| Abstract|| |
Peripheral neuropathy is one of the most common neurological conditions of the nervous system. Hereditary neuropathies (HNs) form an important group with varying degrees of severity, causing a significant disease burden. Accurate diagnosis is essential for management, counseling, and preventing unnecessary extended workups for acquired etiologies and inappropriate treatment. Several hereditary neuropathies have characteristic or diagnostic histologic findings; however, in the era of molecular diagnostics, the role of nerve biopsy in the diagnosis of hereditary neuropathy has reduced significantly. Nevertheless, in sporadic cases, cases without a clear family history, clinical mimics, cases with rare mutations, and genetic variants of unknown significance, a nerve biopsy can confirm the diagnosis, provide an unexpected diagnosis, or direct a targeted molecular testing. HN may be non-syndromic, affecting predominantly the peripheral nervous system or syndromic where it is a part of more widespread neurological or multisystem involvement. This review summarizes the microscopic pathological features in a nerve biopsy in some of the more commonly encountered inherited peripheral neuropathies highlighting their utility in selected cases.
Keywords: Genetics, hereditary neuropathy, nerve biopsy, pathology
|How to cite this article:|
Yasha T C, Sharma S, Gayathri N, Nandeesh N B. Hereditary neuropathies: Genetics and utility of nerve biopsy. Indian J Pathol Microbiol 2022;65, Suppl S1:318-28
|How to cite this URL:|
Yasha T C, Sharma S, Gayathri N, Nandeesh N B. Hereditary neuropathies: Genetics and utility of nerve biopsy. Indian J Pathol Microbiol [serial online] 2022 [cited 2022 May 24];65, Suppl S1:318-28. Available from: https://www.ijpmonline.org/text.asp?2022/65/5/318/345054
| Introduction|| |
The peripheral nervous system (PNS), one of the major components of the nervous system, along with ganglia act as a relay system between the central nervous system and the peripheral parts of the body. The peripheral neuropathies are an etiologically diverse group of diseases affecting peripheral nerves with an occurrence of 1%–2% in the general population and 7%–8% in the elderly population. These can be infectious, immune-mediated, metabolic, toxic, vascular, genetic, or idiopathic with overlapping clinical presentations. In selected cases a nerve biopsy is undertaken to determine the etiology, and this review highlights the relevant histological findings that aid in diagnosis.
Inherited peripheral neuropathies (IPNs) are slowly progressive motor and sensory disorders characterized by distal symmetrical weakness of the limbs, hypo/areflexia, and joint contractures and are more pronounced in lower limbs as compared to upper limbs. The estimated frequency of IPN is approximately 1 in 2500 individuals worldwide. Hospital-based audits from India suggest that hereditary neuropathies account for 4.8% of all cases. The Bangalore Urban–Rural Neuroepidemiology Survey from India showed that the crude prevalence rate of peripheral neuropathy in the general population is approximately 67 per 100,000. The disease commonly occurs in the first two decades of life but can affect all age groups. It causes progressive disability and impairs quality of life, particularly in social and emotional domains with an increased trend toward depressive symptoms.,
IPNs are heterogeneous and can be broadly grouped as non-syndromic and syndromic. In the non-syndromic group, disorders predominantly affect the peripheral nerves or may cause complex inherited neuropathy syndromes wherein neuropathy coexists with other widespread neurological features such as spasticity, ataxia, and impaired global neurodevelopment. Broadly, four entities are recognized: hereditary motor and sensory neuropathy (HMSN) also called Charcot–Marie–Tooth (CMT) disease, which involves both motor and sensory nerves; hereditary neuropathy with liability to pressure palsies (HNPP); hereditary sensory and autonomic neuropathy (HSAN) affecting sensory and autonomic nerve fibers; and distal hereditary motor neuropathy (dHMN) affecting only motor nerves [Table 1]. Deformities in the form of pes cavus and hammer toes can occur due to the long-standing nature of the disease. In the syndromic group, hereditary neuropathy is often a part of a more widespread neurological or multisystem disorder, for example, familial amyloid neuropathy (FAP), mitochondrial disorders, giant axonal neuropathy (GAN), metachromatic leukodystrophy, Krabbe disease, and spinocerebellar ataxia [Table 1]., Presence of family history is an important factor while considering inherited peripheral neuropathies. The inheritance pattern is autosomal dominant, autosomal recessive, X-linked, sporadic, or mitochondrial. In juvenile and adult patients, an acquired cause can manifest an underlying hereditary etiology, which in the absence of family history may be misdiagnosed. Conversely, in hereditary neuropathy, rapid worsening may suggest a superadded acquired cause. Neuropathies are broadly categorized based on electrophysiological studies as
|Table 1: Classification of hereditary neuropathies and the common pathogenic genes|
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- Myelinopathies (primarily affecting Schwann cells in PNS) with predominant involvement of peripheral myelin causing slowing nerve of conduction due to impaired saltatory conduction of nerve impulse. The median nerve conduction velocity is reduced (<35 m/s).
