Giant axonal neuropathy (GAN) - Electrodiagnostic features and differential

M. Mçller

3.3 Electrodiagnostic features and differential

3.3.11 Giant axonal neuropathy (GAN)

Giant axonal neuropathy (GAN) is a rare disease affecting both PNS and CNS. It is traditionally listed and reviewed together with the hereditary neuropathies. The prognosis is poor. Most patients die before the age of 30 years. The lower extremity SNAPs are typically absent and the upper ex-tremity SNAPs are reduced in amplitude. The NCVs and distal latencies are normal or only mildly affected. The data concerning needle electro-myography is poor, but spontaneous activity, high amplitude, polyphasic MUAPs and reduced MUAP recruitment have been described [5].

References

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40 z M. Mçller: 3 Electrodiagnostic evaluation of hereditarypolyneuropathies

Introduction

Pathologic examination of peripheral nerves was the only valuable diagnostic tool for hereditary neuropathies, until molecular genetic diagnosis became available. In fact, after the original description of a duplication of chromo-some 17p11.2, containing the gene coding for peripheral myelin protein 22 (PMP22), in Charcot-Marie-Tooth (CMT) neuropathy type 1A [59, 76], sev-eral other genes involved in the pathogenesis of the different types of CMT and of other inherited neuropathies have been discovered. The diagnosis of hereditary neuropathies now mostly depends upon the search for mutations in all the known genes. The diagnostic biopsy of a peripheral nerve is, there-fore, limited to selected cases of unknown origin, in spite of a wide search for mutations in the most commonly involved genes. Typically, pathologic exam-ination of peripheral nerves is reserved for sporadic cases where an inherited neuropathy is strongly suspected, but acquired neuropathies like chronic in-flammatory demyelinating polyneuropathy (CIDP) need to be excluded.

In general terms, a peripheral nerve has to be sampled in carefully se-lected patients only, because of the potential neurologic deficits following a nerve biopsy. Moreover, the diagnostic value of a nerve biopsy is influ-enced by:

z the small amount of tissue removed;

z the fact that very few nerves in the body are suitable for sampling;

z the availability of a specialized laboratory to process and analyze the tis-z sue;the need of relating histologic measures with physiologic, biochemical

and pharmacological results [26].

The nerve to biopsy has to be accurately chosen. It should be a cutaneous nerve affected by the neuropathic process, easily accessible to neurophysio-logic studies prior to biopsy, constant and superficial in location. The sural nerve meets all these criteria [26]. Alternatively, the radial cutaneous nerve in the arms and the superficial peroneal nerve in the legs may be selected [78]. The sural nerve is a sensory nerve and contains, in the endoneurial space, the axonal processes of neurons located in the dorsal root ganglia,

and nerve biopsy

A. Schenone

surrounded by myelinating and non-myelinating Schwann cells, and by fi-broblasts (Fig. 4.1). Present in the epineurium, perineurium and endoneu-rium are also several blood vessels, whose examination may be useful in case of a suspected vasculitis. Since hereditary neuropathies are normally diffuse processes, two or three nerve fascicles (fascicular biopsy) may be sufficient to provide enough tissue to perform a pathological examination.

However, a careful comparison between fascicular and whole sural nerve biopsies showed no differences in residual deficits between the two surgical procedures [73]. Moreover, when hereditary disorders like familial amyloid polyneuropathy (FAP) are suspected, a whole sural nerve biopsy is neces-sary to detect the amyloidotic deposits.

Postoperative symptoms after sural nerve biopsy are usually mild and include sensory deficit, dysesthesia and pain in the cutaneous distribution of the nerve. Among patients with neuropathy, 93% reported sensory symptoms however, only 33% had mild persistent pain and 19% persistent dysesthesia [34]. Over time, dysesthesia tends to improve and pain to com-pletely subside [34]. The sural nerve is removed under sterile conditions in the operating room by a specialized surgical team. After local anesthesia, the nerve is exposed at the distal calf level, where it lies adjacent to the sa-phenous vein, which is the optimal anatomical landmark for locating the sural nerve. After removal, the nerve is processed for the morphological studies. Normally, the specimen is divided into three segments, each one 2±2.5cm long, which are suspended in glutaraldehyde 2.5%, paraformalde-hyde 4% and frozen in liquid nitrogen, respectively. After fixation, speci-mens are embedded in paraffin or in epoxy resin for examination at the light and electron microscopic levels. One segment of the nerve, fixed in glutaraldehyde and osmium tetroxide, is used for teased fiber preparations z A. Schenone

