Clinical characteristics.

FKBP14 kyphoscoliotic Ehlers-Danlos syndrome (FKBP14-kEDS) is characterized by congenital muscle hypotonia and weakness (typically improving during childhood), progressive scoliosis, joint hypermobility, hyperelastic skin, gross motor developmental delay, myopathy, and hearing impairment. Most affected children achieve independent walking between ages two and four years. A decline of motor function in adulthood may be seen, but affected individuals are likely to be able to participate in activities of daily living in adulthood and maintain independent walking. Occasional features underlying systemic connective tissue involvement include aortic rupture and arterial dissection, subdural hygroma, insufficiency of cardiac valves, bluish sclerae, bladder diverticula, inguinal or umbilical herniae, and premature rupture of membranes during pregnancy. Rarer findings may include bifid uvula with submucous or frank cleft palate, speech/language delay without true cognitive impairment, and rectal prolapse.

Diagnosis/testing.

Clinical diagnostic criteria rely on the finding of congenital muscular hypotonia AND congenital or early-onset kyphoscoliosis in addition to generalized joint hypermobility or further gene-specific and/or supportive clinical features. The diagnosis of FKBP14-kEDS is established in a proband by the identification of biallelic pathogenic variants in FKBP14 by molecular genetic testing.

Management.

Treatment of manifestations: In those with aortic dilatation or vascular dissection, use of beta-blockers may be considered; physical and occupational therapy to address age-dependent decline in muscular strength; standard treatment for severe scoliosis, clubbed foot, osteopenia/osteoporosis, refractive error, hearing impairment, and cleft palate.

Surveillance: Blood pressure measurement at each visit; neurodevelopmental assessment at each visit until adolescence; evaluation by an orthopedic physician as clinically indicated but typically at least annually; periodic ophthalmology and hearing evaluations (e.g., every 2-3 years); DXA scan, echocardiogram with consideration of cardiac MRI, and vascular ultrasonography every 2-5 years.

Agents/circumstances to avoid: Sports that place stress on the joints; contact sports in those with an aortic aneurysm; hypertension.

Pregnancy management: An increased risk for miscarriage, premature rupture of membranes, and rupture of arteries in affected pregnant women should be considered. Delivery in a medical center with a high-risk perinatologist in attendance is recommended.

Genetic counseling.

FKBP14-kEDS is inherited in an autosomal recessive manner. Each sib of an affected individual has 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. Carrier testing for at-risk relatives and prenatal testing for a pregnancy at increased risk are possible if both FKBP14 pathogenic variants have been identified in a family.

Suggestive Findings

FKBP14 kyphoscoliotic Ehlers-Danlos syndrome (FKBP14-kEDS) should be suspected in individuals with kyphoscoliosis, severe congenital muscle hypotonia, and joint hypermobility

Major and minor clinical features of FKBP14-kEDS have been outlined as follows (adapted from Malfait et al [2017] and Giunta et al [2018b]).

Major clinical features

• Congenital muscular hypotonia
• Congenital or early-onset kyphoscoliosis
• Generalized joint hypermobility

Gene-specific minor features

• Early-onset sensorineural, conductive, or mixed hearing impairment (See Clinical Description.)
• Muscle atrophy
• Follicular hyperkeratosis
• Bladder diverticula

Other suggestive findings

• Marfanoid habitus
• Pectus deformity
• Talipes equinovarus
• Skin hyperextensibility
• Easily bruisable skin
• Hernia (umbilical or inguinal)
• Rupture/aneurysm of a medium-sized artery
• Blue sclerae
• Refractive errors (myopia, hypermetropia)
• Osteopenia/osteoporosis

Supportive laboratory findings

• Normal or only slightly elevated serum creatine kinase (CK) level
• Histopathology of muscle biopsies showing nonspecific mild myopathic changes with increased variation in muscle fiber diameter to more pronounced changes with profound fiber atrophy and proliferation of fatty tissue

Note: At the time of writing, muscle biopsy is not required to make the diagnosis of FKBP14-kEDS.