- Axonopathies (primarily affecting axons of peripheral neurons) with predominant involvement of peripheral nerve axons wherein the amplitude of compound muscle action potential is reduced with normal or slightly reduced median nerve conduction velocity (>45 m/s).
- Intermediate with a mixed pattern of nerve injury wherein median nerve conduction velocity ranges from 35 to 45 m/s).
Non-syndromic IPNs are categorized based on clinical features, inheritance pattern, and electrophysiology, with four major groups, namely HMSN (CMT), HNPP, HSAN, and dHMN.,,, HMSN is also called CMT disease, but the terminology is confusing at times as only HMSN 1 and 2 are synonymous with CMT 1 and 2, while the others are not. Following the identification of causative genes, the term “CMT” is now favored. [Table 1] gives the classification of hereditary neuropathies.
There has been a proposal to simplify the classification by including the pattern of inheritance, replacing numerals by “de”/”ax,” and inserting the implicated gene, for example, CMT1A would be AD-CMTde-PMP22dup or spo-CMT if sporadic.
With the advent of high-throughput sequencing technologies, more than 100 genes associated with different cellular functions have been discovered so far. Broadly, the genes identified for demyelinating neuropathies are associated with Schwann cells, while genes identified for axonal neuropathies are predominantly involved in the functions of neurons and axons. A classification based only on genes identified becomes difficult as variations in the same genes can manifest different clinical presentations vis-à-vis variations in that different genes express the same phenotype. The spectrum of pathogenic variants encompasses several important cellular structures and functions, such as myelin structure, cytoskeleton, protein synthesis, axonal transport, and mitochondrial dynamics. The most common genes involved in IPNs grouped according to their functional pathology are represented in [Figure 1].,
|Figure 1: The salient cellular pathomechanisms and the associated causal genes in inherited peripheral neuropathies|
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Myelination involves the process of formation of concentric rings of myelin sheath around the axonal membrane by Schwann cells following a 1:1 relationship with axons. Levels of myelin structural proteins (peripheral myelin protein, myelin protein zero, and myelin basic protein) are elevated along with lipid biosynthesis during the process, which aids in the saltatory conduction at the node of Ranvier. Myelin sheath facilitates the conduction of nerve impulses and simultaneously provides protection and nutritional support to axons. Any insult to the nerve can trigger demyelination, which can also lead to neurodegeneration. The most common demyelinating CMT (CMT1A) is caused by PMP22 duplication, which is a result of unequal meiotic crossover facilitated by the presence of flanking homologous repeat sequences. The effect of this altered protein structure due to PMP22 duplication has also been studied in mice models, and it was observed the gene dosage alone is sufficient to cause the disease. On the contrary, PMP22 point mutations associated with more severe clinical phenotype than CMT1A were identified in patients with Dejerine–Sottas syndrome.,
Axons are the other equally important component of the peripheral nervous system as they provide significant metabolic and energetic support required to maintain the electrochemical gradient for neurotransmission. As neuronal homeostasis depends on the transport of various cargoes between soma and the axons with the help of molecular motors such as KIF1B and DYNC1H1 proteins, mutations in genes involved in axonal transport impairs this highly regulated process of movement of organelles such as mitochondria, other vital cargoes such as mRNA transcripts, microtubules, and trophic factor signaling complexes. In addition to the transport of mitochondria, mitochondrial dynamics and mitochondrial endoplasmic reticulum crosstalk has been reported to be one of the requirements for axonal transport. MFN2 is the most commonly involved gene in mitochondrial dynamics along with other related genes such as GDAP1, OPA1, and SURf1. These are associated with axonal CMT subtypes.,
Approach to genetic testing
The advancement in the high throughput sequencing technologies has provided a new dimension for the reclassification of CMT. Clinical examination equipped with neurophysiological assessment remains the opening wedge. This information is helpful not only in choosing from the various gene panels available but also during the data analysis to identify the plausible causative variants. Based on the nerve conduction studies, the demyelinating cases can be subjected first to identify PMP22 duplication/deletion. If found negative, other genes such as GJB1 and MPZ can be screened using Sanger sequencing before going for targeted CMT1 panel. Similarly, axonal neuropathy cases can be screened for commonly implicated genes such as MFN2, GDAP1, and MORC2 by using Sanger sequencing before proceeding for targeted CMT2 panel.
