Congenital Muscular Dystrophies

Keywords

Congenital muscular dystrophies, merosin-deficient, dystroglycanopathies, collagen VI deficiency, SEPN1 CMD, Walker-Warburg syndrome, muscle-eye-brain disease, Fukuyama CMD

Introduction

The identification of several new genes responsible for different forms of congenital muscular dystrophies (CMD) has dramatically expanded our knowledge of the spectrum of these conditions and their pathogenesis, allowing a better characterization of the individual forms and of the specific pathomechanisms underlying them.

Definition of CMD

Classically, the term congenital muscular dystrophy includes a group of genetically, clinically, and biochemically distinct entities sharing clinical and pathological features such as early presentation of weakness and hypotonia and dystrophic features on muscle biopsy. The term congenital implies origin of the pathological process during fetal life, which distinguishes these variants from later onset, postnatal conditions such as Duchenne muscular dystrophy (DMD) and limb girdle muscular dystrophies (LGMDs). It is nevertheless appreciated that the onset of the clinical symptoms might be delayed as not all affected children have frank symptoms at birth, but they will start developing weakness, contractures, and delayed motor milestones in the first year of life.

This “expanded” definition can be confusing, however, when assessing children in whom allelic mutations result in a broad spectrum of clinical syndromes, ranging from severe CMDs with clear prenatal onset to mild LGMD variants with onset in childhood, adolescence, or adulthood. This is particularly confusing when the severity of their conditions overlaps between CMD and LGMD and in the presence of central nervous system involvement, when delayed motor milestones could be attributed to muscle weakness, or global developmental delay, or a combination of the two. This is best exemplified by mutations in genes responsible for secondary dystroglycanopathy, such as FKRP, FKTN, POMT1, POMT2 , or ISPD . Many authors have therefore followed a pragmatic approach that includes in the CMD definition children with onset of weakness prenatally or before ambulation is acquired, while LGMD, in contrast, refers to all other cases with later onset weakness, with overt clinical signs after ambulation has been acquired at an appropriate age.

The definition of CMD should also take into account that some disorders classically labeled as CMD do not always have the typical dystrophic pattern but only milder myopathic changes, at least in the muscle available for the routine diagnostic procedures. Indeed, in some conditions such as rigid spine syndrome or rigid spine muscular dystrophy 1 (RSMD1), a phase of “myopathic” changes can precede dystrophic abnormalities, or some muscles (such as the paraspinal muscles) may have more severe involvement than others.

A comprehensive definition of CMD could therefore be that of a heterogeneous group of genetically, clinically, and biochemically distinct entities sharing clinical and pathological features, with onset of clinical signs at birth or typically in the first 12 months. In some variants such as Ullrich CMD and SEPN1-related myopathies, the onset can be delayed even beyond the end of the first year. Muscle biopsy reveals a dystrophic or myopathic pattern without other diagnostic features suggestive of a structural congenital myopathy or a metabolic disorder.

Classification

The first attempt at a rational classification of CMD was proposed in 1994 by the International Consortium on CMD, which recommended recognizing forms of CMD with structural brain changes from a classical or “pure” CMD without structural brain abnormalities. The original cornerstone in the CMD classification came after the identification of the deficiency of the extracellular matrix protein merosin (more specifically the laminin α2 chain of the merosin trimer) in a subset of CMD cases, followed shortly thereafter by the isolation of the responsible gene ( LAMA2 , encoding for the laminin α2 chain of the merosin trimer) on chromosome 6. This allowed the subdivision of the “pure” form into two groups, merosin-deficient and merosin-positive CMD, according to the presence or deficiency of merosin. The term merosin-positive CMD then became commonly used to describe variants other than merosin-deficient CMD; however, this term is no longer used as the majority of patients in this category now have well described conditions, with specific patterns of abnormal protein expression and/or with specific primary genetic defects. The most common subtypes in this category are the Ullrich phenotype with distal laxity, RSMD1, and LMNA-CMD, well-recognized variants in which the genetic basis has been identified.