Fig. 4.1. Normal sural nerve. Semithin section showing a normal number of myelinated and unmyelinated fibers. The distribution of myelinated fibers is typically bimodal. Toluidine blue;

bar: 15 lm

(Fig. 4.2). The examination of teased fibers allows the study of a single myelinated fiber over a distance of 3±15internodes and is useful to identify the incidence of fibers undergoing Wallerian degeneration and segmental demyelination or remyelination. Sometimes specific changes, such as the sausage-like thickenings of the myelin sheath, which are typical of the

he-Fig. 4.2. Normal sural nerve. Teased fiber preparation. Several normallymyelinated internodes and nodes of Ranvier (arrows) maybe seen along a single fiber (A through D). O.T. 2%

Fig. 4.3. Normal human sural nerve. A normal myelinated fiber and several unmyelinated fibers (arrows) are present. Electron microscopy. Lead citrate and uranyl acetate; bar: 0.5 lm

reditary neuropathy with liability to pressure palsy (HNPP), are easier to detect in teased fibers than in paraffin- or in epoxy-embedded sections.

Electron microscopic examination (Fig. 4.3) is useful when the study of un-myelinated fibers or a search for specific inclusions in Schwann cells, as observed in hereditary storage disorders, are necessary. Immunohistochem-ical techniques may be performed, mainly in acquired neuropathies, to de-tect and characterize inflammatory infiltrates. On paraffin embedded sec-tions, specific stains, like Congo red or immunolabeling with antitransthy-retin or anti-light chain antibodies are useful to detect endoneurial amy-loid deposits and to distinguish between FAP and neuropathy due to a monoclonal gammopathy.

Morphometry of sural nerve biopsies can be used to determine the num-ber, density, diameter distribution and shape of myelinated and unmyeli-nated fibers. In the past, morphometric studies were helpful in correlating morphological and electrophysiological findings in hereditary motor and sensory neuropathies (HMSN) [42] and to separate CMT1 (also called HMSN 1) from Djerine-Sottas syndrome (DSS, also called HMSN 3) [70].

The ratio of axon diameter to fiber diameter (g-ratio) is a useful measure-ment to grade hypomyelination and to detect signs of axonal atrophy in sural nerve biopsies. In case of hypomyelination the g-ratio is higher (> 0.75), whereas axonal atrophy is suspected when g-ratios are <0.4.

4.1 Charcot-Marie-Tooth disease type 1 (CMT1)

The neuropathological phenotype of patients affected by CMT1 is charac-terized by diffuse demyelinating changes. This finding is in agreement with the neurophysiological studies showing reduced motor and sensory nerve conduction velocities (<38 m/s).

4.1.1 Charcot-Marie-Tooth disease type 1A (CMT1A)

Sural nerve biopsies of CMT1A patients, due to the 17p11.2 duplication, show rather stereotyped abnormalities. There is a reduction in myelinated fiber density, ranging from moderate to severe [44]. Unmyelinated fibers may be normal or slightly affected [44]. Compared to normal nerves, there is an increase in transverse fascicular area, which may also be demon-strated by non-invasive ultrasonographic techniques (Fig. 4.4) [64]. Onion bulbs, made up of thinly or near normally myelinated axons surrounded by concentric layers of Schwann cell cytoplasm, are frequently present in sural nerves of CMT1A patients (Fig. 4.5). Up to 85% of myelinated fibers may consist of onion bulbs [44]. Some onion bulbs may contain several myelinated or unmyelinated axons that are believed to be nerve sprouts.

The occasional presence of large axons devoid of a myelin sheath, some-z A. Schenone

times surrounded by onion bulb formation, confirms the demyelinating na-ture of the pathological process. This is even more evident in the teased fi-ber preparation, where most of the remaining fifi-bers show segmental de-myelination and/or rede-myelination (Fig. 4.6).