The targeted panels comprise a limited number of genes associated with a particular phenotype. These panels may fail to identify uncommon findings, especially in a disease like hereditary neuropathy wherein intragenic and intergenic phenotypic heterogeneity may play a significant role. Whole-exome sequencing covers a complete coding sequence of the genome and opens up avenues to identify novel variants. The cases negative on targeted panels when subjected to whole-exome sequencing provided a diagnostic yield ranging from 19% to 45%. With the increasing accessibility and reducing cost of next-generation sequencing technologies, whole-genome sequencing is being preferred as it covers almost the complete genome and can detect structural variants, breakpoints in intronic or intergenic areas, and single base exchange variants.,
Utility of nerve biopsy
Nerve biopsy includes microscopic analysis of an affected nerve segment to understand its involvement in the pathologic process and provide information suitable for management. The sural nerve is the most commonly biopsied nerve as it is purely sensory and thus gives no motor deficits. Its superficial location above the deep muscle fascia makes it easily accessible. As it is an invasive procedure, it has to be considered wisely wherein the histological findings when correlated with clinical observations will outweigh the consequences of the biopsy.
Biopsy specimens should be of adequate size and quality for accurate diagnosis. The glutaraldehyde fixed tissue is subjected to paraffin embedding and light microscopy, special stains, and immunohistochemistry. A portion of epoxy resin embedded tissue is processed for both light and transmission electron microscopy. Nerve teased preparation of individual nerve fibers is performed only in a few centers, and its utility in routine diagnosis is minimal. The biopsy examination begins with identifying any inflammation, vascular changes, amyloid deposits, and alteration in myelin sheaths and axons. Appropriate special stains such as Congo red, Cresyl violet, and PAS and immunohistochemistry are applied.,,,
The resin-embedded semithin sections viewed on light microscopy can provide more information on the damage to the axon, myelin sheath, and the Schwann cells, such as altered axon density, myelin ovoids, onion bulbs, regenerating clusters, and osmiophilic granules of Fabry disease. Electron microscopy of ultrathin sections provides additional information of inclusion morphology and importantly of unmyelinated fibers, which are identified only on electron microscopy. Along with the chief players, the axons and the Schwann cells, other supporting cells such as mast cells, endothelial cells, pericytes, and lymphocytes are seen in the sural nerve biopsy. These cells can provide information on the blood–nerve barrier morphology and hence can give clues toward autoimmune neuropathies.,
Though nerve biopsy is not the first line of investigation for inherited peripheral neuropathies, it can prove beneficial in the management of some inherited peripheral neuropathies, such as confirming the diagnosis in sporadic cases or in case of neuropathies of unknown origin. Molecular testing (NGS/WES) with clinical and electrophysiological features has largely reduced the role of nerve biopsy in IPN. However, in a proportion of familial and sporadic cases, the molecular information is insufficient for confirmation, particularly when variants of uncertain significance are found. Thus, in suitably selected cases, nerve biopsy has been found to be useful at all ages.,, Helpful/diagnostic biopsy findings include onion bulbs, myelin abnormalities, tomaculae, storage material, characteristic disease-associated inclusions, amyloid, and giant axons, among others. The biopsy findings also help in directing the molecular testing. Although skin biopsy may show abnormalities, it is not suitable for routine diagnosis due to scanty dermal nerve twigs, and improper orientation; thus, a sensory nerve biopsy is still preferable. Nerve biopsy is also helpful to rule out the differential diagnosis of suspected inflammation or other interstitial pathology. The nerve biopsy has shown yield ranging from 60% to 77% in patients diagnosed with hereditary neuropathies in different studies reported so far.,,,,, The utility of nerve biopsy and its yield for acquired causes ranges from 40% to 60% in various studies, the more common causes being vasculitis, leprous neuropathy, diabetes, CIDP, and paraprotein and metabolic causes.
