Calpainopathy

Clinical characteristics.

Calpainopathy is characterized by symmetric and progressive weakness of proximal limb-girdle muscles. Clinical findings of calpainopathy include the tendency to walk on tiptoe, difficulty in running, scapular winging, waddling gait, laxity of the abdominal muscles, Achilles tendon shortening, and scoliosis. Affected individuals typically do not have cardiac involvement or intellectual disability.

Three autosomal recessive calpainopathy phenotypes have been identified based on the distribution of muscle weakness and age at onset:

• Pelvifemoral limb-girdle muscular dystrophy (LGMD) (Leyden-Möbius LGMD) phenotype, the most frequently observed calpainopathy phenotype, in which muscle weakness is first evident in the pelvic girdle and later in the shoulder girdle, with onset that may occur as early as before age 12 years or as late as after age 30 years
• Scapulohumeral LGMD (Erb LGMD) phenotype, usually a milder phenotype with infrequent early onset, in which muscle weakness is first evident in the shoulder girdle and later in the pelvic girdle
• HyperCKemia, usually observed in children or young individuals, in which individuals are asymptomatic and have high serum creatine kinase (CK) concentrations

The autosomal dominant form of calpainopathy is clinically variable, ranging from almost asymptomatic to wheelchair dependence after age 60 years in a few individuals; phenotype is generally milder than the recessive form.

Diagnosis/testing.

The diagnosis of calpainopathy is established by identification of biallelic pathogenic variants in CAPN3 or a dominantly acting heterozygous CAPN3 pathogenic variant by molecular genetic testing. Muscle biopsy showing absent or severely reduced calpain-3 on immunoblot analysis can confirm the diagnosis if molecular testing is inconclusive.

Management.

Treatment of manifestations: Physical therapy and stretching exercises to promote mobility and prevent contractures; supervised strengthening and gentle low-impact aerobic exercise; nutrition management as needed to maintain appropriate weight for height; mobility aids such as canes, walkers, orthotics, and wheelchairs to help maintain independence; knee-ankle-foot orthoses while sleeping to prevent contractures; positioning and seating devices to prevent scoliosis; surgery for foot deformities, scoliosis, and Achilles tendon contractures as needed; scapular fixation as needed for scapular winging; annual influenza vaccine; prompt treatment of chest and respiratory infections; nocturnal ventilator assistance as needed; respiratory aids to treat chronic respiratory insufficiency in late stages of the disease; social, emotional, and family support for care decisions.

Surveillance: Monitor muscle strength, joint range of motion, and orthopedic complications annually; assess for nocturnal hypoventilation annually; pulmonary evaluation as needed with forced vital capacity assessed in the sitting and supine position; examination of cardiac function in those with advanced disease as needed; assess need for social work support at each visit.

Agents/circumstances to avoid: Strenuous and excessive muscle exercise; obesity and excessive weight loss; physical trauma, bone fractures, and prolonged immobility. Avoid succinylcholine and halogenated anesthetic agents when possible; avoid cholesterol-lowering agents (e.g., statins) when possible.

Evaluation of relatives at risk: It is appropriate to clarify the status of apparently asymptomatic older and younger at-risk relatives of an affected individual in order to identify as early as possible those who would benefit from initiation of evaluation and subsequent surveillance.

Genetic counseling.

Calpainopathy is typically inherited in an autosomal recessive manner. Less commonly, calpainopathy is inherited in an autosomal dominant manner.

• Autosomal recessive inheritance: If both parents are known to be heterozygous for a pathogenic variant associated with autosomal recessive calpainopathy, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once the CAPN3 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives is possible.
• Autosomal dominant inheritance: Each child of an individual with autosomal dominant calpainopathy has a 50% chance of inheriting the CAPN3 pathogenic variant.

Once the CAPN3 pathogenic variant(s) have been identified in an affected family member, prenatal and preimplantation genetic testing for calpainopathy are possible.

Suggestive Findings

Calpainopathy should be suspected in individuals with the following clinical, laboratory, imaging, and electromyogram (EMG) findings.