Supportive imaging findings. MRI of the lower limbs that may demonstrate fatty degeneration of multiple muscle groups, including rectus femoris and soleus

Establishing the Diagnosis

Proposed minimal clinical diagnostic criteria for FKBP14-kEDS include the following [Malfait et al 2017]:

• Congenital muscular hypotonia AND congenital or early-onset kyphoscoliosis; PLUS
• Either or both of the following:
  • Generalized joint hypermobility
  • Three minor criteria (from either Gene-specific minor features or Other suggestive findings)

However, the diagnosis of FKBP14-kEDS is established in a proband by identification of biallelic pathogenic (or likely pathogenic) variants in FKBP14 by molecular genetic testing (see Table 1).

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) Identification of biallelic FKBP14 variants of uncertain significance (or of one known FKBP14 pathogenic variant and one FKBP14 variant of uncertain significance) does not establish or rule out the diagnosis.

Gene-targeted testing requires that the clinician determine which gene(s) are likely involved, whereas genomic testing does not. Because the phenotype of FKBP14-kEDS 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 a phenotype indistinguishable from many other inherited generalized connective tissue disorders are more likely to be diagnosed using genomic testing (see Option 2).

Clinical Description

FKBP14 kyphoscoliotic Ehlers-Danlos syndrome (FKBP14-kEDS) is characterized by congenital muscle hypotonia and weakness that typically improves during childhood, progressive scoliosis, joint hypermobility, hyperelastic skin, gross motor developmental delay, myopathy, and hearing impairment [Baumann et al 2012, Giunta et al 2018a]. Occasional features underlying systemic connective tissue involvement include aortic rupture and arterial dissection, subdural hygroma (potentially due to subdural bleeding or spontaneous intracranial hypotension), insufficiency of cardiac valves, bluish sclerae, bladder diverticula, inguinal or umbilical herniae, and premature rupture of membranes during pregnancy [Murray et al 2014, Dordoni et al 2016, Giunta et al 2018a].

A range of clinical severity is observed in individuals with FKBP14-kEDS for each of the systems discussed in this section [Baumann et al 2012, Brady et al 2017, Giunta et al 2018a].

Prenatal. Pregnancy involving an affected fetus is often characterized by reduced fetal movements and an increased risk of premature rupture of membranes.

Muscular features. Individuals with FKBP14-kEDS typically present at birth with congenital hypotonia and weakness. They often show feeding problems, and some will need airway management or respiratory support. Most affected infants have poor head control. With rare exceptions, motor developmental delay and reduced motor strength are common features in childhood, although muscle weakness typically improves during childhood [Giunta et al 2018a]. Most children achieve independent walking between ages two and four years. A decline of motor function in adulthood appears to represent an age-dependent feature in the natural course of this condition, but affected individuals are likely to be able to participate in activities of daily living in adulthood and maintain independent walking.

Wider phenotypic variability of the muscular features may exist, as suggested by the presence of early-onset muscle disease with severe involvement of the lower-limb muscles in one recently described affected individual [Castori et al 2019].

Skeletal/joint features

• Kyphoscoliosis is a hallmark of FKBP14-kEDS and is usually severe and progressive. Two thirds of affected individuals manifest kyphoscoliosis before age one year (range: birth to 7 years in 15 affected individuals described by Giunta et al [2018a]). Progressive kyphoscoliosis may respond to bracing, but often surgery is needed. Severe kyphoscoliosis may lead to restrictive lung disease without need for assisted ventilation.
• Pronounced joint hypermobility (mean value of Beighton score 8/9) is seen in 23/23 affected individuals [Giunta et al 2018a] for the small joints and 21/23 for the large joints. Joint hypermobility usually decreases with age.
  • Hypermobility may result in recurrent joint dislocations/sprains or chronic pain (5/23 affected individuals reported).
  • Foot deformities that include congenital or postural talipes and pes planus / planovalgus have been found in 23/23 of affected individuals.
• Despite significant joint hypermobility, congenital contractures are present in up to one third of affected individuals and may impact the fingers, wrist, elbows, or knees (7/23). Congenital hip dislocation is present in 4/17 of affected individuals.
• Fractures probably due to osteopenia/osteoporosis from immobility occurred in 3/23.
• Atlantoaxial subluxation/instability has been reported in three individuals [Dordoni et al 2016, Giunta et al 2018a, Castori et al 2019].