Salient findings of nerve biopsy,,,,,
Demyelination: Demyelination appears as a varying degree of loss of myelin from the internode regions along myelinated peripheral nerve fibers. This is associated with reduced G-ratio (ratio of axon to myelinated fiber diameter), resulting in inappropriately thin myelin for axon diameter. Lack of compact myelin lamellae, disproportionately thick myelin sheaths (hypermyelination, tomaculae), and abnormal myelin folds are other alterations.
Onion bulbs: Numerous concentric layers of Schwann cells cytoplasmic processes around axons due to repeated segmental demyelination and remyelination appear as onion bulbs [Figure 2]. The processes are surrounded by basal lamina. These can be associated with increased fascicular area and collagen, and the amount of increased collagen can be a correlative measure to the duration of the disease. The pattern of onion bulbs can be correlated with the subtype of demyelinating neuropathy such as a generalized pattern in hereditary neuropathies compared with mixed or diffuse patterns in acquired neuropathies, and these can be differentiated by immunohistochemistry using Tenascin-C., The size of the onion bulbs is variable from large to small and ill-formed. In long-standing demyelinating neuropathies, the Schwann cell cytoplasmic processes are attenuated/lost and the onion bulb consists only of basal lamina.
|Figure 2: Hereditary neuropathy (Non-syndromic). (a–f) CMT1/HMSN1. The thickened sural nerve contains expanded nerve fascicles (a), which are filled with large concentric Schwann cell units “onion bulbs” (b). The overlapping cytoplasmic Schwann cell layers (red) are separated by collagen (blue) and are well made out in this trichrome stain for collagen (c). The K-pal stain for myelin highlights the moderate and uniform reduction in myelinated fiber density. Most of the fibers are thinly myelinated (d). The toluidine blue-stained resin embedded semithin section further highlights the large onion bulbs (e). Electron microscopy reveals the multiple concentric layers of Schwann cells around a demyelinating fiber that has a thin myelin sheath (f). (g–j) CMT4 with abnormally folded myelin (CMT-AFM). The enlarged nerve fascicle is filled with small onion bulbs (g). The irregularly thickened contours of several myelinated fibers are caused by the abnormal outpouches and infoldings of the myelin. Several fibers are also thinly myelinated (h). The myelin morphology and the onion bulbs are better appreciated in the semithin section (i), while the EM graph (j) clearly elucidates the strikingly abnormal loops of myelin. Inset shows a thinly myelinated fiber with an onion bulb. (k and l) Hereditary neuropathy with pressure palsies (HNPP). A single enlarged tomaculous fiber (arrow) is seen in the HE section (k). The semithin section (i) reveals several abnormally thickened hypermyelinated fibers on a background of a demyelinating neuropathy containing thinly myelinated fibers. (m)CMT2. The myelin stain shows uniform depletion of fibers and the predominant involvement of large diameter fibers, accompanied by presence of regenerating clusters (arrow). (n and o) Hereditary sensory and autonomic neuropathy (HSAN): The semithin section (n) shows mainly large-diameter fibers indicating a prominent loss of small myelinated fibers. The accompanying loss of unmyelinated fibers is brought out well by electron microscopy (o) which shows loss of both small myelinated and unmyelinated fibers. A rare persistent unmyelinated fiber is marked (arrow)|
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Abnormally folded myelin (AFM): These are thickened myelin sheaths with striking abnormal outfoldings and inpouchings of the myelin [Figure 2]. They are most often due to defects in myelin adhesion and compaction. They differ from tomacule, which are focally hypermyelinated segments with smooth outer contours.