Clinical findings

• Proximal muscle weakness (pelvic and/or shoulder girdle) with early onset (age <12 years), adult onset, or late onset (age >30 years)
• Symmetric atrophy and wasting of proximal limb and trunk muscles; calf hypertrophy is rarely and sometimes only transiently present [Fardeau et al 1996].
• Scapular winging, scoliosis, Achilles tendon contracture, and other joint contractures (including hip, knee, elbow, finger, and spine)
• Waddling gait; tiptoe walking; difficulty in running, climbing stairs, lifting weights, and getting up from the floor or from a chair
• Sparing of facial, ocular, tongue, and neck muscles
• Absence of cardiomyopathy and intellectual disability
• Back pain and myalgia; present in 50% of individuals with autosomal dominant calpainopathy [Vissing et al 2016]

Serum creatine kinase (CK) concentration is elevated (5-80 times normal) in autosomal recessive calpainopathy from early infancy, particularly during the active stage of the disease. Serum CK concentration decreases with disease progression, as muscles become progressively atrophic [Urtasun et al 1998]. In autosomal dominant calpainopathy, some individuals presented with normal CK levels [Vissing et al 2016].

Muscle imaging findings

• CT reveals early and striking wasting of muscles predominantly from the posterior compartment of the thighs (but also hip adductors and quadriceps) [Fardeau et al 1996, Urtasun et al 1998], with sparing of the sartorius and gracilis muscles. The difference in distribution of muscle involvement between calpainopathy and other LGMDs (e.g., sarcoglycanopathies, dysferlinopathies) makes muscle imaging analysis an important element in clinical diagnosis [ten Dam et al 2012] (see Differential Diagnosis).
• MRI shows the selective muscle involvement and, in some instances, reveals edema-like changes on STIR sequences [Mercuri et al 2005, Borsato et al 2006, Stramare et al 2010, Kana et al 2014]. Muscle MRI is a useful tool in characterizing the degree of muscle atrophy and in some instances disease severity [Straub et al 2012, Díaz-Manera et al 2015].
• In the dominant form of calpainopathy, the paraspinal muscles are almost completely lost and replaced by fat [Prahm et al 2017].

Electromyogram (EMG) pattern is typically myopathic (showing small polyphasic potentials), although a normal EMG can also be observed in presymptomatic individuals. Myotonia and spontaneous discharges are not present.

Establishing the Diagnosis

The diagnosis of calpainopathy is established in a proband with suggestive findings and one of the following identified by molecular genetic testing (see Table 1):

• Biallelic pathogenic (or likely pathogenic) variants in CAPN3
• Heterozygous dominantly acting CAPN3 pathogenic variant

Note: (1) Per ACMG/AMP variant interpretation guidelines, the terms "pathogenic variants" and "likely pathogenic variants" are synonymous in a clinical setting, meaning that both are considered diagnostic, and both can be used for clinical decision making [Richards et al 2015]. Reference to "pathogenic variants" in this section is understood to include any likely pathogenic variants. (2) The identification of variant(s) of uncertain significance cannot be used to confirm or rule out the diagnosis.

Molecular genetic testing approaches can include use of a multigene panel, comprehensive genomic testing (exome sequencing, genome sequencing), and single-gene testing depending on the phenotype (see Table 1).

The clinical and laboratory findings in individuals with calpainopathy overlap with other forms of LGMD and other muscular dystrophies. Therefore, use of a multigene panel or comprehensive genomic testing is recommended [Thompson & Straub 2016]. If biallelic CAPN3 pathogenic variants or a heterozygous dominantly acting CAPN pathogenic variant are not identified, other including muscle imaging and muscle biopsy with protein immunoanalysis (see Muscle Biopsy, Calpain-3 immunoblot analysis) testing should be considered.

Other Molecular Testing Options

Single-gene testing may be considered if a multigene panel and/or more comprehensive genomic testing is not available and calpainopathy appears to be a likely diagnosis based on clinical findings. Sequence analysis of CAPN3 is performed first, followed by gene-targeted deletion/duplication analysis if only one or no pathogenic variant is found.