Another major breakthrough came in the last decade following the discovery that various forms of CMD are characterized by a profound depletion of the glycosylation of α-dystroglycan. The underlying genetic defects in these disorders, which are collectively described as dystroglycanopathies , are mutations in known or putative glycosyltransferase enzymes, which are involved in the process of constructing an unusual glycostructure on α-dystroglycan.

In the last few years, CMDs have been classified according to combined clinical, genetic and pathological approaches :

1.

Forms of CMD due to mutations in genes encoding for structural proteins of the basal lamina or extracellular matrix or receptors for extracellular matrix proteins. This category includes mutations in the collagen 6 genes, laminin α2 (merosin), in integrin α7 and the more recent variant due to integrin α9 gene deficiency. Mutations in DAG1 (dystroglycan 1 gene) have very recently also been described and clearly belong to this subgroup.
2.

Forms secondary to genes encoding for putative or confirmed glycosyltransferases that affect the glycosylation of α-dystroglycan. These include Fukuyama CMD, muscle-eye-brain (MEB) disease, Walker-Warburg syndrome (WWS), MDC1C and MDC1D, and other phenotypes associated with mutations in one of the 15 known genes.
3.

Defects of nuclear envelope proteins (LMNA and nesprin).
4.

Defects of proteins with thus far unknown function localized in the endoplasmic reticulum, which include the form of CMD with rigid spine syndrome secondary to mutations in the SEPN1 gene.
5.

CMD with mitochondrial structural abnormalities (CMDmt).

Table 29.1 provides a list of the known genes responsible for CMD and of the phenotypes associated with mutations in each of them. The locations of a number of proteins described in this chapter are shown in Figure 29.1 . In this chapter, we will specifically concentrate on new findings related to diagnosis, clinical management, and differential diagnosis of the most common forms of CMD.

Table 29.1

Classification of Congenital Muscular Dystrophies: Clinical, Biochemical, and Genetic Characteristics

Biochemical Defect	Locus	Gene	Disease Phenotype(s)
Extracellular Matrix Proteins	6q22-23	LAMA2	Primary merosin deficiency (MDC1A)
Extracellular Matrix Proteins	21q22.3 2q37	COL6A1 COL6A2 COL6A3	Ullrich CMD
External Sarcolemmal Proteins	12q13	ITGA7	Integrin α7-related CMD
External Sarcolemmal Proteins	3p23-21	ITGA9	Integrin α9-related CMD
Dystroglycan and Glycosyltransferase Enzymes	9q34.1	POMT1	WWS, MEB, CMD with cerebellar involvement, CMD with mental retardation and microcephaly
	1q32-34	POMGnT1	WWS, MEB, CMD with cerebellar involvement
	14q24.3	POMT2	WWS, MEB, CMD with cerebellar involvement, CMD with mental retardation and microcephaly
	19q13.3	FKRP	WWS, MEB, CMD with cerebellar involvement, CMD with mental retardation and microcephaly, CMD with no mental retardation and normal brain MRI
	9q31	FCMD	Fukuyama CMD
	22q12.3-13.1	LARGE	WWS, MEB, white matter changes
	1q12-q21	DPM2/DPM3	CMD with mental retardation and severe epilepsy
	7p21.2	ISPD	WWS, LGMD
	3p22.1	GTDC2	WWS
	11q13.2	B3GALNT2	WWS, MEB
	3p21.23	GMPPB	CMD with mental retardation and severe epilepsy, LGMD
	3p21	DAG1	Primary dystroglycanopathy, LGMD with early onset and mental retardation, normal brain MRI
	8p11.21	SGK196	MEB
	1q42	–	MDC1B
Endoplasmic Reticulum Protein	1p35-36	SEPN1	CMD with spinal rigidity (RSMD1)
Nuclear Envelope Proteins	6q25	SYNE1 (nesprin 1)	CMD with adducted thumbs
Nuclear Envelope Proteins	1q21.2	LMNA	Congenital laminopathy
Sarcolemmal and Mitochondrial Membrane Protein	22q13	CHKB	Mitochondrial CMD (CMDmt)

The classification is based on combined clinical, genetic, and pathological data.