Tomacula: These are hypermyelinated sausage-shaped swellings of the myelin sheath first described by Behse and Buchthal in 1972. Myelin proteins such as myelin-associated glycoprotein (MAG), PMP22, expressed in the peripheral nervous system carries the HNK-1 epitope responsible for cell adhesion. These are involved in myelin sheath maintenance, and their altered expression results in hypermyelination, redundant loop formation, increased myelin lamellae, transnodal myelination, one myelin sheath formation by two Schwann cells, and disruption of the myelin sheath. This process is best seen on teased fiber preparation as focal, cylindrical sausage-like thickening of myelin sheath at the paranode or other circumscribed portions of the internode. Tomaculae are not only hallmark features of demyelinating neuropathy such as HNPP [Figure 2] but can also be seen in IgM paraproteinemic neuropathy with relatively less thick myelin sheaths.,,
Axonopathy: The neurons undergo axonal degeneration and loss in one of the three ways: a) Wallerian degeneration and Wallerian-like degeneration via NAD+ and SARM1
b) apoptosis-induced axonal degeneration involving global deprivation of nerve growth factor and dual leucine zipper kinase signaling cascade
c) axonal pruning, which is characterized by selective and local deprivation of nerve growth factor.
The axons are primarily affected in axonal neuropathies with secondary myelin changes. The axonal loss is usually homogenous across the fascicles and can be seen in both myelinated and unmyelinated nerve fibers, unlike in cases of HSAN and amyloid neuropathy wherein unmyelinated fibers are relatively more affected contributing to the autonomic involvement. Large-diameter fibers are affected first. In addition to primary axonal loss, secondary axonal degeneration is seen in cases of demyelinating neuropathy. When the neuronal soma is healthy, the reparative process in the axons results in axonal sprouting that is identified by regenerating clusters, defined as three or more thinly myelinated fibers within a single Schwann cell unit.,
Bands of Bungner: Axonal loss followed by dedifferentiation and cluster of proliferated Schwann cells results in the formation of tubular structures called bands of Bungner that are ~3 microns in size and surrounded by a single basal lamina. These indicate denervated empty Schwann cell units.
Regenerating clusters: These are groups of three or more thinly myelinated axons of uneven size within a single Schwann cell unit, due to axonal sprouting as a reparative process of chronic axonopathy [Figure 2].
Giant axons: These are characterized as large axons with densely packed neurofilaments [Figure 2]. Normally, the neurofilament light chains are degraded by gigaxonin protein via the ubiquitin–proteasome pathway. As these neurofilament light chains are a structural component of the axons, accumulation of these in the neurons due to unstable gigaxonin results in abnormally large and dysfunctional axons with impaired signal transmitting capacity.
Mitochondrial abnormalities: Mitochondrial morphology is best evaluated at the ultrastructural level and is significant in cases of axonal and intermediate hereditary neuropathies. Abnormal intra-axonal mitochondria with abnormal or loss of cristae can be identified predominantly in myelinated nerve fibers and Schwann cell cytoplasm. In nerve biopsy, fragmented mitochondria may be evident within the axon and/or the Schwann cell cytoplasm.,
Deposits and inclusions: Amyloid deposits, cellular inclusions as in Fabry disease, MLD, and APBD.