Note: Targeted analysis for pathogenic variants can be performed first in individuals of Amish ancestry or individuals from communities with an identified founder variant (e.g., Tlaxcala, Mexico; Mòcheni community, Italy; La Réunion Island; Chioggia, Italy; Guipúzcoa Province, Spain) (see Table 6).

Table 1.

Molecular Genetic Testing Used in Calpainopathy

Clinical Description

Calpainopathy is characterized by symmetric and progressive weakness of proximal limb-girdle muscles, symmetric muscle atrophy of the proximal limb and trunk muscles, scapular winging, scoliosis, and joint contractures. The age at onset of muscle weakness ranges from two to 40 years. Early motor milestones are usually normal. Significant intra- and interfamilial clinical variability is seen [Richard et al 1999, Fanin & Angelini 2015].

Three phenotypes of autosomal recessive calpainopathy have been identified based on the distribution of muscle weakness and age at onset:

• Pelvifemoral limb-girdle muscular dystrophy (LGMD) (Leyden-Möbius LGMD) phenotype, the most frequently observed calpainopathy phenotype. Muscle weakness is first evident in the pelvic girdle and later in the shoulder girdle. Onset can be early (age <12 years), adult (age 12-30 years), or late (age >30 years). Individuals with early onset and rapid disease course usually have pelvifemoral LGMD.
• Scapulohumeral LGMD (Erb LGMD) phenotype. Muscle weakness is first evident in the shoulder girdle and later in the pelvic girdle. Early onset is infrequent; the disease course is variable, but usually milder than that in the pelvifemoral phenotype.
• HyperCKemia. HyperCKemia may be considered a presymptomatic stage of calpainopathy, as it is usually observed in children or in young persons with recessive calpainopathy [Fanin et al 2009a, Kyriakides et al 2010]. Asymptomatic individuals may develop symptoms of muscle weakness later.

The first clinical findings of calpainopathy are usually:

• Tendency to walk on tiptoe;
• Difficulty in running;
• Scapular winging.

Early stage of the disorder. The following are frequently observed:

• Waddling gait and slight hyperlordosis
• Symmetric weakness [Matsubara et al 2007] of proximal more than distal limb muscles, trunk, and periscapular muscles. In particular, the gluteus maximus, thigh adductors, and posterior compartment of the limbs may be severely affected [Fardeau et al 1996, van der Kooi et al 1996, Dinçer et al 1997, Topaloğlu et al 1997, Urtasun et al 1998]; facial and neck muscles are usually spared.
• Scapular winging
• Marked laxity of the abdominal muscles [Bushby 1999, Pollitt et al 2001]
• Early Achilles tendon shortening and scoliosis

Variable findings include the following:

• Muscle pain, exercise intolerance, and elevated lactate levels in some individuals similar to that seen in a pseudometabolic myopathy [Pénisson-Besnier et al 1998, Pollitt et al 2001]
• Eosinophilic myositis with increased serum CK, an early and transient feature that is not present in older individuals [Brown & Amato 2006, Krahn et al 2006b, Krahn et al 2011]
• Significant atrophy of the calf muscle or more rarely calf hypertrophy
• Rhabdomyolysis (and/or myoglobinuria) triggered by physical exercise; occasionally observed in asymptomatic individuals or in individuals with mild muscle involvement [Lahoria & Milone 2016]

Advanced stage of the disorder. Commonly observed findings:

• The inability to climb stairs, rise up from a chair, lift weights, or get up from the floor
• Joint contractures (in the hips, knees, elbows, and fingers)

Occasionally observed findings:

• Rigid spine [Pollitt et al 2001]
• Foot drop [Burke et al 2010]
• Respiratory insufficiency with reduced lung vital capacity to 30%-50% due to deficiency in diaphragmatic function, weakness in thoracic and abdominal muscles, and scoliosis [Fardeau et al 1996, Urtasun et al 1998]. Compromised respiratory function was observed in 11% of individuals, showing forced vital capacity lower than 50% [Richard et al 2016]. Some of those with severe respiratory insufficiency required the use of respiratory aids [Mori-Yoshimura et al 2017].