Morphometric studies show that larger fibers are more affected than smaller ones. The mean internodal length is lower than in normal nerves, meaning that remyelination occurs in nerve fibers of patients affected by Fig. 4.4. Ultrasonographyof median nerve in a CMT1A patient. With high frequencytransdu-cers, peripheral nerves show a typical fascicular structure in transverse sections. In CMT1A (A) the nerve and the fascicles (arrows) appear enlarged compared to a normal control (B)

Fig. 4.5. Sural nerve biopsyfrom a CMT1A patient. At lower magnification (A) the densityof the myelinated fibers appears to be decreased from normal. At higher magnification (B) onion bulbs (arrows), made up of concentric proliferation of Schwann cell cytoplasm around normally or thinlymyelinated fibers, can be seen. Semithin section, toluidine blue; bars: 10 lm

CMT1A. The observation of myelinated fibers with a reduced axon diame-ter as compared to myelin thickness suggests the presence of an associated axonal impairment [28]. The relevance of axonal atrophy in CMT1A nerves has been confirmed in xenografts of sural nerves from CMT1A patients into nude mice sciatic nerves [77].

CMT1A phenotypes caused by mutations in the PMP22 gene are rare. In these cases the neuropathological picture may be more severe than in typi-cal duplication cases, but specific changes in sural nerve biopsies have not been detected.

Sometimes, inflammatory changes may be observed in sural nerve biop-sies of CMT1A patients [38, 99]. This observation suggests the possibility of a genetic susceptibility to immune mediated demyelination in certain CMT families and explains previous reports on steroid-responsive inherited neuropathies [28].

4.1.2 Charcot-Marie-Tooth disease type 1B (CMT1B)

CMT1B, caused by point mutations in the MPZ gene, has a more severe clinical and neuropathological phenotype than CMT1A. In typical cases, the sural nerve biopsy shows a variable loss of myelinated fibers, onion bulbs made up of concentric layers of Schwann cell cytoplasm surrounding thinly myelinated fibers and segmental demyelination in teased fibers [44].

Focal thickenings of the myelin sheath have been observed in CMT1B pa-tients (Fig. 4.7) [31, 36]. Although not specific, focally folded myelin seems to be particularly frequent in a subgroup of patients harboring mutations in the extracellular domain of MPZ [31]. Focally folded myelin is also a hallmark of some very rare forms of CMT4. In other families, ultrastruc-z A. Schenone

Fig. 4.6. Sural nerve biopsyfrom a CMT1A patient. Teased fiber preparation. Several internodes lacking of myelin (demyelinated internodes) can be seen along a single fiber (A through F).

O.T. 2%

tural examination shows uncompacted myelin in several fibers (Fig. 4.8), thus, suggesting two divergent neuropathological phenotypes in CMT1B, the first dominated by myelin thickenings and the second by loosening of myelin lamellae [36].

Morphometric studies show, like in CMT1A, a preferential loss of larger fibers. Total transverse fascicular area is not as enlarged as in typical CMT1A.

Most of the remaining fibers, outside of the myelin thickenings, have high g-ratios, thus, confirming the severity of the demyelinating process [36].

Fig. 4.7. Sural nerve biopsyfrom a CMT1B patient. The densityof myelinated fibers is from moderatelyto severelydecreased from normal. Some fibers show focal thickenings of the mye-lin sheath (arrow). Toluidine blue; bar: 10 lm

Fig. 4.8. Sural nerve biopsyfrom a CMT1B patient. Electron microscopyshows, in some thinly myelinated fibers, uncompaction of myelin lamellae (arrows). Bar: 0.1 lm

4.1.3 Charcot-Marie-Tooth disease type 1C (CMT1C)

CMT1C, due to mutations in the LITAF/SIMPLE gene, shares clinical and electrophysiological features with typical CMT1 [91]. The neuropathologi-cal phenotype is also suggestive of a hypertrophic demyelinating neuropa-thy. In particular, myelin loss and onion bulbs similar to those observed in CMT1A have been reported in a family with CMT1C [92].

4.1.4 Charcot-Marie-Tooth disease type 1D (CMT1D)

Mutations in the EGR2 zinc-finger transcription factor have been demon-strated in a few cases of CMT1, DSS and congenital hypomyelination (CH) phenotypes [100]. Patients with CMT1D show a typical CMT1 phenotype [81, 96] and display neuropathological changes ranging from severe to rel-atively mild fibers loss and demyelination. Onion bulbs, although present, are not as prominent as in CMT1A (Fig. 4.9). Fibers showing a reduced axon diameter compared to myelin thickness have also been observed, in a CMT1D family, and may suggest an associated axonal atrophy [96].