Genotype-Phenotype (Pathology) correlation
[Table 2] summarizes the pathological alterations in the nerve biopsy associated with specific genetic alterations.
|Table 2: Microscopic findings in nerve biopsy associated with mutant genes|
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[Figure 2] and [Figure 3] demonstrate some of the characteristic microscopic nerve biopsy findings in the more common hereditary neuropathies.
|Figure 3: Hereditary neuropathy (Syndromic). (a–d) Familial amyloid polyneuropathy (FAP): Subperineurial (a) and endoneurial perivascular (b) eosinophilic, amorphous amyloid deposits show Congo red staining (c) and birefringence under polarized light (d). Small myelinated fiber loss is noted on the myelin stain (e). (f–h) Metachromatic leukodystrophy (MLD). On HE stain (f), few endoneurial perivascular macrophages show abnormal reddish granules of the sulfatide storage material (arrow). The storage material in the macrophages and Schwann cells reveals golden brown metachromasia on staining with acidified cresyl violet (g). The Kpal myelin stain (h) shows numerous thinly myelinated fibers and degenerating myelin with remodeling. (i–l) Giant axonal neuropathy (GAN). Numerous bloated giant axons are seen in the HE stain (i), while the myelin stain reveals the thinly stretched out myelin sheaths around the giant axons (j). Neurofilament immunostaining (k) labels the abnormal axons and indicates the closely packed intermediate fibers. The semithin section (l) shows the giant axons with thin myelin as well as completely denuded axons. (m and n) Fabry disease. The semithin section (m) reveals numerous dense osmiophilic granules in several perineurial cells (arrows). Similar inclusions are present in the epineurial vascular smooth muscle cells and endothelial cells (arrows) (n), the latter causing vascular luminal occlusion. (o-r) Adult polyglucosan body disease (APBD). A single large basophilic intraaxonal polyglucoasan body (PB) is seen on the HE stain (o). Several PBs (circled) in a single sural nerve fascicle are highlighted by the PAS-diastase stain in (p) and Lugol's iodine in (q). EM graph reveals non-lysosomal dense accumulation of glycogen granules in (r)|
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Familial Amyloid Polyneuropathy (FAP)
It is a rare autosomal dominant disorder associated with the deposition of beta-pleated amyloid fibrils in several organs, particularly in the peripheral nerve. The most common pathogenic variants involve transthyretin (TTR) and less often the apolipoprotein A1 and gelsolin genes. TTR Val30Met is the most frequent pathogenic variant among the more than 100-point mutations in FAP. It causes a length-dependent small fiber neuropathy with loss of temperature and pain, and the presence of autonomic dysfunction. Cardiac and renal involvement is frequent., As transthyretin is also produced in the retina and choroid plexus (the main source is the liver) involvement of the vitreous and leptomeninges is documented. Isolated carpal tunnel syndrome is another manifestation. Cranial neuropathies are a feature of gelsolin-related FAP. The nerve fiber loss is initially of unmyelinated fibers, followed by small myelinated fibers, and in the terminal stages by large fiber loss. Regenerating clusters are typically absent when there is no other concomitant disorder.
Diagnosis is by genetic testing, biopsy, and amyloid typing. Nerve, skin, salivary gland, and abdominal fat biopsies are commonly performed with sensitivity rates of 80%, 70%, 90, and 14%–83%, respectively. Several sections/biopsies may be required, and it is also somewhat dependent on the experience of the pathologist in detecting small deposits; moreover, a negative biopsy does not exclude the diagnosis. The nerve deposits are most often around the endoneurial vessels in the interstitium and rarely around epineurial blood vessels. They are Congo red positive and show apple-green birefringence under polarized light [Figure 3]. It is important to differentiate FAP from acquired paraprotein-associated primary amyloidosis where immunoglobulin light chain fibrils form the amyloid. Immunostaining for transthyretin may help; however, light chain staining is very unsatisfactory and other tests will be required.