Uncommon findings include cardiomyopathy. In most individuals, cardiac symptoms that precede cardiac morbidity are not present (e.g., chest pain, lower limb edema, palpitations), and cardiac abnormalities may only be identified by echocardiography or electrocardiography. A systematic cardiac evaluation in affected individuals using cardiovascular MR showed no cardiac involvement, even in individuals of advanced age with severe disease [Quick et al 2015]. A few individuals have presented with non-life-threatening cardiac abnormalities [Richard et al 2016], atrial fibrillation, or variably impaired left ventricular function [Mori-Yoshimura et al 2017].

Note: Intellectual disability is not associated with this disorder. Macroglossia, described in affected individuals from a genetic isolate in the Alps [Fanin et al 2012], also does not to be associated with calpainopathy.

Progression and variability. The asymptomatic stage may be relatively long in some affected individuals, especially in females. In some individuals with calpainopathy, the onset of symptoms or the worsening of symptoms may be influenced by environmental factors, such as infectious disease, strenuous physical exercise, drug treatment, a traumatic event, or pregnancy [Sáenz et al 2005].

The disease is invariably progressive, and loss of ambulation occurs approximately ten to 30 years after the onset of symptoms (range: ages 10-48 years) [Richard et al 1999, Zatz et al 2003, Sáenz et al 2005, Angelini et al 2010, Gallardo et al 2011, Richard et al 2016]. In general, loss of independent ambulation occurs earlier in individuals with childhood onset [Gallardo et al 2011].

A more rapid progression was observed in males than in females [Richard et al 2016]. In a natural history study, a higher proportion of females remained ambulatory as compared to males (72% vs 48%) [Richard et al 2016]. Males are more susceptible to muscle fiber atrophy and have increased muscle weakness and clinical disability [Fanin et al 2014].

Intrafamilial variability in the clinical phenotype has been reported: in sibs with the same pathogenic variants the age at onset and the clinical course can vary considerably [Schessl et al 2008].

Autosomal dominant calpainopathy has a variable clinical phenotype, ranging from almost asymptomatic to wheelchair dependent after age 60 years in a small number of individuals [Vissing et al 2016]. A prominent feature of such individuals is back pain and myalgia (present in more than 50% of heterozygotes for CAPN3 pathogenic variant c.643_663del21). The average age of onset of muscle weakness is 34 years, 16 years later than individuals with autosomal recessive calpainopathy. The clinical phenotype of autosomal dominant calpainopathy is generally milder than autosomal recessive calpainopathy.

Genotype-Phenotype Correlations

There are no consistent genotype-phenotype correlations in calpainopathy, although null homozygous variants are generally associated with a severe phenotype and absent calpain-3 protein in muscle [Richard et al 1999].

Individuals who are compound heterozygous for CAPN3 variant c.1746-20C>G and another variant consistently present with a phenotype of mild-to-moderate severity. This variant is most frequently identified in individuals from northern and western regions of Russia and may originate from this region [Mroczek et al 2022].

CAPN3 variants in proximity to the calmodulin-binding site, which are predicted to interfere with proteolytic activation, are associated with autosomal dominant calpainopathy [González-Mera et al 2021].

Penetrance

Nearly full penetrance is observed by adulthood. Serum CK concentration is usually increased until the advanced stage of the disease.

Nomenclature

Calpainopathy was originally called LGMD2A because it was the first form of autosomal recessive LGMD to be mapped [Beckmann et al 1991]. The designation LGMDR1 has been proposed in revised nomenclature (LGMDR refers to genetic types of LGMD showing autosomal recessive inheritance).

Vissing et al [2016] proposed that autosomal dominant calpainopathy associated with CAPN3 pathogenic variant c.643_663del21 be designated LGMD1I in the current nomenclature (LGMD1 refers to genetic types of LGMD showing dominant inheritance), and designated LGMDD4 in revised nomenclature [Straub et al 2018] (LGMDD refers to genetic types of LGMD showing autosomal dominant inheritance).

