Annals of Clinical & Laboratory Science 31:140-146 (2001)
© 2001 Association of Clinical Scientists
Proximal Myotonic Myopathy: Clinical, Neuropathologic, and Molecular Genetic Features
Stephan Eisenschenk1,
William J. Triggs1,
Gary S. Pearl2 and
Amyn M. Rojiani3
1 Department of Neurology, University of Florida College of Medicine, Gainesville, Florida
2 Department of Pathology, Orlando Regional Medical Center, Orlando, Florida
3 Department of Pathology, University of South Florida College of Medicine, Tampa, Florida
Address correspondence to Amyn M. Rojiani, M.D., Ph.D., Department of Pathology (Neuropathology), University of South Florida College of Medicine, 12901 Bruce B. Downs Blvd. (MDC11), Tampa, FL 33612, USA; tel 813 974 8750; fax 813 974 5536; e-mail arojiani{at}com1.med.usf.edu
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Abstract
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The primary genetic abnormality in myotonic dystrophy (DM) is an expansion of the CTG trinucleotide repeat on chromosome 19q. Recently, patients with similar clinical features, but without this genetic alteration, have been designated as proximal myotonic myopathy (PROMM). We describe two additional cases of PROMM, both of whom presented with clinical features suggestive of myotonic dystrophy. The patients had electromyographic (EMG) evidence of myotonia, normal cardiac evaluation, and no cataracts. Genetic analysis of peripheral blood leukocytes revealed no expansion of the trinucleotide repeat by polymerase chain reaction (PCR) and Southern blot analysis. Muscle biopsies in both cases were significant with features suggestive of myotonic dystrophy, such as large numbers of fibers containing multiple internal nuclei, occasional nuclear chains, and fiber atrophy, although sarcoplasmic masses and ring fibers were absent. These cases illustrate the clinical and neuropathologic findings of PROMM and underline the importance of correlating these aspects with genetic studies in patients with myotonic muscle disorders.
(received 15 November 2000; accepted 22 November 2000)
Keywords: proximal myotonic myopathy (PROMM), myotonic dystrophy (DM), trinucleotide repeats, polymerase chain reaction, Southern blot
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Introduction
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The myotonic muscle disorders are a heterogeneous group of diseases with myotonia. While these processes have clinical manifestations that help to differentiate them from one another, they also share features that sometimes make them difficult to distinguish on clinical grounds [1]. Recent advances in genetics and molecular biology have improved the identification and diagnosis of myotonia congenita [2,3], hyperkalemic periodic paralysis [4], paramyotonia congenita [5], and myotonic dystrophy [6–10].
Myotonic dystrophy (DM) is by far the most common inherited muscular dystrophy that affects adults. The disorder is characterized by myotonia, progressive muscle weakness, cataracts, and cardiac abnormalities as well as frontal balding and gonadal insufficiency in males. The primary genetic abnormality responsible for myotonic dystrophy has been identified as an expanded trinucleotide repeat (CTG) in the DM gene on chromosome 19. This finding greatly facilitates the confirmation of clinically apparent cases, and it has been detected in the vast majority of clinical cases [8,11–13]. In recent years, there have been descriptions of individuals and families with clinical manifestations similar to myotonic dystrophy that lack expansion of the CTG trinucleotide repeat and that have demonstrated variable phenotypic expression [12–15]. Ricker et al [16,17] excluded the probability of myotonic dystrophy and other myotonic disorders in a large subset of families and described a new myotonic disorder, ie, "proximal myotonic myopathy" (PROMM).
We describe two additional cases of PROMM. Both patients presented with some features of myotonic dystrophy. However, following further studies, including pathologic examinations and analysis for the CTG trinucleotide repeats on chromosome 19q, both patients were diagnosed as PROMM.
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Case Presentations
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Patient 1.
The first patient is a 68-yr-old white male who presented with insidiously progressive difficulty rising from a seated position. He had also noted intermittent stiffening for "many years." His medical history was significant for well-controlled insulin-dependent diabetes mellitus, coronary artery bypass grafting, and resection of an adrenal adenoma. The patient has had no children. There was no prior family history of neuromuscular disease.
On evaluation, the patient exhibited moderate gynecomastia. Testicular size was normal. There was no evidence of cardiac dysrhythmia. No cataracts were noted. Cranial nerve exam revealed mild bilateral ptosis and moderate neck flexor and rotational weakness. On motor examination, there was moderate proximal weakness of upper and lower extremities with normal strength of distal musculature. There was a positive Gower’s sign. No evidence of muscle hypertrophy was noted. Sensory examination was normal for all modalities except for mild vibratory loss involving the distal lower extremity.