Metachromatic leukodystrophy (MLD)
MLD is an autosomal recessive, lysosomal storage disorder caused by deficient arylsulfatase A (ARSA) activity resulting in the accumulation of sphingolipid (cerebroside sulfate/sulfatide) in the brain, peripheral nerve (Schwann cells, macrophages), and viscera (distal renal tubules, gall bladder, and liver). More than 250 causative pathogenic variants in the ARSA gene have been identified. ARSA activity is also reduced in multiple sulfatase deficiency caused by the pathogenic SUMF1 gene variants. The normal level of ARSA but pathogenic gene variants in the prosaposin or ARSA activator proteins and the Saposin-B/Sap1 also cause disease., A pseudo-deficiency of ARSA is observed when a synthetic substrate is used for testing in the lab and should not be construed as disease. Of the three forms of MLD, namely, late infantile, juvenile, and adult, it presents most commonly as the late infantile variant with features of developmental delay, regression, ataxia, white matter changes, and peripheral neuropathy. The juvenile and adult variants are less frequent and present with attention deficits, scholastic difficulties, and psychiatric manifestations.,, Pathologically, metachromatic storage material is present in the neurons, oligodendrocytes, viscera, and nerves. Nerve shows segmental demyelination, remodeling of myelin into thin myelinated fibers, and fiber loss. The storage material is at times visible on routine HE stain as reddish granular material in the macrophages and occasionally the Schwann cells. Staining a cryosection of the fixed nerve tissue with acidified Cresyl violet reveals the golden brown metachromasia in endoneurial macrophages and Schwann cells [Figure 3]. Nerve biopsy plays a useful role as it is definitive and is positive even in the early subclinical stages. Ultrastructurally, the characteristic inclusions include zebra, tuff stone, and prismatic and membrane cellular bodies.,
Other Leukodystrophies: The role of nerve biopsy for diagnosis of other leukodystrophies is minimal as the ultrastructural findings are neither specific for the disease nor is it sensitive as they are not consistently present, and it is not recommended for diagnosis.
Globoid cell Leukodystrophy/Krabbe disease/(GLD): GLD is an autosomal recessive disorder caused by pathogenic mutations of the galactosyceramidase (GALC) gene resulting in the deficiency of galactosylceramidase (GALC), a lysosomal hydrolase, resulting in CNS and PNS demyelination and accumulation of psychosine that is toxic to the oligodendrocytes and Schwann cells. It presents as infantile (most common), juvenile, and adult-onset disease., On light microscopy, the nerve biopsy shows non-specific features of demyelination, few onion bulbs, and occasional macrophages. The large perivascular multinucleate globoid macrophages seen in the brain are not present in the nerve. On electron microscopy, the typical inclusions are seen in only a few cases and consist of tubular, prismatic, needle-like, or membrane-bound structures in Schwann cells. Diagnosis is by detecting low blood enzyme levels/activity, increased serum psychosine levels, and gene testing. Current modes of therapy include hemopoietic stem cell transplant (HMSC) and ERT.
Adrenoleukodystrophy: Childhood ALD and later-onset adrenomyeloneuropathy (AMN) are X-linked recessive disorders caused by pathogenic variants of the adenosine triphosphate [ATP]-binding cassette transporter superfamily D member 1 (ABCD1) gene and is characterized by defective metabolism and accumulation of very-long-chain fatty acids (VLCFA). Demyelinating neuropathy is often present in the later-onset disease and the histology is that of a demyelinating neuropathy; however, nerve biopsy does not play a significant role. Inclusions in the nerve are scanty and EM may rarely demonstrate bi and trilaminar aggregates and increased Reich/Pi granules. Diagnosis is generally by genetic/biochemical testing.
An autosomal recessive peroxisomal disorder caused by biallelic pathogenic variants in the PHYH or PEX7 gene, neuropathy is seen in late-childhood onset patients and is associated with deafness, anosmia, ataxia, and ichthyosis. It is diagnosed by increased levels of phytanic acid and VLCFA in the blood. Early diagnosis is essential for instituting suitable dietary and other symptomatic management. Nerve biopsy shows a chronic demyelinating neuropathy with small onion bulbs.