As both recessive and dominant forms are associated with CAPN3 pathogenic variants, calpainopathy is the preferred term for this disorder.

Prevalence

Calpainopathy is the most common form of LGMD [Bushby & Beckmann 2003, Guglieri et al 2008], accounting for 30% of LGMD worldwide (range: 4%-80% depending on the geographic region) [Chou et al 1999, Zatz et al 2000].

A study in northeastern Italy estimated that calpainopathy has a prevalence of approximately 1:100,000 inhabitants (corresponding to a carrier frequency of ~1:160) [Fanin et al 2005]. Another study in southern Italy estimated the prevalence of calpainopathy at 1:42,700 inhabitants (corresponding to a carrier frequency of ~1:103) [Piluso et al 2005]. Three general population screening studies of the most common CAPN3 pathogenic variant (c.550delA) in Lithuania, Croatia, and Poland identified carrier frequencies of 1:175, 1:133, and 1:124, respectively [Canki-Klain et al 2004, Dorobek et al 2015, Inashkina et al 2016].

Higher prevalence rates have been calculated in genetically isolated communities; the prevalence of the disease has been estimated at 48:1,000,000 in La Réunion Island [Fardeau et al 1996], 69:1,000,000 in Basque country [Urtasun et al 1998], 1,900:1,000,000 in the Mòcheni community in the Alps [Fanin et al 2012], 4,300:1,000,000 in the Tlaxcala village in central Mexico (with a carrier frequency of 1:11) (see Table 6) [Pantoja-Melendez et al 2017], and 13,000:1,000,000 among the Amish population of Indiana [Young et al 1992, Richard et al 1995].

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with calpainopathy, the evaluations summarized in Table 3 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Table 3.

Recommended Evaluations Following Initial Diagnosis in Individuals with Calpainopathy

Treatment of Manifestations

Table 4.

Treatment of Manifestations in Individuals with Calpainopathy

Surveillance

To monitor existing manifestations, the individual's response to supportive care, and the emergence of new manifestations, the evaluations in Table 5 are recommended.

Table 5.

Recommended Surveillance for Individuals with Calpainopathy

Agents/Circumstances to Avoid

Strenuous and excessive muscle exercise should be discouraged as it exacerbates muscle necrosis and could precipitate the onset of weakness or accelerate muscle wasting. Although individuals with minimal muscle weakness and functional limitation may be able to perform strenuous exercise, in some instances this may result in rhabdomyolysis and myoglobinuria [Lahoria & Milone 2016], which may lead to severe complications such as acute kidney failure and compartment syndrome.

Body weight should be controlled to avoid obesity as well as excessive weight loss (atrophy of muscles can be accelerated by loss of muscle proteins).

Physical trauma, bone fractures, and prolonged immobility can induce disuse atrophy and thus should be avoided.

Although no association of the disease with malignant hyperthermia is reported, the use of succinylcholine and halogenated anesthetic agents should be avoided when possible (see Malignant Hyperthermia Susceptibility).

While the specific mechanism whereby cholesterol-lowering agents (e.g., statins) may produce muscle damage causing pain or weakness is unknown, such drugs should be avoided when possible.

Evaluation of Relatives at Risk

It is appropriate to clarify the status of apparently asymptomatic older and younger at-risk relatives of an affected individual in order to identify as early as possible those who would benefit from initiation of evaluation and subsequent surveillance. Evaluations can include:

• Molecular genetic testing if the pathogenic variant(s) in the family are known;
• Neurologic examination for muscle weakness if the pathogenic variant(s) in the family are not known.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Pregnancy Management

Women with calpainopathy do not have impaired uterine smooth muscle strength or function and typically have uncomplicated pregnancies. A higher incidence of abnormal fetal presentation was reported in pregnant women with LGMD who were wheelchair bound [Awater et al 2012]. Epidural blockade can be difficult in those with severe spine deformities and appropriate general anesthesia may be necessary. About half of persons with LGMD reported deterioration of clinical symptoms in pregnancy [Awater et al 2012].

