Fibrillary gliosis varies from slight to intense. The astrocytes are sometimes hypertrophied. The gray matter is also involved in the process; myelin sheaths are reduced in number or are absent, but the cortical cytoarchitecture as well as the individual nerve cells are normal, although a certain loss of and damage to nerve cells may be seen. In the cerebellar cortex, loss of Purkinje cells and granule cells may be evident.

Whereas most PMD patients do not show signs of primary axonal degeneration, evidence of length-de- pendent axonal degeneration is seen in patients with a PLP gene deletion or null mutation.

Myelin is present in normal amounts for age in the PNS, including spinal roots and cranial nerves with the exception of the optic nerve in most PMD patients. However, in some patients the peripheral nerves are affected and myelin loss is found.

The transitional type shows abnormalities intermediate in severity between the connatal and the classical types.

Electron microscopy demonstrates that the few oligodendrocytes present contain an excess of cyto- plasmic dense bodies and a poorly developed endoplasmic reticulum. There is condensation of nuclear chromatin, strongly suggestive of apoptosis, as has been found in animal models.

Information on the neuropathology of SPG2 is limited. A severe myelin deficiency has been reported in the spinal cord, contrasting with a mild lack of myelin

(3)

in the cerebral hemispheric white matter with sparing of the U fibers.

34.3 Chemical Pathology

Chemical analysis of the remaining myelin in PMD reveals that its lipid content is greatly reduced and its protein content, conversely, relatively increased. Cere- brosides are reduced in quantity and sulfatides are greatly reduced in quantity. The remaining glycolipids contain an abnormally high proportion of glucose with a proportional reduction in galactose.

The myelin ganglioside content and cholesterol content are near normal. Phospholipids are increased, particularly sphingomyelin, choline phosphoglycerides, and inositol phosphoglycerides, whereas ethanolamine phosphoglycerides and plasmalogens are reduced. Protein analysis shows that proteolipid protein is absent, myelin basic protein is decreased, whereas so-called Wolfgram proteins are increased.

The chemical composition of whole white matter depends on the amount of myelin that is present. The water content is abnormally high. Those lipids that are generally recognized as myelin lipids are either absent or markedly reduced. No or very little sulfatide is present. A marked reduction is seen in cerebroside, cholesterol, and phospholipids. The cholesterol:phospholipid:cerebroside ratio is similar to the ratio in brain prior to myelination. Gangliosides are found in normal concentrations. No or only a very small amount of cholesterol esters are present. The protein composition of white matter is also altered with absence of proteolipid protein and reduction of other myelin proteins.

In contrast to white matter, the chemical composition of gray matter is much closer to normal. The concentration of sulfatides and cerebrosides is decreased, but the concentration of phospholipids, cholesterol, and gangliosides is normal or close to normal.

34.4 Pathogenetic Considerations

For many decades the pathogenesis of the lack of myelin in the CNS in PMD has been a matter of debate. The original contention of Merzbacher was that the lack of myelin sheaths was due to faulty or absent myelination. Subsequently, many authors have classified PMD among the leukodystrophies and described the histopathological findings as tigroid demyelination. However, several histological and chemical findings are not consistent with demyelination, but are consistent with a defect in myelin depo- sition. Histological examination fails to reveal signs of active demyelination. The small amounts of myelin degradation products are in conformity with at best a

very slow breakdown of myelin. Oligodendrocytes are found to be decreased in number, show morphologi- cal abnormalities, and appear inactive. The chemical findings of no or at most a low level of cholesterol esters in the white matter is not in agreement with active demyelination. The cholesterol:phospholipid:

cerebroside ratio in PMD corresponds with the ratio in the brain prior to myelination. The low ratio of ethanolamine phosphoglycerides to choline phosphoglycerides in PMD is an indication of a poor state of maturation of the brain, as the ratio increases with maturation. The high glucose content of glycolipids is also an indication of the immature state of the brain;

as maturation proceeds, glucolipids are replaced by galactolipids. The topography of the myelin present is in conformity with an arrest of myelination, which apparently occurs before birth (connatal form) or within the first year of life (classical and transitional form). So, the severity of the clinical phenotype seems to be related to the degree of myelin deficiency.

PMD caused by a defect in the gene coding for pro- teolipid protein (PLP). The PLP gene is localized on the long arm of the X chromosome (Xq21.33–Xq22).

It codes for PLP as its major gene product and addi- tionally for DM 20 due to alternate splicing of mRNA.

