Clinical characteristics.

Hereditary myopathy with early respiratory failure (HMERF) is a slowly progressive myopathy that typically begins in the third to fifth decades of life. The usual presenting findings are gait disturbance relating to distal leg weakness or nocturnal respiratory symptoms due to respiratory muscle weakness. Weakness eventually generalizes and affects both proximal and distal muscles. Most affected individuals require walking aids within a few years of onset; some progress to wheelchair dependence and require nocturnal noninvasive ventilatory support about ten years after onset. The phenotype varies even among individuals within the same family: some remain ambulant until their 70s whereas others may require ventilator support in their 40s.

Diagnosis/testing.

The diagnosis of HMERF is established in a proband with typical clinical findings and/or a heterozygous pathogenic variant in the region of TTN that encodes the 119th fibronectin-3 domain of titin on molecular genetic testing.

Management.

Treatment of manifestations: Management is supportive. For distal leg weakness, use of ankle-foot orthoses can optimize independent ambulation early in the disease course; later in the disease course other mobility aids (canes, walkers, or wheelchairs) may be required. Noninvasive ventilation with bilevel positive airway pressure (BiPAP) or continuous positive airway pressure (CPAP) may be indicated for nocturnal hypoventilation initially, followed by mechanical ventilatory support as needed. Influenza vaccination, occupational therapy, and social service support are important.

Surveillance: Reassessment of muscle strength and clinical status annually by a neurologist; pulmonary function testing every six to 12 months, or guided by individual findings.

Pregnancy management: Although the onset of symptoms usually occurs after the age of childbearing, a pregnant woman with early manifestations of HMERF or at risk for HMERF should be considered high-risk because of the associated respiratory muscle weakness and the increased physiologic demands of pregnancy. Consultation with a high-risk maternal-fetal medicine specialist is recommended when possible.

Genetic counseling.

HMERF is inherited in an autosomal dominant manner with variable expressivity. Most individuals diagnosed with HMERF have an affected parent; to date, de novo pathogenic variants have not been reported in any individuals with genetically confirmed HMERF. Each child of an individual with HMERF has a 50% chance of inheriting the pathogenic variant. If the pathogenic variant has been identified in an affected family member, predictive testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible.

Suggestive Findings

Diagnosis of hereditary myopathy with early respiratory failure (HMERF) should be suspected in individuals with the following:

• Adult-onset muscle disease with onset typically between ages 30 and 50 years (range 22-71 years)
  • The first symptoms usually relate to weakness of the distal leg muscles and may include foot drop or frequent falls.
  • Weakness may also involve the proximal muscles of the lower extremities, proximal and/or distal muscles of the upper extremities, and axial muscles.
  • Affected individuals may appear quite muscular even when weakness is present [Pfeffer et al 2014a].
    In particular, hypertrophy of the calf muscles is frequently reported [Ohlsson et al 2012]; however, atrophy of the calf muscles has also been reported [Pfeffer et al 2012] and may reflect a more advanced disease stage at the time of examination.
  • Serum creatine kinase is usually mildly elevated (range: normal to 1,000 units/L).
• Evidence of respiratory muscle weakness early in the disease course
  Note: Since affected individuals may not report symptoms, they need to be specifically asked about orthopnea, dyspnea on exertion, and excessive daytime sleepiness.
• Family history consistent with autosomal dominant inheritance

Note: Muscle MRI findings and muscle pathology studies can identify supportive evidence but may not be specific to this disorder (see Clinical Description).

Establishing the Diagnosis

The diagnosis of HMERF is established in a proband with typical clinical findings and/or a heterozygous* pathogenic variant in TTN identified by molecular genetic testing (see Table 1).

Note: All HMERF-associated TTN pathogenic variants are located in the 119th fibronectin-3 domain of titin, which corresponds to the following (see Molecular Genetics):

• NM_001267550.2: exon 343
• NM_001256850.1: exon 293
• NM_133378.4: exon 292

* Rare individuals have been reported to be homozygous for the TTN pathogenic variant p.Pro30091Leu; such individuals have been born to parents who are both heterozygous and clinically asymptomatic or subclinically symptomatic (i.e., muscle abnormality demonstrable on imaging) [Palmio et al 2014]

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

Gene-targeted testing requires that the clinician determine which gene(s) are likely involved, whereas genomic testing does not. Because the phenotype of HMERF is broad, individuals with the distinctive findings described in Suggestive Findings are likely to be diagnosed using gene-targeted testing (see Option 1), whereas those with atypical findings in whom the diagnosis of HMERF has not been considered are more likely to be diagnosed using genomic testing (see Option 2).

Clinical Description

Hereditary myopathy with early respiratory failure (HMERF) is a slowly progressive myopathy that typically begins in the third to fifth decades of life [Edström et al 1990, Pfeffer et al 2012].

Genotype-Phenotype Correlations

Although clinical variability is observed with HMERF-related TTN variants, no relationship between the pathogenic variant and phenotype is evident.

Penetrance

Penetrance appears to depend on the pathogenic variant.