4.1.5 Djerine-Sottas syndrome (DSS)

DSS may be considered, from the neuropathological point of view, as a severe variant of demyelinating CMT. Originally, the distinction between DSS (HMSN III) and CMT1 (HMSN I) was based on the presence of a significantly lower density of myelinated fibers > 8 lm in diameter, a greater frequency of z A. Schenone

Fig. 4.9. Sural nerve biopsyfrom a CMT1D patient. The densityof myelinated fibers is moder-atelydecreased from normal. Several onion bulbs (arrows) maybe seen, sometimes surrounding thinlymyelinated fibers. Semithin section, toluidine blue; bar: 10 lm

onion bulbs and higher g-ratios in DSS [70]. Molecular genetic studies made this distinction less important. In fact, mutations in the PMP22, MPZ, EGR2 and PRX genes may cause either a CMT1 or a DSS phenotype [11, 12, 45, 71, 96]. Sural nerve biopsies have been performed in most of these cases, but genotype-phenotype correlations based on neuropathological features are not possible, because specific changes for each genotype have not been ob-served. Patients with DSS show a diffuse loss of myelinated fibers (Fig. 4.10). Unmyelinated fiber density is also, but less severely, decreased.

Onion bulbs are made up of multiple layers of Schwann cell cytoplasm with or without a central axon (denervated onion bulbs). Several larger axons de-void of myelin may also be observed. Hypertrophy of peripheral nerves is a common feature and enlargement of nerve roots has been found in a family with a MPZ point mutation [89]. Focally folded myelin, especially at para-nodes, has been observed in a patient with mutations in the PRX gene [94].

4.1.6 Congenital hypomyelination (CH)

CH is characterized by a defect in myelination, probably due to a primary failure in myelin formation. It may be clinically undistinguishable from se-vere, early onset CMT1 and DSS. Sural nerve biopsy (Fig. 4.11), although not diagnostic, shows a severe loss of myelinated fibers with all the resid-ual axons lacking a myelin sheath or surrounded by very thin myelin and onion bulbs made up of multiple laminae of double layered Schwann cell basement membranes (basal lamina onion bulbs). However, most authors considered CH as a more severe variant of DSS. Molecular genetic studies confirmed that mutations in the same genes known to cause DSS or severe Fig. 4.10. Sural nerve biopsyfrom a DSS patient. The densityof myelinated fibers is severely decreased from normal. Onion bulbs formed bySchwann cell processes wrapping around thinly myelinated axons may be seen (arrow). Semithin section, toluidine blue; bar: 5 lm

CMT1, like PMP22, MPZ and EGR2, can be found in CH families [32, 62, 100]. Neuropathological genotype-phenotype correlations are virtually im-possible in all these cases.

4.1.7 Hereditary neuropathy with liability to pressure palsy (HNPP) HNPP is clinically characterized by recurrent episodes of peripheral nerve palsies, due to mechanical compression of the nerve trunks. Various patho-logic changes have been described in association with HNPP. However, the most common abnormality, in sural nerve biopsies, is constituted by focal thickenings of the myelin sheath, named tomacula by the sausage-like ap-pearance they give to the myelinated fiber in longitudinal sections or in teased fiber preparations (Fig. 4.12). Typical tomacula have been described as focal enlargements of the myelin sheath, in the internodal or paranodal segment of the fiber, which are between 40 and 250 lm long (Fig. 4.13) [101]. In transverse sections, tomacula are characterized by an extremely thickened myelin sheath wrapping around an axon of reduced diameter. It is not known whether this is due to a constriction of the axon by the thickened myelin or to an ongoing axonal atrophy. However, the observa-tions that neurofilament density is increased in the narrowed area of the z A. Schenone

Fig. 4.11. Sural nerve biopsyfrom a patient with a CH phenotype and a mutation in the P0 gene. The densityof myelinated fibers is severelydecreased. Several axons are either demyeli-nated (thick arrows) or surrounded byan extremelythin myelin sheath (thin arrows). Toluidine blue; bar: 10 lm

axon [101], lower g-ratios correlate to the levels of PMP22 mRNA in HNPP patients [82] and that xenografts of HNPP nerves in mice sciatic nerves re-sult in axonal atrophy [77] suggest that axonal impairment does occur in HNPP. Rarely, tomacula may be absent in transverse semithin sections of HNPP sural nerves. Therefore, the analysis of longitudinal sections and teased fiber preparations is needed to identify these peculiar myelin abnor-malities. Sometimes, focal thickenings of the myelin sheath have been ob-served in other hereditary neuropathies, particularly those due to P0 muta-tions [31]. However, in the author's experience, typical tomacula, as de-fined above, may be observed only in HNPP sural nerves.