DM 20 is identical to PLP but 35 amino acids shorter.

The oligodendrocyte is the predominant cell in which the PLP gene is expressed in the CNS. PLP and DM 20 are produced in the endoplasmic reticulum and routed through the Golgi to the plasma membrane. PLP is a major myelin membrane protein.

Myelin basic protein and PLP normally constitute more than 80% of the total CNS myelin proteins.

Myelin basic protein accounts for 30–40% of the total myelin protein, PLP for 40–50%. PLP and its less abundant isoform DM 20 are strongly hydrophobic transmembrane proteins. They are myelin-specific and almost entirely confined to the CNS. They are, however, minor constituents of the PNS and compose less than 1% of the mass of PNS myelin proteins. The compact lamellar structure of myelin is organized and stabilized by the two main myelin proteins, myelin basic protein as a peripheral membrane protein, and PLP as a strongly hydrophobic integral membrane protein. Myelin basic protein contributes to the compaction of the major dense lines and PLP to the tight apposition of the intraperiod lines in the myelin sheath. DM 20 is the predominant isoform in oligodendrocyte progenitors and is expressed before PLP in the developing brain. DM 20 has biological functions in the maturing CNS that are distinct from the role of PLP.

Although PLP and DM 20 have been studied exten- sively, their biological functions are still not known in detail. Mice in which PLP gene expression has been inactivated demonstrate only subtle defects in the ultrastructure of CNS myelin, demonstrating that

Chapter 34 Pelizaeus–Merzbacher Disease and X-linked Spastic Paraplegia Type 2 274

(4)

neither PLP nor DM 20 is necessary for normal myelin assembly. These mice develop widespread wallerian degeneration of CNS axons, demonstrating that PLP expression in oligodendrocytes is necessary for the maintenance of normal axonal integrity, probably through oligodendrocyte–axonal interactions.

Many different mutations in the PLP gene related to PMD have been identified. Gene duplications are the most common cause of PMD and account for over 60% of the mutations. Furthermore, missense mutations, insertions, deletions, and nonsense mutations have been found. Point mutations do not only occur in the coding regions of the gene, but may also affect splice sites and noncoding regions of the gene. The severity of the clinical phenotype is related to the na- ture of the mutations. The most benign course of PMD is seen in PLP gene deletions or null mutations, whereas most missense mutations lead to a serious disease with neonatal or prenatal onset. Gene duplications lead to a disease of intermediate severity (usually the classical phenotype). The disease severity in duplications is proportional to the degree of overex- pression of the PLP gene. In some mutations only PLP is altered whereas DM 20 is produced correctly, resulting in the more benign phenotype of SPG2. SPG2 may also be seen in PLP null mutations or point mutations in some nonconserved regions of the PLP gene.

The mode of action of changes in or absence of PLP has not yet been fully elucidated. There is exper- imental support for the concept that missense mutations cause conformational changes of the protein,

“misfoldings” that prevent proper processing of PLP and DM 20 after biosynthesis in the endoplasmic reticulum. The accumulation of the mutated proteins in the endoplasmic reticulum of oligodendrocytes leads to the so-called unfolded protein response, which sets into motion an apoptotic cascade. The greater the accumulation of mutated proteins, the more intense the unfolded protein response and the higher the likelihood of apoptosis of oligodendrocytes. Oligodendroglial cell death leads to hypomyeli- nation. In mutations in which PLP is mutated but nor- mal DM 20 is produced, the trafficking of mutated PLP to the cell surface is disrupted, but the trafficking of DM 20 is normal, a situation that leads to milder disease. Much more myelin is produced than in classical PMD, but the myelin is less stable. Myelin instability and loss may contribute to the clinical phenotype.

In gene duplications, the excessive biosynthesis of PLP and DM 20 has deleterious effects on oligodendroglia, leading to CNS hypomyelination and subsequent demyelination, but there is no evidence of activation of the unfolded protein response. Over- expression of the PLP gene leads to arrested maturation and death of oligodendrocytes. Overexpressed PLP is routed to late endosomes/lysosomes and caus- es sequestration of cholesterol in these compart-

ments. Oligodendroglia may be particularly sensi- tive to an imbalance in the synthesis and turnover of myelin components due to their high rate of myelin synthesis. Considering the premature death of oligodendrocytes in PLP gene duplications, it is likely that a death program is triggered early in the disease course.