Nerve conduction studies demonstrated subtle evidence of a polyneuropathy, including mildly reduced sural nerve potential amplitude and axon reflexes. There was physiologic evidence of a superimposed carpal tunnel syndrome. Needle electromyography revealed myotonia and mild, diffuse evidence of a myopathy, including reduced motor unit potential amplitude and duration, and early recruitment. A quadriceps muscle biopsy was obtained and a blood sample was submitted for molecular genetic studies.
Patient 2.
The second patient is a 41-yr-old white female who initially presented for evaluation of neck pain. She noted progressive stiffening of her limbs over the past two years and difficulty arising from chairs and climbing stairs. Her medical history is unremarkable. Review of her family history disclosed a brother who has frontal balding, limb stiffness, and cataracts. The patient’s father died at age 68-yr with an undiagnosed neuromuscular disease.
On examination, cardiac evaluation showed no dysrhythmia. Cataracts were absent. She demonstrated intact higher cortical functions and cranial nerves were normal. Evaluation of motor strength demonstrated mild proximal weakness with prominent shoulder slumping. There was evidence of contraction induced myotonia. No percussion myotonia was present. Sensation, tendon reflexes, and gait were normal.
Serum creatine kinase levels were unremarkable. Electrocardiograms were normal. Electrodiagnostic testing showed myotonic discharges in proximal muscles. A muscle biopsy was obtained and a blood sample was submitted for molecular genetic studies.
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Methods
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Muscle biopsy.
Skeletal muscle biopsies, performed on both patients, were processed in diagnostic surgical neuropathology laboratories. Formalin-fixed paraffin embedded sections as well as cryostat sections were employed. The tissue sections were stained by the H & E, modified trichrome, NADH-TR, PAS, ORO, myophosphorylase, and ATPase (pH 9.4, 4.6 and 4.3) procedures and examined by light microscopy.
Molecular genetic studies.
Blood samples were sent to a commercial laboratory (Athena Diagnostics, Worcester, MA) for molecular genetic analysis of leukocytes, specifically for CTG repeats (myotonic dystrophy). The following is an overview of the test and interpretation of the results as outlined by the laboratory. Samples were analyzed by the polymerase chain reaction (PCR) and by Southern blot autoradiography. A specific banding pattern, visualized on the PCR and Southern blot autoradiographs, is used to calculate the number of CTG trinucleotide repeats. The use of both PCR and two Southern blot analyses results in 99% accuracy in detection of the CTG trinucleotide repeat size; which has been shown to correlate with the prognosis in DM. The PCR assay provides fine resolution of the smaller CTG repeats such as would be found within the normal range (5–37 CTG repeats), the borderline normal range (38–49 CTG repeats), and the permutation range (50–99 CTG repeats). The Southern blot assay, on the other hand, identifies large expansions (100–1000s range).
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Results
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Skeletal muscle biopsies in both patients revealed similar histopathologic findings and are described together. Both biopsies were examined as cryostat sections cut in the transverse plane, as well as formalin-fixed, paraffin-embedded sections seen in both cross and longitudinal sections.
The most prominent histologic feature in H&E stained sections was a striking number of internal nuclei. This was present in most fibers and multiple nuclei were frequently present within individual fibers. There was a moderate variation in myofiber diameter with occasional smaller, somewhat angulated fibers scattered throughout the sample (Fig. 1
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Fig. 1. The large number of internal nuclei, involving most fiber, is readily evident in these H&E-stained, cryostat sections. Original magnification 200x and 400x (Patients 1 and 2, left and right panels, respectively).
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Nuclear chains, ie, linear arrangements of multiple nuclei, were frequently present and were best seen in longitudinally sectioned paraffin-embedded material (Fig. 2). No myofiber-grouped atrophy was noted. There was a mild increase in endomysial connective tissue. Ring-binden (ring fibers) and subsarcoplasmic pads, frequent features of myotonic dystrophy, were not seen in any of the sections. Necrotic fibers were not identified and no inflammatory infiltrate was seen in any sections. Similarly, the microvasculature was normal with no inflammation or deposits.
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Fig. 2. Cryostat sections stained by the modified Gomorri trichrome method showed the internal nuclei and a mild increase in endomysial connective tissue, but did not reveal other abnormalities such as ragged red fibers or inclusions. Original magnification 400x and 200x (Patients 1 and 2, left and right panels, respectively).