Abbreviations : CMD, congenital muscular dystrophy; CMDmt, CMD with mitochondrial structural abnormalities; LGMD, limb girdle muscular dystrophy; MEB, muscle eye brain; MRI, magnetic resonance imaging; RSMD1, rigid spine muscular dystrophy 1; WWS, Walker-Warburg syndrome; MDC, muscular dystrophy congenital.

Merosin-Deficient CMD

In a recent UK survey, merosin-deficient CMD (MDC1A) accounted for 22% of all the molecularly defined CMD subtypes, with a lower frequency than that reported in the early studies reporting this. Similar figures have been reported in the USA and in a recent Italian population study (Mercuri, personal communication). Patients with absent merosin invariably present at birth or in the first few weeks of life with hypotonia, weakness, and, in some cases, contractures. The maximal motor ability achieved is sitting or, in rare instances, walking with support. Weakness generally affects upper limbs more severely than lower limbs. There are often contractures such as flexion deformity at the hips, knees, elbows, and ankles, followed by rigidity and scoliosis of the spine. Another clinical sign is the progressive limitation of eye movements and more specifically of the upward gaze, a feature typically noted in the second decade of life.

Respiratory function is invariably reduced in the first decade and night time hypoventilation is frequently observed in early childhood. Feeding problems are also frequent.

Diagnosis is generally performed after the onset of clinical signs, as the muscle biopsy shows a classical dystrophic picture and immunofluorescence techniques can readily demonstrate the reduction or absence of laminin α2 chain. In most cases, the protein is totally absent or only present in traces. A severe inflammatory infiltrate can sometimes be seen, and this explains why in the past some of these children received a diagnosis of infantile inflammatory myopathy.

Since a secondary, partial merosin deficiency can also be found in other forms of CMD, such as dystroglycanopathies, the diagnosis of merosin-deficient CMD should be genetically confirmed by studying the laminin α2 chain ( LAMA2 ) gene, mapped to chromosome 6q22-23, especially in cases in which there is residual protein expression.

In patients with mutations in the LAMA2 gene and partial merosin deficiency, the severity of the phenotype is often milder compared to patients with absent merosin. Patients with partial primary merosin deficiency usually have normal motor milestones in the first years of life, and the onset of symptoms may be delayed and occur in the second decade ; these patients are also much less likely to require enteral feeding and ventilatory support.

Patients with both complete or partial absence of merosin have diffuse white matter changes on brain magnetic resonance imaging (MRI) affecting both hemispheres ( Figure 29.2A ), with sparing of the internal capsule, corpus callosum, basal ganglia, thalami, and cerebellum. These changes are best visualized on MRI after 6 months. The changes on brain MRI are thought to be the result of an increase in water content rather than being a sign of demyelination and are generally not associated with clinical signs of CNS involvement. While until recently these changes were thought to be specific of MDC1A, the same pattern has also been found in patients with dystroglycanopathies who also often have secondary partial merosin deficiency on muscle biopsy. In these cases, genetic confirmation is needed for differential diagnosis. While the white matter abnormalities are essentially invariably found, we recently came across an individual with mild LGMD due to LAMA2 mutations in whom the brain MRI did not show the typical changes.

Cognitive and visual function are generally normal with the exception of the small percentage (~<10%) of cases with associated structural brain changes affecting the occipital regions ( Figure 29.2B ). The molecular explanation for this rare but consistent complication in MDC1A is not known.