Gene deletions and null mutations do not lead to accumulation of mutated protein in the endoplasmic reticulum, do not cause increased oligodendrocyte cell death and arrest of myelination, and lead to a mild phenotype. It is intriguing that in these conditions the formation of compact myelin can proceed, while PLP and DM 20 are absent. In null mutations axonal abnormalities with wallerian degeneration are found in the CNS. This suggests that PLP has a role in glial–axon communication and is somehow necessary for axonal maintenance. In patients with absent PLP expression, a demyelinating peripheral neuro- pathy has been reported. It has been demonstrated that PLP but not DM 20 is necessary for peripheral nerve function, suggesting that the PLP-specific domain plays an important role in this respect.

The cause of the secondary neurological deterioration in PMD is insufficiently clarified. It is possible that the small amount of myelin present is unstable, leading to a very slow breakdown. In some patients histopathology provides evidence for axonal loss.

There seems to be an inverse relationship between initial clinical severity of the disease (and the severity of the myelin deficit) and axonal loss.Axonal loss is especially seen in patients with milder disease and more myelin, for instance in patients with a null mutation.

As a rule PMD affects hemizygous males. However, from the beginning the occurrence of PMD has also been noticed in girls and women. This phenomenon has been ascribed to highly unfortunate X inactivation. Some female patients may have transient neurological abnormalities of variable severity as children but gradually improve over a period of several years.

This functional restoration is attributed to ongoing myelination by oligodendrocytes, in which the healthy X chromosome is not inactivated. However, in most patients the occurrence cannot be explained by mechanisms related to X inactivation. It has become apparent that female patients are more common in families with milder forms of PMD or SPG2. Women heterozygous for PLP mutations are mosaic: one pop- ulation of oligodendrocytes expresses normal PLP and synthesizes normal myelin sheaths, whereas the other population expresses a mutant form of PLP. The lack of phenotype in most female PMD carriers can be attributed to the phenomenon that oligodendrocytes in which the mutant X chromosome is activated become apoptotic and are replaced by healthy oligodendrocytes. Most of the resulting population con-

(5)

sists of healthy oligodendrocytes at the peak of myelination. In contrast, in female carriers of a mild mutation the mutant oligodendrocytes survive and com- pete with normal oligodendrocytes. They produce structurally flawed myelin sheaths, which are suscep- tible to degradation. Because the affected females maintain a population of normal oligodendrocytes, they never display as marked a phenotype as male members of the same family.

34.5 Therapy

Apart from supportive care there is presently no effective therapy for PMD and SPG2. Transplantation of myelin forming cells is a promising strategy.

34.6 Magnetic Resonance Imaging

CT scanning is of little help in the diagnosis of PMD, showing only atrophy.

In contrast, the MRI pattern in PMD is usually highly suggestive of the disorder. In most cases, MRI shows an arrest of myelination in a stage that is in itself normal (Figs. 34.1 and 34.2). There is a correla- tion between the amount of myelin present and the clinical severity of the disease. In some cases of connatal PMD, no myelin at all is seen. The T2-weighted images show a high signal intensity of all unmyelinated white matter structures, whereas these structures have a low signal intensity on T1-weighted images. In fact, no high signal intensity areas are seen on T1- weighted images in these cases. In cases of classical PMD myelin is present in (parts of) the brain stem, (parts of) the cerebellar white matter, (parts of) the posterior limb of the internal capsule, the thalamus, and the globus pallidus. Often the pyramidal tracts in the brain stem lack myelin while the brain stem is otherwise better myelinated. In some cases additional myelin is present in the directly periventricular part of the corona radiata, in the subcortical white matter and cortex of the pre- and postcentral gyri, and in the directly periventricular part of the optic radiation. The myelinated structures have a low signal intensity on T2-weighted images and a high signal intensity on T1-weighted images. The pattern described is a normal stage of myelination for a neonate or an infant in the first few months of life, but not normal for the age of the patient. In addition, the cerebral white matter is variably but often markedly reduced in volume with a mild enlargement of the ventricular system, a thin corpus callosum, folding of the cortex in thin, deep gyri, and enlargement of the subarach- noid spaces (Fig. 34.1). The appearance of the white matter is often not completely identical to normal unmyelinated white matter, but may be somewhat

speckled, possibly reflecting the presence of some myelin in a tigroid pattern. Atrophy of brain stem and cerebellum may be striking. If MRI is performed dur- ing the first few months of life, the images are not di- agnostic as they merely show some atrophy and delay of myelination or may even be near-normal in appearance, but repeated MRI confirms the absence of progress of myelination. The described pattern of myelin deficiency in a boy who is a few years old is highly suggestive of PMD. Proton MRS of the cerebral white matter may give normal results. In some pa- tients an increased concentration of N-acetylaspar- tate is found, probably related to denser axonal pack- ing in the absence of normal amounts of myelin.