For the common p.Cys30071Arg variant, penetrance appears to be complete, although individuals with very late-onset disease have been described (as late as age 71 years); therefore, it is possible that some affected individuals may die from other causes before the disease becomes manifest.

The p.Pro30091Leu variant appears to have reduced penetrance [Pfeffer et al 2014a] in at least one family, where only one of two heterozygous family members developed the disease.

Because the other pathogenic variants have only been described in a few individuals to date [Izumi et al 2013, Toro et al 2013, Palmio et al 2014, Uruha et al 2015, Yue et al 2015, Palmio et al 2019], data are insufficient to draw conclusions regarding their penetrance; however, current observations suggest complete penetrance.

Nomenclature

Hereditary myopathy with early respiratory failure (HMERF) has previously been termed:

• Myopathy with respiratory failure and myofibrillar aggregates [Kinoshita et al 1975, Edström et al 1990];
• Hereditary cytoplasmic body myopathy with early respiratory failure [Jerusalem et al 1979];
• Hereditary inclusion body myopathy with early respiratory failure [Chinnery et al 2001].

The authors prefer the term "myofibrillar myopathy-titinopathy" [Pfeffer et al 2014a] because of the clinical, MRI, and pathologic similarities of HMERF with the myofibrillar myopathies. For pragmatic purposes this term is useful because future cases of HMERF are most likely to be identified among persons with myofibrillar myopathy.

Prevalence

The prevalence of HMERF is not known, but it is most likely under-recognized because of its broad phenotypic spectrum and relatively recent discovery of its underlying genetic etiology.

Two studies have indicated that about 5% of persons with an undiagnosed myofibrillar myopathy have a TTN pathogenic variant and a phenotype consistent with HMERF [Toro et al 2013, Pfeffer et al 2014a]. This suggests that HMERF is a fairly common subtype of myofibrillar myopathy, which itself is rare. Of note, the estimated prevalence of desminopathy in the northeastern United Kingdom (accounting for 3% of myofibrillar myopathy in that population [Pfeffer et al 2014a]) is 0.17:100,000 [Norwood et al 2009].

Evaluations Following Initial Diagnosis

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

Treatment of Manifestations

At present no disease-modifying therapy exists. Management is supportive. Because of the rarity of this disorder, no formal treatment guidelines have been developed, although general recommendations based on clinical experience are provided in Table 4.

Surveillance

No specific guidelines are in place for surveillance of this disorder; general recommendations are provided in Table 5.

Evaluation of Relatives at Risk

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

Pregnancy Management

Information is insufficient to determine if particular issues in HMERF relate to pregnancy. In general, onset of symptoms occurs after the age of childbearing. However, a pregnant woman with early manifestations of HMERF or at risk for HMERF should be considered at high risk because of the associated respiratory muscle weakness and the increased physiologic demands of pregnancy. Consultation with a high-risk maternal-fetal medicine specialist is recommended when possible.

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for 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

Hereditary myopathy with early respiratory failure (HMERF) is inherited in an autosomal dominant manner with variable expressivity.

Note: The TTN pathogenic variant, p.Pro30091Leu, is associated with extremely variable expressivity. Individuals heterozygous for p.Pro30091Leu may have mild clinical manifestations or only subclinical manifestations (i.e., muscle abnormality demonstrable on imaging) while individuals homozygous for the variant are reported to have more severe (and earlier onset) disease manifestations [Palmio et al 2014]. For this reason, the terms "semirecessive" and "semidominant" have been proposed to describe the mode of inheritance associated with the p.Pro30091Leu pathogenic variant [Palmio et al 2014, Tasca & Udd 2018].

Risk to Family Members

Parents of a proband

• Most individuals diagnosed with HMERF are heterozygous for a TTN pathogenic variant inherited from an affected parent.
• To date, de novo pathogenic variants have not been reported in any individuals with genetically confirmed HMERF.
• Molecular genetic testing is recommended for the parents of a proband with an apparent de novo pathogenic variant.
• If the pathogenic variant found in the proband cannot be detected in the leukocyte DNA of either parent, possible explanations include germline mosaicism in a parent or a de novo pathogenic variant in the proband. Neither germline mosaicism nor de novo mutation has been reported; therefore, it is unknown whether germline mosaicism or de novo pathogenic variants occur in this disorder.
• The family history of some individuals diagnosed with HMERF may appear to the negative because of failure to recognize the disorder in family members, early death of the parent before the onset of symptoms, a milder phenotype, or late onset of the disease in the affected parent. Therefore, an apparently negative family history cannot be confirmed until appropriate evaluation and/or molecular genetic testing has been performed on the parents of 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 have the TTN pathogenic variant identified in the proband, the risk to the sibs of inheriting the pathogenic variant is 50%. Note: The HMERF phenotype may vary among individuals within the same family.
• If the proband has a known TTN 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 the parents have not been tested for the TTN pathogenic variant but are clinically unaffected, the risk to the sibs of a proband appears to be low. However, sibs of a proband with clinically unaffected parents are still presumed to be at increased risk for HMERF because of the possibility of reduced penetrance in a heterozygous parent or the theoretic possibility of parental germline mosaicism.
• If both parents are heterozygous for the p.Pro30091Leu TTN pathogenic variant, sibs have a 50% chance of inheriting one pathogenic variant and having mild or subclinical manifestations of HMERF and a 25% chance of inheriting two pathogenic variants and having severe disease manifestations.