The density of myelinated fibers may be normal or slightly reduced from normal. Onion bulbs are usually absent or observed occasionally. Rarely, HNPP sural nerves show a severe reduction of myelinated fibers and a large number of onion bulbs. These patients, although carrying the 17p11.2 deletion, develop a clinical and neurophysiological phenotype undistin-guishable from CMT1 [61].

The teased fiber preparation shows a variable degree of segmental de-myelination and rede-myelination. This corresponds to the electrophysiologic observation of slowing of conduction velocity in clinically unaffected Fig. 4.12. Sural nerve biopsyfrom a HNPP patient. The densityof myelinated fibers is within normal limits. Several focal thickenings of the myelin sheath (tomacula) (arrows) may be seen.

Toluidine blue; bar: 10 lm

Fig. 4.13. Sural nerve biopsyfrom an HNPP patient. Teased fiber preparation. Several thicken-ings of the myelin sheath (tomacula) may be seen along the fibers. Sometimes tomacula are up to 250 lm long (arrows). O.T. 2%

nerves [101]. At the electron microscopic level, tomacula look like redun-dant loops of myelin with irregularly folded lamellae, which are also enor-mously increased in number.

Madrid and Bradley identified several mechanisms by which focal thick-enings are formed, several years before the discovery of the genetic ab-normality underlying HNPP [60]. These include branching and duplication of the mesaxon (the Schwann cell membrane adjacent to the axon), trans-nodal myelination, the participation of two or more Schwann cells in the formation of the myelin sheath, and the degeneration of myelin in the adaxonal or intramyelin regions between an intact outer layer of the myelin sheath and the axon. The knowledge that a reduced dosage of the PMP22 gene is present in HNPP patients [83, 97] is not sufficient to understand the mechanisms of tomacula formation and the relationship between these structural changes and mechanic compression. However, it is possible that low levels of PMP22 affect the structural integrity of the myelin sheath and make it more susceptible to damage from external trauma [88].

4.2 Charcot-Marie-Tooth disease type 4(CMT4)

The autosomal recessive forms of CMT1 are traditionally called CMT4. The different forms of CMT4 are extremely rare. They are neuropathies character-ized by a neuropathological phenotype, which in some cases, like CMT4B1, is characterized by extremely peculiar changes.

4.2.1 Charcot-Marie-Tooth disease type 4A (CMT4A)

CMT4A is caused by mutations in the ganglioside-induced differentiation-associated protein-1 gene (GDAP1). Neuropathological findings in CMT4A range from an axonal to a demyelinating phenotype [7, 20]. Irrespective of the prevalence of axonal or demyelinating changes, a severe loss of myeli-nated fibers, especially affecting the larger fibers has been found in all sur-al nerve biopsies of CMT4A patients. Onion bulbs have been described, but in most cases they are scattered and atypical, since they enclose regenerat-ing axons and show only few concentric layers of Schwann cell cytoplasm [10, 87]. Demyelinated axons and abundant onion bulbs have been ob-served in only a few affected individuals [7]. In most families, neither de-myelinated axons nor abnormalities in myelin compaction could be found [10, 67, 87]. However, some cases do show signs of de-remyelination along with axonal changes [67]. Morphometric studies confirm the morphologi-cal observations and show that larger fibers (>8 lm) are always lacking, whereas there are more smaller fibers (<3 lm) [67, 87]. These observations may suggest that axonal regeneration is a prominent feature of CMT4A.

z A. Schenone 52

The g-ratio ranges from normal (0.7) to low values (<0.4), suggesting that an axonal atrophy is present in some fibers [10, 87].

In agreement with these observations, we found in sural nerves of patients carrying a M116R mutation at exon 4 of the GDAP1 gene [E. Di Maria, per-sonal communication] a loss of myelinated axons, especially affecting larger fibers. Several of the remaining fibers showed a reduction in myelin thick-ness. Sometimes, thinly myelinated axons surrounded by Schwann cell pro-cesses concentrically organized to form small onion bulbs are also observed (Fig. 4.14). Occasionally, clusters of small regenerating fibers may be found.