Dystroglycanopathies

The term dystroglycanopathies includes a genetically heterogeneous group of muscle disorders, with hypoglycosylated α-dystroglycan (ADG) on muscle biopsy, frequently associated with concomitant central nervous system pathology and more rarely with eye pathology. Mutations in dystroglycan itself have been very recently reported but are rare, hence almost invariably patients in this group of conditions will have mutations in genes involved in the glycosylation of ADG rather than in dystroglycan ( DAG1 ) itself. A more appropriate terminology is therefore of secondary dystroglycanopathies for all conditions in which DAG1 is genetically intact.

So far, defects in a total of 15 putative and demonstrated glycosyltransferases or accessory proteins of glycosyltransferases have been shown to cause a dystroglycanopathy phenotype, with several new genes reported in the last two years ( Table 29.1 ). The main pathway related to defects in glycosylation of ADG relates to O-mannosylation; however, recently a few cases with mutations in genes involved also in the N-glycosylation pathways have been reported also.

Patients with primary or secondary dystroglycanopathies have common features, such as predominant involvement of the upper limbs and markedly elevated CK. The spectrum of clinical severity ranges from forms with severe structural muscle, eye, and brain involvement (WWS), muscle-eye-brain disease (MEB), and Fukuyama congenital muscular dystrophy (FCMD), to mild cases with late onset and no brain or eye involvement. The classification of these forms has changed over the years in order to take into account not only the complexity of genetic defects but also the heterogeneity of the phenotypes. While the early studies following the identification of the first genes involved mainly focused on distinct known phenotypes, namely MEB, WWS, and FCMD, or on their absence in patients with normal brain MRI scans, it soon became obvious that the spectrum of CNS involvement is much more complex. Recent brain MRI studies have highlighted that a number of patients have posterior fossa lesions, such as cerebellar dysplasia or hypoplasia, occurring in isolation or variably associated with brainstem and pons abnormalities, suggesting that infratentorial structures have a particular susceptibility to the underlying disease process in the majority of the dystroglycanopathies with brain involvement. We have proposed a simplified clinical classification system comprising seven broad phenotypic categories (five for the CMD forms), with patients subdivided according to the presence/absence and degree of structural and functional brain involvement, and motor functional abilities ( Tables 29.2 and 29.3 ).

Table 29.2

Dystroglycanopathy Core Phenotypic Categories for Congenital Muscular Dystrophies

Phenotypic Classifications	Description
Onset
Further subdivisions based on degree of structural or functional brain involvement
Congenital onset weakness
WWS/WWS-like	Onset is prenatal or at birth. Patients have severe structural brain abnormalities including complete agyria or severe lissencephaly with only rudimentary cortical folding, marked hydrocephalus, severe cerebellar involvement, and complete or partial absence of the corpus callosum. Eye abnormalities including congenital cataracts, microphthalmia, and buphthalmos are common. Motor development is typically severely impaired with most patients attaining head control only.
MEB/FCMD-like	These categories are merged due to the overlapping phenotypic features. Included in this group are patients with CMD and structural brain defects less severe than those seen with WWS. MRI findings include pachygyria with preferential frontoparietal involvement, polymicrogyria, cerebellar hypoplasia, cerebellar dysplasia, and frequent flattening of the pons and brainstem. Eye abnormalities are often present in this group and include congenital glaucoma, progressive myopia, retinal atrophy and juvenile cataracts. Individuals may rarely acquire the ability to walk, although this is delayed. Patients typically speak only a few meaningful words.
CMD CRB (CMD with cerebellar involvement)	CMD with mental retardation and MRI evidence of cerebellar involvement as the only structural brain abnormality. Cerebellar abnormalities may include cysts, hypoplasia, or dysplasia. Cerebellar cysts in the absence of supratentorial white matter or cortical involvement have been encountered most commonly in patients with defects in FKRP or POMGNT1 .
CMD MR (CMD with mental retardation)	CMD with mental retardation and a structurally normal brain. This category includes patients with isolated microcephaly or minor white matter changes or entirely normal brain on MRI.
CMD no MR (CMD with no mental retardation)	Patients within this group have no evidence of abnormal cognitive development. This category includes patients who have not had neuroimaging performed.