Offspring of a proband

• Each child of an individual with heterozygous HMERF-associated pathogenic variants has a 50% chance of inheriting the TTN pathogenic variant.
• All offspring of an individual with biallelic p.Pro30091Leu pathogenic variants will be heterozygous for the TTN pathogenic variant.

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

Related Genetic Counseling Issues

Predictive testing (i.e., testing of asymptomatic at-risk individuals)

• Predictive testing for at-risk relatives is possible once the TTN pathogenic variant has been identified in an affected family member.
• Potential consequences of such testing (including, but not limited to, socioeconomic changes and the need for long-term follow up and evaluation arrangements for individuals with a positive test result) as well as the capabilities and limitations of predictive testing should be discussed in the context of formal genetic counseling prior to testing.

Predictive testing in minors (i.e., testing of asymptomatic at-risk individuals age <18 years)

• For asymptomatic minors at risk for adult-onset conditions for which early treatment would have no beneficial effect on disease morbidity and mortality, predictive genetic testing is considered inappropriate, primarily because it negates the autonomy of the child with no compelling benefit. Further, concern exists regarding the potential unhealthy adverse effects that such information may have on family dynamics, the risk of discrimination and stigmatization in the future, and the anxiety that such information may cause.
• For more information, see the National Society of Genetic Counselors position statement on genetic testing of minors for adult-onset conditions and the American Academy of Pediatrics and American College of Medical Genetics and Genomics policy statement: ethical and policy issues in genetic testing and screening of children.

In a family with an established diagnosis of HMERF it is appropriate to consider testing of symptomatic individuals regardless of age.

Considerations in families with an apparent de novo pathogenic variant. When neither parent of a proband with an autosomal dominant condition has the pathogenic variant identified in the proband or clinical evidence of the disorder, the pathogenic variant is likely de novo. However, non-medical explanations including alternate paternity or maternity (e.g., with assisted reproduction) and undisclosed adoption could also be explored.

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 or at risk.

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 in unknown).

Prenatal Testing and Preimplantation Genetic Testing

Once the TTN pathogenic variant has been identified in an affected family member, prenatal testing for a pregnancy at increased risk 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

Titin is the molecular scaffold protein that spans half of the sarcomere. Pathogenic variants in different domains of titin can result in different disorders affecting muscle tissue, with combinations of cardiomyopathy, proximal myopathy, distal myopathy, and respiratory failure, the presentation of which can range from congenital to very late onset.

Pathogenic variants causing HMERF are all located within the 119th fibronectin-3 domain of titin (see exon designations below). The function of this domain and disease mechanism of HMERF are unknown. Genetic constructs expressing the FN119 domain with HMERF-associated variants demonstrated reduced solubility compared to normal [Hedberg et al 2014], suggesting that myofibrillar aggregates may cause disease pathogenesis and the myopathologic resemblance to myofibrillar myopathy.

Mechanism of disease causation. The mechanism of disease causation is presumably gain of function, similar to the mechanism causing other myofibrillar myopathies; however, this has not to date been formally studied or proven for HMERF.

TTN-specific laboratory technical considerations. All HMERF-associated pathogenic variants reside in a single TTN exon, which contains non-repetitive sequence.

The FN119 domain exon corresponds to the following:

• NM_001267550.2: exon 343
• NM_001256850.1: exon 293
• NM_133378.4: exon 292

Acknowledgments

GP is an Assistant Professor and clinical neurologist at the University of Calgary and Department of Clinical Neurosciences. He is the recipient of funding from the Canada Foundation for Innovation, and Muscular Dystrophy Canada. PFC is an Honorary Consultant Neurologist at Newcastle upon Tyne Foundation Hospitals NHS Trust, a Wellcome Trust Senior Fellow in Clinical Science (084980/Z/08/Z), and a UK NIHR Senior Investigator.

PFC receives additional support from the Wellcome Trust Centre for Mitochondrial Research (096919Z/11/Z), the Medical Research Council (UK) Centre for Translational Research in Neuromuscular Diseases, and EU FP7 TIRCON, and the National Institute for Health Research (NIHR) Newcastle Biomedical Research Centre based at Newcastle upon Tyne Hospitals NHS Foundation Trust and Newcastle University.

Revision History

• 14 April 2022 (gm) Revision: correction of nomenclature for 119th fibronectin-3 domain of titin (see Establishing the Diagnosis and Molecular Genetics) [Chauveau et al 2014]
• 19 March 2020 (ha) Comprehensive update posted live
• 27 February 2014 (me) Review posted live
• 5 December 2013 (gp) Original submission

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