Arrest of myelination may be seen in other conditions, such as severe asphyxia or late congenital infec- tions, but as a rule in these conditions additional focal brain lesions are present which are lacking in PMD. Most conditions other than PMD, having an adverse effect on myelination, lead to delayed but slowly progressive myelination, with advancement of the degree of myelination on each repeat MRI if made after a sufficiently long interval. Hence, repeated MRI is of help in establishing the diagnosis of PMD. Seri- ous and permanent hypomyelination is also present in some DNA repair disorders and sialic acid storage disorders. These can be ruled out on the basis of clinical findings and appropriate biochemical tests. In addition, patients with Cockayne syndrome typically have calcium deposits within the basal ganglia, a phenomenon lacking in PMD.

In cases of mild PMD, related to gene deletions or null mutations, more myelin is present in the cerebral hemispheres and corpus callosum (Figs. 34.3 and 34.4). The pattern of myelination may be patchy and myelination may involve the subcortical areas in particular. In a family with multiple males with mild PMD related to a null mutation (initiation codon mutation) we found considerable although incom- plete initial myelination of the cerebral hemispheres (Fig. 34.3), with subsequent cerebral atrophy and diffuse loss of myelin in a way seen in primary neuro- degenerative disorders, suggesting underlying axonal degeneration and secondary loss of myelin (Fig. 34.4).

In MRS of one of these patients a strikingly decreased level of N-acetylaspartate was found within the cere- bral white matter, in agreement with axonal degener- ation. Decreases in N-acetylaspartate within the cerebral white matter have also been found in other patients lacking PLP.

Some female carriers of PMD have been shown to have multiple foci of increased signal intensity in the cerebral white matter, but others have not. MRI is not suitable as a tool for carrier identification. In some more seriously affected females, an MRI pattern of profound myelin deficit may be seen, similar to the pattern observed in male patients.

(6)

In SPG2 MRI abnormalities vary. Diffuse serious white matter changes may be seen as in males with classical PMD, with a high signal of the white matter on T2-weighted images and a low signal on T1-weighted images. In some cases the MRI is suggestive of diffuse or patchy mild hypomyelination with a mildly el- evated signal on T2-weighted images, but also a high

signal on T1-weighted images. In other patients widespread or more limited focal lesions are seen within otherwise well myelinated white matter. Proton MRS of affected white matter in SPG2 patients has demon- strated decreased levels of N-acetylaspartate, indica- tive of axonal damage or loss.

Fig. 34.1. Boy, 29 months old, with classical PMD. The T1- weighted images (third row) show presence of myelin in the central areas. On the T2-weighted images as well, some myelin is seen in the cerebellum and brain stem.The pattern of myelin

presence is consistent with arrest of myelination soon after birth. There is some cerebral atrophy. Courtesy of Dr. J.J.M. van Collenburg, Zwolle, The Netherlands

(7)

Fig. 34.2. A 6-year-old boy with classical PMD. The T1-weighted images reveal some myelin in the central white matter (third row), but the T2-weighted images show that the white

matter has a high signal intensity throughout. The brain stem and cerebellar white matter contain more myelin,but less than normal. It is striking that this patient has no atrophy

(8)

Fig. 34.3. This 2.5-year-old boy has a relatively mild form of PMD and an initiation codon mutation (Sistermans et al. 1996).

The T1-weighted images (third row) suggest an advanced stage of myelination, but the T2-weighted images demonstrate that myelination is far from complete. Most cerebral

hemispheric white matter has a high signal; some deep white matter in the parieto-occipital region has a low signal.The corpus callosum and brain stem also have a low signal intensity on the T2-weighted images

(9)

Fig. 34.4. The same boy as in Fig. 34.3, 8 years later. He has lost myelin. The corpus callosum and deep parieto-occipital white matter now have a high signal on the T2-weighted images.

The T1-weighted images (third row) show a loss of contrast

between white and gray matter, indicative of diffuse myelin loss. Compared to 8 years ago, there is some diffuse cerebral atrophy