An association of axonal and demyelinating features has also been recently reported in other CMT4A families [85]. Therefore, although the number of mutations in the GDAP1 gene described so far is too low for reliable geno-type-phenotype correlations, it is possible that CMT4A, similarly to CMTX, combines axonal and demyelinating changes. This may be due to a combined, negative effect of GDAP1 mutations on neurons and Schwann cells [93]. How-ever, further studies on the function of this protein are needed to explain these mixed changes.

4.2.2 Charcot-Marie-Tooth disease type 4B1 and 4B2 (CMT4B1, CMT4B2)

Mutations in different members of the myotubularin-related gene family cause CMT4B [5, 13]. In particular, CMT4B1 is due to mutations in the myotubularin related protein-2 (MTMR2) [13] and CMT4B2 to mutations in the MTMR13 gene [5]. However, sural nerve biopsy findings do not dif-Fig. 4.14. Sural nerve biopsyfrom a CMT4A patient. The densityof myelinated fibers is de-creased from normal. Several thinlymyelinated fibers maybe seen, occasionallysurrounded by concentric layers of the Schwann cell cytoplasm forming a small onion bulb (arrow). Semithin section, toluidine blue; bar: 10 lm

z A. Schenone 54

Fig. 4.15. Sural nerve biopsyfrom a CMT4B patient. The densityof myelinated fibers is se-verelydecreased from normal. The majorityof fibers show the typical abnormalities of the myelin sheath, which is redundant and irregularly folded (arrows). Semithin section, toluidine blue; bar: 10 lm

Fig. 4.16. Sural nerve biopsyfrom a CMT4B patient. Electron microscopyshows complex out-foldings of the myelin sheath surrounding the axon (asterisk). Bar: 1 lm

fer between the two types of CMT4B and are characterized by typical ab-normalities of the myelin sheath, which is redundant and irregularly folded (Figs. 4.15, 4.16). These changes are called myelin outfoldings. Myelin out-foldings were first related to autosomal recessive hereditary motor and sen-sory neuropathies and described in detail by Onishi et al. in 1989 [69]. In CMT4B up to 90% of the fibers presents myelin outfoldings, discriminating this disorder from all the other autosomal dominant or recessive forms of CMT [75, 80]. A profound loss of myelinated fibers is also observed. Occa-sionally, small onion bulbs may be seen [80].

4.2.3 Charcot-Marie-Tooth disease type 4C (CMT4C)

The neuropathological features of CMT4C, which has been recently related to mutations in a gene encoding a SH3/TPR domain protein [86], have been known since 1999, when the phenotypic characterization of an auto-somal recessive demyelinating CMT linked to chromosome 5q23-q33 was described [35]. The sural nerve biopsy shows a predominantly demyelinat-ing neuropathy with a severe loss of large myelinated fibers (>8 lm), ab-normally thin myelin sheaths in the remaining fibers and extensive onion bulbs formation [53]. Onion bulbs made up of concentric, basal membrane layers surrounding a demyelinated axon are frequently seen (basal lamina onion bulbs).

4.2.4 Charcot-Marie-Tooth disease type 4D (CMT4D)

Sural nerve biopsy findings, in CMT4D, also known as hereditary motor and sensory neuropathy of the Lom type (HMSN-L), are suggestive of a se-vere demyelinating neuropathic process [52]. In fact, a profound loss of myelinated fibers and prominent hypertrophic changes, characterized by frequent onion bulbs consisting of multiple layers of Schwann cell cyto-plasm and basal lamina around thinly myelinated or unmyelinated axons, have been described [17, 52, 54]. Interestingly, onion bulbs are more evi-dent in younger patients and seem to subside with aging [52]. Why there is a regression of the onion bulbs in CMT4D is unknown; however, it may be related to the axonal loss which is striking in this hereditary neuropathy [54]. With electron microscopy, various abnormalities both in the axons and in the Schwann cells of CMT4D patients have been shown, like axonal inclusions consisting of tubular or small curvilinear profiles, pleomorphic material in the adaxonal Schwann cell cytoplasm and uncompaction of the myelin lamellae adjacent to the axon [54]. However, none of these ultra-structural abnormalities seem to be specific of CMT4D.

4.2.5 Charcot-Marie-Tooth disease type 4E (CMT4E)

Point mutations in the EGR2 gene, besides causing autosomal dominant CMT1, DSS and CH phenotypes, may also be responsible for severe demye-linating autosomal recessive CMT, now classified as CMT4E [55]. However, the neuropathological phenotype does not allow clear differentiation of CMT4E from autosomal dominant CH. In fact, the sural nerve biopsy shows a diffuse loss of myelinated fibers with absence of myelin in vir-tually all the remaining fibers [100].