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Mode of Inheritance

Calpainopathy is typically caused by biallelic pathogenic variants and inherited in an autosomal recessive manner. Less commonly, calpainopathy is caused by a heterozygous, dominantly acting pathogenic variant and inherited in an autosomal dominant manner [Vissing et al 2016, González-Mera et al 2021].

Autosomal Recessive Inheritance – Risk to Family Members

Parents of a proband

• The parents of an individual with autosomal recessive calpainopathy are presumed to be heterozygous for a CAPN3 pathogenic variant.
• If a molecular diagnosis has been established in the proband, molecular genetic testing is recommended for the parents of the proband to confirm that both parents are heterozygous for a CAPN3 pathogenic variant and to allow reliable recurrence risk assessment.
• If a pathogenic variant is detected in only one parent and parental identity testing has confirmed biological maternity and paternity, it is possible that one of the pathogenic variants identified in the proband occurred as a de novo event in the proband or as a postzygotic de novo event in a mosaic parent [Jónsson et al 2017]. If the proband appears to have homozygous pathogenic variants (i.e., the same two pathogenic variants), additional possibilities to consider include:
  • A single- or multiexon deletion in the proband that was not detected by sequence analysis and that resulted in the artifactual appearance of homozygosity;
  • Uniparental isodisomy for the parental chromosome with the pathogenic variant that resulted in homozygosity for the pathogenic variant in the proband.
• Individuals who are heterozygous for a pathogenic variant associated with autosomal recessive calpainopathy are asymptomatic and are not at risk of developing the disorder.

Sibs of a proband

• If both parents are known to be heterozygous for a pathogenic variant associated with autosomal recessive calpainopathy, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
• Intrafamilial variability in the clinical phenotype has been reported: in sibs with the same pathogenic variants, the age at onset and the clinical course can vary considerably [Schessl et al 2008, Landires et al 2020].
• Individuals who are heterozygous for a pathogenic variant associated with autosomal recessive calpainopathy are asymptomatic and are not at risk of developing the disorder.

Offspring of a proband

• Unless an affected individual's reproductive partner also has calpainopathy or is a carrier, offspring will be obligate heterozygotes for a pathogenic variant in CAPN3.
• Carrier testing for the reproductive partner of an affected individual should be considered, particularly if consanguinity is likely and/or if both partners are of the same ethnic background. Higher prevalence rates have been calculated in genetically isolated communities (see Prevalence).

Other family members. Each sib of the proband's parents is at a 50% risk of being heterozygous for a CAPN3 pathogenic variant associated with autosomal recessive calpainopathy.

Carrier detection. Carrier testing for at-risk relatives requires prior identification of the CAPN3 pathogenic variants in the family.

Note: Carrier testing for the reproductive partner of a known carrier should be considered, particularly if consanguinity is likely and/or if both partners are of the same ethnic background. Higher prevalence rates have been calculated in genetically isolated communities (see Prevalence).

Autosomal Dominant Inheritance – Risk to Family Members

Parents of a proband

• All individuals reported to date with autosomal dominant calpainopathy whose parents have undergone molecular genetic testing have inherited a CAPN3 pathogenic variant from a heterozygous parent. Clinical variability has been reported within families [Vissing et al 2016] and a heterozygous parent may or may not have clinical features of calpainopathy.
• To date, de novo occurrence of the autosomal dominant form of calpainopathy has not been reported.
• If a molecular diagnosis has been established in the proband and the proband appears to be the only affected family member (i.e., a simplex case), molecular genetic testing is recommended for the parents of the proband to confirm their genetic status and to allow reliable recurrence risk counseling.
• If the pathogenic variant identified in the proband is not identified in either parent and parental identity testing has confirmed biological maternity and paternity, the following possibilities should be considered:
  • The proband has a de novo pathogenic variant.
  • The proband inherited a pathogenic variant from a parent with germline (or somatic and germline) mosaicism. Note: Testing of parental leukocyte DNA may not detect all instances of somatic mosaicism and will not detect a pathogenic variant that is present in the germ cells only.
• If the proband has a known CAPN3 pathogenic variant that cannot be detected in the leukocyte DNA of either parent, possible explanations include a de novo variant in the proband or germline mosaicism in a parent (though theoretically possible, no instances of germline mosaicism have been reported).
• The family history of some individuals diagnosed with autosomal dominant calpainopathy may appear to be negative because of early death of the parent before the onset of symptoms, late onset of the disease in the affected parent (see Penetrance), or subclinical manifestations of calpainopathy in a heterozygous parent [Vissing et al 2016]. Therefore, an apparently negative family history cannot be confirmed unless molecular genetic testing has demonstrated that neither parent is heterozygous for the pathogenic variant identified in the proband.