Eyes. Refractive errors, myopia, and hypermetropia are moderately frequent, present in about two thirds of affected individuals. Blue sclerae are present in about one third of affected individuals.

Ears. Hearing impairment can manifest at birth, in early infancy, or even later in life [Giunta et al 2018a]. Sensorineural hearing impairment is the most frequent, present in about half of affected individuals; conductive hearing loss is present in up to one quarter. Hearing impairment (either conductive or sensorineural) may manifest later in life or remain subclinical, thus necessitating periodic investigations (see Management).

Cardiovascular. Vascular complications in adulthood and their possible occurrence in childhood suggests that cardiovascular investigations in the routine assessment and follow up of affected individuals is indicated (see Management). Cardiovascular complications can be congenital (septal defects in a minority) or acquired (usually mild mitral or pulmonary valve insufficiency or dilatation of the ascending aorta).

Additionally, artery dissections occurred in two adult individuals (internal carotid artery and celiac artery) [Murray et al 2014, Giunta et al 2018a] and a pseudoaneurysm rupture occurred in one child (hypogastric artery) [Dordoni et al 2016].

Skin and integument. All individuals described to date have had a subjective finding of soft skin texture. Hyperextensibility was found in 17/23. Atrophic and hypertrophic scarring are seen in fewer than half of affected individuals, as is easy bruising. Additional findings may include follicular hyperkeratosis and crisscross palms/soles.

Other findings

• Inguinal and/or umbilical hernia in about half of affected individuals (11/23), sometimes with redundant umbilical skin.
• Bifid uvula with submucous cleft palate or frank cleft palate (7/23)
• Speech or language delay (7/20); true intellectual disability is rare and may be unrelated in children of consanguineous relationships.
• Visceral complications, including large bladder diverticula (3/19) and (rarely) rectal prolapse.

Prognosis. It is unknown if life span in individuals with FKBP14-kEDS is reduced. One reported individual is alive at age 53 years [Giunta et al 2018a], demonstrating that survival into adulthood is possible. Since many adults with disabilities have not undergone advanced genetic testing, it is likely that adults with this condition are underrecognized and underreported.

Genotype-Phenotype Correlations

Genotype-phenotype correlations that predict risk for specific complications or clinical severity have not been reported to date.

Pathophysiology

The pathomechanism of FKBP14-kEDS is only partially understood [Baumann et al 2012]. Pathology findings include the following:

• Normal collagen biosynthesis and secretion of collagen types I, III, and V
• Disarray of the main components of the extracellular matrix (i.e., collagen type I, III, and VI; fibronectin; tenascins; thrombospondin) by indirect immunofluorescence on skin fibroblast from affected individuals
  Type V collagen is organized in an extracellular network that is similar to control fibroblasts.
• Loss of the main receptors of collagens and fibronectin, α2β1 and α5β1 integrins.
• Marked enlargement of the ER cisterns with accumulation of flocculent material in skin cells of affected individuals by transmission electron microscopy

See also Molecular Pathogenesis.

Nomenclature

FKBP14-kEDS was initially referred to as a variant of Ehlers-Danlos syndrome with progressive kyphoscoliosis, myopathy, and hearing loss. Since the development of the 2017 EDS Nosology [Malfait et al 2017], it is known as kEDS-FKBP14, FKBP14-kEDS, and FKBP14-related kEDS.

Prevalence

FKBP14-kEDS is rare; the exact prevalence is unknown. From its first description in 2012, 30 individuals are known to the authors at the time of review (2019). A disease incidence of approximately 1:100,000 live births is a reasonable estimate. Prevalence does not vary by race or ethnicity.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with FKBP14 kyphoscoliotic Ehlers-Danlos syndrome (FKBP14-kEDS), the evaluations summarized in Table 3 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Treatment of Manifestations

Surveillance

Standardized medical surveillance guidelines for individuals with FKBP14-kEDS have not been published.