4.2.6 Charcot-Marie-Tooth disease type 4F (CMT4F)

CMT4F is due to mutations in the PRX gene [94]. PRX encodes two PDZ domain proteins, L- and S-periaxin, which are required for the maintenance of periph-eral nerve myelin. To date, most of the mutations responsible for an autosomal recessive demyelinating neuropathy affect L-periaxin and only one family shows a homozygous mutation in the region common to both forms and there-fore affecting both L- and S-periaxin [94]. However, the neuropathological findings do not allow a distinction between mutations of the L- or S-periaxin.

The sural nerve biopsies in patients carrying mutations in the PRX gene, even if the clinical phenotype may vary from a severe demyelinating sensory motor neuropathy to a milder, mainly sensory phenotype, show similar features. A severe loss of myelinated fibers accompanied by prominent onion bulb forma-tion is always present [12, 23, 40, 94]. Onion bulbs are made up of Schwann cell cytoplasm and basal lamina. Hypermyelination and tomacula are also fre-quently observed in CMT4F [40, 94]. The paranodal region shows incomplete myelination and separation of multiple terminal myelin loops from the axon, suggesting that PRX may play an important role in mediating Schwann cell-axon adhesion at the node of Ranvier [94]. Finally, it is important to underscore that the PRX null mice develop a neuropathic process which is strikingly similar to the human pathology, supporting the view that this animal model may lead to a better understanding of the human disease [37].

4.3 X-linked Charcot-Marie-Tooth disease (CMTX)

CMTX is in nearly all cases due to mutations in the GJB1 gene, coding for Cx32, a protein belonging to the family of gap junction proteins and highly expressed in the Schwann cells at the nodes of Ranvier and Schmidt-Lanter-man incisures. Cx32 mediates the transport of low molecular weight sub-stances from the adaxonal to the outer myelin lamellae [6]. This type of CMT is also known as CMTX1 [55]. In fact, neurophysiological studies reveal a slowing of conduction velocities intermediate between a demyelinating and an axonal CMT. However, given the peculiar localization and function of z A. Schenone

Cx32, the relationship between Schwann cells and axon should be particularly affected in CMTX1 patients and the compact myelin relatively preserved [88].

Sural nerve biopsies show that axonal features are predominant in CMTX1 patients [44]; however, concomitant demyelination has been de-scribed [41, 79, 84]. Myelinated fiber density is reduced, unmyelinated fi-bers are relatively well preserved and the density of small myelinated fifi-bers is less affected than that of larger ones, due to the frequency of axonal sprouting (Fig. 4.17) [84]. Onion bulbs, sometimes surrounding normally myelinated fibers, may be seen. Teased fiber preparations show an in-creased rate of remyelinating fibers, mainly secondary to axonal damage.

Rarely, fibers undergoing segmental demyelination have been described [44, 79]. Due to the pattern of inheritance, males are normally more af-fected than females and this is also evident in sural nerve biopsies [41].

According to the distribution of Cx32, electron microscopy analysis reveals unusual findings at the nodal and paranodal regions. Widening of the Schmidt-Lanterman incisures and of the nodes of Ranvier have been de-scribed along with a separation of the myelin sheath from the axon leaving a clear periaxonal space that appeared either empty or contained vesicular material and whose significance is still unknown [41].

An extremely rare form of CMTX is linked to chromosome Xq24-q26.1 (CMTX2) [74]. In the original report this type of CMTX was described as an axonal HMSN II with additional clinical features (deafness and mental retardation) [18]. The sural nerve biopsy shows typical axonal changes, like a slight reduction of myelinated fibers density with axonal loss and sprout-ing, but no onion bulb formation or segmental demyelination.

Fig. 4.17. Sural nerve biopsyfrom a CMTX1 patient. The densityof myelinated fibers is moder-atelydecreased from normal. Several clusters of thinlymyelinated fibers (arrows) maybe seen.

Fibers with thin myelin sheath occasionally surrounded by concentric layers of Schwann cell cy-toplasm to form small onion bulbs (arrow heads) are occasionallypresent. Semithin section, tol-uidine blue; bar: 10 lm

Dalam dokumen HereditaryPeripheralNeuropathies G.Kuhlenbåumer z F.Stægbauer z E.B.Ringelstein z P.Young (Eds.) (Halaman 53-101)