Sibs of a proband. The risk to the sibs of the proband depends on the genetic status of the proband's parents:

• If a parent of the proband is affected and/or is known to be heterozygous for the CAPN3 pathogenic variant identified in the proband, the risk to sibs is 50%. Significant intrafamilial clinical variability has been observed between family members heterozygous for a pathogenic variant associated with autosomal dominant calpainopathy [Vissing et al 2016].
• If the proband has a known CAPN3 pathogenic variant that cannot be detected in the leukocyte DNA of either parent, the recurrence risk to sibs is estimated to be 1% because of the theoretic possibility of parental germline mosaicism [Rahbari et al 2016].
• If both parents are clinically unaffected but their genetic status is unknown, the sibs of a proband are still at increased risk for calpainopathy because of the possibility of reduced penetrance in a heterozygous parent or the theoretic possibility of parental germline mosaicism.

Offspring of a proband. Each child of an individual with autosomal dominant calpainopathy has a 50% chance of inheriting the CAPN3 pathogenic variant.

Other family members. The risk to other family members depends on the status of the proband's parents: if a parent is affected/has the CAPN3 pathogenic variant, the parent's family members may be at risk.

Related Genetic Counseling Issues

See Management, Evaluation of Relatives at Risk for information on evaluating at-risk relatives for the purpose of early diagnosis and treatment.

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are heterozygous, or are at risk of being heterozygous (e.g., asymptomatic relatives of known affected individuals).

DNA banking. Because it is likely that testing methodology and our understanding of genes, pathogenic mechanisms, and diseases will improve in the future, consideration should be given to banking DNA from probands in whom a molecular diagnosis has not been confirmed (i.e., the causative pathogenic mechanism is unknown). For more information, see Huang et al [2022].

Prenatal Testing and Preimplantation Genetic Testing

Once the CAPN3 pathogenic variant(s) have been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing. While most centers would consider use of prenatal testing to be a personal decision, discussion of these issues may be helpful.

Molecular Pathogenesis

Calpain-3 is the muscle-specific member of a family of Ca⁺⁺-activated neutral proteases that cleave proteins into short polypeptides. Calpain-3 is expressed predominantly in skeletal muscle. Upon stimulation, calpain-3 both activates and inactivates itself rapidly through autocatalysis. In the sarcomeres, calpain-3 directly binds to titin [Keira et al 2003] and changes its localization from the M-lines to the NA2 regions as the sarcomeres extend. Calpain-3 is thought to process proteins involved in signaling pathways, transcription factors, calcium transport, and cytoskeletal proteins as part of a process called sarcomere remodeling, in which the synthesis of novel proteins is balanced by the degradation of misfolded proteins [Baghdiguian et al 1999, Baghdiguian et al 2001, Kramerova et al 2005, Duguez et al 2006, Kramerova et al 2007, Beckmann & Spencer 2008, Benayoun et al 2008, Kramerova et al 2008, Sáenz et al 2008, Fanin et al 2009c, Ono et al 2010, Ermolova et al 2011].