Agents/Circumstances to Avoid

Avoid the following:

• For children with severe joint hypermobility, sports that place stress on the joints
• High blood pressure
• For individuals with aortic aneurysm, contact sports

Evaluation of Relatives at Risk

It is appropriate to clarify the genetic 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 prompt initiation of treatment and surveillance measures. Molecular genetic testing can be used if the pathogenic variants in the family are known.

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

Pregnancy Management

No pregnancies in women with FKBP14-kEDS have been reported to date. An increased risk for miscarriage, premature rupture of membranes, and rupture of arteries in affected pregnant women should be considered. Delivery in a medical center with a high-risk perinatologist in attendance is recommended.

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

FKBP14 kyphoscoliotic Ehlers-Danlos syndrome (FKBP14-kEDS) is inherited in an autosomal recessive manner.

Parents of a proband

• The parents of an affected child are obligate heterozygotes (i.e., carriers of one FKBP14 pathogenic variant).
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Sibs of a proband

• At conception, each sib of an affected individual has 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.
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Offspring of a proband. The offspring of an individual with FKBP14-kEDS are obligate heterozygotes (carriers) for a pathogenic variant in FKBP14.

Other family members. Each sib of the proband's parents is at a 50% risk of being a carrier of a FKBP14 pathogenic variant.

Carrier Detection

Carrier testing for at-risk relatives requires prior identification of the FKBP14 pathogenic variants in the family.

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, clarification of carrier status, 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 carriers, or are at risk of being carriers.

Prenatal Testing and Preimplantation Genetic Testing

Once the FKBP14 pathogenic variants 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

FKBP14 (transcript variant 1) encodes a 211-amino acid FKBP22 (alias FKBP14) protein, which contains a signal peptide of 18 residues and forms a dimer of identical subunits [Boudko et al 2014]. The protein consists of three domains:

• The PPIase catalytic domain (aa 45-135)
• The first EF-hand1 domain (aa 135-170)
• The second EF-hand2 domain (aa 179-211)

FKBP22 is an ER resident protein that belongs to the FK506-binding protein (FKBP) class of immunophilins, which have been implicated in catalyzing cis-trans-isomerization of peptidyl-prolyl peptide bonds and are supposed to accelerate protein folding. FKBP22 catalyzes the folding of type III collagen and interacts with type III collagen, type VI collagen, and type X collagen, but not with type I collagen, type II collagen, or type V collagen [Ishikawa & Bächinger 2014]. Remarkably, pathogenic variants in type III and type VI collagens cause the vascular type of EDS (vEDS) and COL6-related muscular dystrophies, respectively. Therefore, the clinical features of vascular abnormalities and myopathy documented in the affected individuals clearly correlates with the interaction of FKBP14 with type III and VI collagens [Giunta et al 2018a] (see also Pathophysiology).

Mechanism of disease causation. The majority of FKBP14 pathogenic variants are loss-of-function variants [Baumann et al 2012, Giunta et al 2018a]. The common c.362dupC frameshift, which is found with a frequency of approximately 70%, is the most common pathogenic variant [Baumann et al 2012, Giunta et al 2018a] and has been linked to the same haplotype in all individuals tested [Murray et al 2014].

A missense variant, p.Met48Lys [Giunta et al 2018a], and an in-frame deletion, p.Glu191del [Dordoni et al 2016], have also been reported. Mapping of the missense variant p.Met48Lys onto the protein crystal structure near the potential PPIase active site of FKBP22 supports complete or partial loss of function of FKBP14 as a further disease mechanism in addition to loss of protein [Giunta et al 2018a].

Western blot analysis using a FKBP14 mouse polyclonal antibody showed deficiency of FKBP14 in two individuals with FKBP14-kEDS [Baumann et al 2012].

FKBP14-specific laboratory technical considerations. FKBP14 consists of three small exons, a rather large fourth exon, which includes the 3' UTR, and three rather large introns. Transcript variants 2 and 3 are noncoding.

Literature Cited

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Revision History

• 23 May 2019 (ma) Review posted live
• 5 October 2018 (cg) Original submission