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Cryostat sections stained by the modified Gomorri trichrome method showed the internal nuclei but did not reveal any other abnormality (eg, ragged red fibers) in either biopsy (Fig. 3a,b). Oxidative enzyme histochemistry revealed normal intermyofibrillar architecture with no structural abnormalities.
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Fig. 3a (left panel). There was moderate variation in myofiber diameter with occasional smaller, angulated fibers (arrows). H&E-stained cryostat section. Original magnification 200x (Patient 2). Fig. 3b (right panel). Sections from both biopsies revealed nuclear chains as seen in this example (arrows). H&E-stained, paraffin section. Original magnification 400x (Patient 1).
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Enzyme histochemistry for ATPase, preincubated at pH 4.3, 4.6, and 9.4 showed normal "checkerboard" pattern of fiber types, with mild type 2 fiber atrophy in one biopsy, while the other case revealed a similar type 2 fiber atrophy and was associated with a type 1 fiber predominance (Fig. 4).
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Fig. 4. ATPase enzyme histochemistry revealed a normal "checkerboard" pattern of fiber types, with mild type 2 fiber atrophy (darker staining fibers, arrows). In Patient 1, this was associated with type 1 fiber predominance. Cryostat sections, ATPase, preincubated at pH 9.4, original magnification 200x (Patient 2, left panel), 100x (Patient 1, right panel).
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Abnormal deposits of glycogen or lipid were not present and myophosphorylase activity was detected in both biopsies.
Genetic analysis of DNA in peripheral blood leukocytes revealed no increase in CTG repeats in either case, based on the PCR and Southern blot analyses.
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Discussion
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Identification of the CTG trinucleotide expansion on the long arm of chromosome 19 [7,9,10] and utilization of PCR and Southern blot techniques have provided the ability to detect up to 99% of individuals with phenotypic manifestations consistent with myotonic dystrophy with expansions greater than 50 repeats [6,9]. Nonetheless, patients with clinical manifestations consistent with myotonic dystrophy, but without expanded CTG repeats, have been described [12,13,15]. Ricker et al [16,17] described a large subset of families with a multisystem disease process involving abnormalities of skeletal muscle, heart, lens, and liver, but distinct from myotonic dystrophy; they termed this disease process proximal myotonic myopathy (PROMM).
The principal differences between myotonic dystrophy and PROMM are listed in Table 1. Our patients lacked the full systemic manifestations of DM. In contrast, they demonstrated clinical and pathologic findings consistent with PROMM and lacked (CTG)n trinucleotide repeats indicative of myotonic dystrophy. Although patients with paramyotonia congenita or myotonia congenita may develop muscle weakness and wasting, our patients did not have other symptoms suggestive of either of these nondystrophic myotonias [1]. Notably, there was no history of localized, cold-induced, prolonged myotonia, or the exacerbation with activity that is commonly associated with paramyotonia congenita. Furthermore, our patients have continued to demonstrate progressive weakness, which is rare in myotonia congenita.
Abruzzese et al [18] reintroduced the theory that mutation(s) other than the (CTG)n repeat may also produce the myotonic dystrophy phenotype. These authors described several families without the (CTG)n repeat, but with phenotypic myotonic dystrophy, including prominent weakness in distal limb and facial muscles, cardiac conduction abnormalities, and prominent ring fibers in muscle biopsy [18]. Such clinical and pathological features were absent in our patients. Nonetheless, phenotypic variability in myotonic dystrophy and PROMM [16,17,19,20] suggests that the distinction between these myotonic disorders may sometimes be tenuous. The mild facial weakness and possible infertility in Patient 1 illustrate this point. Nonetheless, this patient’s clinical and pathologic findings are most consistent with PROMM, and not myotonic dystrophy.
Our Patient 2 had a family history consistent with an autosomal dominant inheritance, compatible with either myotonic dystrophy or PROMM. Normalization of the trinucleotide repeat has been described in asymptomatic offspring of patients with myotonic dystrophy [11], but not in clinically affected offspring.
This report reemphasizes the clinical literature and diagnostic features of this extremely rare entity. Clinical awareness of this disorder in conjunction with skeletal muscle biopsy examinations and analyses of molecular genetic features are the only means by which this diagnosis can be rendered. Although the symptomatology of myotonic dystrophy and PROMM may overlap, PROMM appears to have a different physiologic basis than the previously described myotonic syndromes [16,17,21]. Our experience suggests that histopathologic examination may facilitate the distinction between myotonic dystrophy and PROMM, prompting further molecular studies. Further investigations are indicated to improve our understanding of both myotonic dystrophy and PROMM, and to clarify the nature of the responsible genetic alterations.
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Abstract
Introduction