Most individuals with calpainopathy have complete or partial calpain-3 protein deficiency on muscle biopsy. In 10%-30% of individuals with calpainopathy, muscle biopsy shows a normal amount of protein [Talim et al 2001, de Paula et al 2002, Fanin et al 2004, Groen et al 2007, Milic et al 2007, Fanin et al 2009b] even though calpain-3 may have lost its autocatalytic activity and may be functionally inactive [Fanin et al 2003, Fanin et al 2007b].

The mobility of calpain-3 between the sarcomeric M-lines and the cytosol may have a key role in physical stress, and it is compromised in calpainopathy when its protease activity has been lost. An impairment of calpain proteolytic activity results in sarcomere remodeling by promoting ubiquitin-mediated degradation of sarcomeric proteins [Duguez et al 2006]. This degradation process may depend on ubiquitous calpains in the initial stage, and on the ubiquitin-proteasome system (UPS) in the later stages [Kramerova et al 2005, Beckmann & Spencer 2008, Rajakumar et al 2013]. Impaired sarcomere remodeling would also affect myoblast fusion and repair, as well as the regenerative capacity of muscle in calpainopathy. The activation of the muscle atrophy process appears to depend mainly on the induction of the UPS [Fanin et al 2013].

Mechanism of disease causation

• The majority of CAPN3 pathogenic variants are loss-of-function variants and result in recessive disease.
• The mechanism by which CAPN3 pathogenic variant c.643_663del21 results in autosomal dominant calpainopathy has been proposed to be a dominant-negative effect [Vissing et al 2016]. Since the active calpain-3 is a homodimer, the aberrant protein could polymerize with the wild type protein and render the complex inactive.

CAPN3-specific laboratory technical considerations. Alternate CAPN3 promoters and alternative splicing result in multiple transcript variants encoding different isoforms [De Tullio et al 2003, Kawabata et al 2003]. Muscle tissue expresses only one isoform (the full-length transcript), whereas leukocytes express four different transcripts (produced by alternative splicing of exons 6, 15, and 16), all of which lack exon 15. Since peripheral blood instead of muscle tissue has increasingly been used to obtain mRNA for cDNA sequencing, the results of the two analyses could be discordant in some instances [Blázquez et al 2008].

Many deep intronic pathogenic variants that disrupt the correct splicing can be overlooked by sequencing of genomic DNA [Krahn et al 2007]; their identification may require the sequencing of cDNA obtained from muscle or blood tissues [Krahn et al 2006a, Blázquez et al 2008, Nascimbeni et al 2010]. Deep intronic variants causing a pseudoexonization of an intronic sequence have been reported [Blázquez et al 2008, Blázquez et al 2013].

A small number of CAPN3 pathogenic variants are associated with autosomal dominant calpainopathy [Vissing et al 2016, González-Mera et al 2021]. Of note, one of these variants (c.643_663del21) has been identified in compound heterozygosity with other pathogenic CAPN3 variants [Richard et al 1997, Groen et al 2007, Sáenz & López de Munain 2017].

Table 6.

Notable CAPN3 Pathogenic Variants

Author Notes

Coalition to Cure Calpain 3
15 Compo Parkway
Westport, CT 06880
Web page: www.curecalpain3.org
Email: gro.3niaplaceruc@ofni

Acknowledgments

Dr Corrado Angelini acknowledges the previous contribution to this chapter by Dr Marina Fanin, a major contributor to calpain research, who first described the intronic variant in calpain-3 in a genetic isolate in the Alps [Fanin et al 2012]. Dr Fanin unfortunately died in April 2022.

Author History

Corrado Angelini, MD (2005-present)
Marina Fanin, PhD; University of Padova (2005-2022)

Revision History

• 1 December 2022 (sw) Comprehensive update posted live
• 3 August 2017 (ha) Comprehensive update posted live
• 5 July 2012 (me) Comprehensive update posted live
• 8 July 2010 (cd) Revision: deletion/duplication analysis available clinically
• 3 December 2007 (me) Comprehensive update posted live
• 15 December 2005 (ca) Revision: prenatal diagnosis available
• 10 May 2005 (me) Review posted live
• 29 November 2004 (ca) Original submission

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