Giant Axonal Neuropathy (GAN)
This is an autosomal recessive disorder caused by more than 30 pathogenic variants of the gigaxonin (GAN) gene located on chromosome 16. Gigaxonin is a ubiquitin E3 ligase substrate adaptor and is involved in the degradation or turnover of intermediate filament proteins. GAN is characterized by peripheral neuropathy, kinky/frizzy hair, and CNS involvement. Patients later develop pyramidal signs, dysarthria, nystagmus, facial weakness, and mental retardation. Radiological findings include white matter changes sparing the subcortical U fibers. Nerve biopsy reveals several large, bloated axons of more than 100 microns diameter surrounded by thinned-out myelin [Figure 3]. With increasing duration, there is axonal loss, demyelination, and loose onion bulbs. Ultrastructurally, the axons are packed with the neurofilament intermediate filament. Peripheral tissues such as skin also contain vimentin intermediate filament clusters in several cell types and on occasion may serve to aid in diagnosis without the nerve biopsy.
Fabry disease (FD)
FD is a multisystem X-linked disorder caused by pathogenic variants in the GLA gene, resulting in deficient action of a-galactosidase A (a-Gal-A) and accumulation of globotriaosylceramide (GL-3) and globotriaosylshingosine (lyso-GL-3) in viscera, peripheral nerve, and dorsal root ganglia. The accumulation in the endothelial cells leads to vascular occlusion and a generalized vasculopathy involving the brain, cardia, and kidney. Among the two clinical phenotypes, the “classical” phenotype occurs in males in childhood/adolescence, with >1% enzyme activity with features of painful small fiber neuropathy, dermal angiokeratomas, corneal abnormalities (ventricillata), renal involvement, and unexplained stroke., The “late-onset” phenotype occurs in males with varying residual enzyme activity and most often with cardiac involvement. Heterozygous females are often normal or have variable phenotypes that are occasionally pronounced. The diagnosis is by enzyme assay and molecular genetic testing. Histologically, the nerve reveals a small fiber neuropathy. The storage material is not visible in routine paraffin-embedded sections as it is dissolved during processing. It is seen only in resin-embedded tissue processed for semithin sections and electron microscopy as it preserves the lipid. On light microscopy of the semithin sections, osmiophilic granules are present in the vascular endothelium, perineurial cells, and vascular smooth muscle cells [Figure 3]. Ultrastructurally, they are homogenous or lamellated bodies.,, Awareness and prompt diagnosis are crucial as enzyme replacement therapy (ERT) is available that results in a better outcome when instituted at an early age.
Adult polyglucosan body disease (APBD)
APBD is an autosomal recessive disorder first described by DiMauro in 1980 and is defined by the presence of polyglucosan bodies (PB) in the neurons, astrocytes, and axons. Often starting after the age of 40 years, it is characterized by multilevel nervous system involvement resulting in cognitive impairment, pyramidal signs, peripheral neuropathy, and neurogenic bladder. White matter changes in the brain and cord atrophy are frequently present. APBD is characterized by decreased activity of the glycogen brancher enzyme (GBE) that is usually a result of pathogenic variants in the GBE1 gene. The GBE1 gene is also affected in GSDIV with which APBD is allelic.,, The nerve biopsy shows several intra-axonal basophilic polyglucosan bodies that stain for PAS-diastase and Lugol's iodine [Figure 3]. Occasionally, PB may be found as an age-related phenomenon, but the presence of a single large, or multiple (two or more in one fascicle) raises the possibility of APBD. Like Lafora bodies, the PB of ABPD may also be found in axillary skin biopsies, though it is not a sensitive tool for diagnosis.,
This brief review highlights the role of nerve biopsy and its findings in various types of hereditary neuropathies. In the era of molecular genetics, it remains of utility in selected familial and sporadic cases to confirm/provide diagnosis or direct the genetic testing.
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T C Yasha
Department of Neuropathology, National Institute of Mental Health and Neurosciences, Bengaluru, Karnataka
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]