Classical EDS (cEDS)

cEDS represents the paradigmatic form of the Ehlers-Danlos syndromes, with the defining features of generalized joint hypermobility, marked hyperextensibility of generally soft, doughy skin, as well as skin fragility with poor wound healing and atrophic scarring [6]. Many affected individuals suffer from chronic musculoskeletal and joint pain. Additional features include easy bruising with lasting hemosiderin deposits and other skin signs such as molluscoid pseudotumors, subcutaneous spheroids and piezogenic papules. There may be a history of hernias and mild aortic dilatation, but aortic rupture and other serious cardiovascular events are uncommon.

With an estimated incidence of approximately 1:20 000 [27], cEDS is the most common monogenic form of EDS. It is usually caused by deficiency or structural abnormality of type V collagen, a heterotrimeric fibrillar collagen that forms the scaffold on which type I collagen is deposited for fibril formation (homozygous loss of COL5A1 expression is embryonic lethal with no normal collagen fibrils formed) and plays an important role in the assembly and regulation of fibrillogenesis. Inheritance is autosomal dominant; more than 90 % of individuals with cEDS carry heterozygous pathogenic variants in one of the two type V collagen genes, COL5A1 or COL5A2. The majority of individuals with cEDS have pathogenic variants in COL5A1, both loss-of-function (LoF, null) and missense (structural) variants without evidence of major phenotypic differences. COL5A2 variants are generally missense/structural and may be associated with a more severe clinical manifestation [11, 38]. Rarely a cEDS phenotype may be due to specific variants in type I collagen genes, in particular arginine-to-cysteine variants which may be associated with an increased probability of vascular events. The main differential diagnosis is classical-like EDS (clEDS); some individuals with hEDS show skin manifestations that may resemble cEDS, but careful clinical evaluation usually allows reliable distinction between the two conditions.

Genetic diagnosis of cEDS involves complete sequence analysis plus deletion/duplication testing of COL5A1 or COL5A2 as well as COL1A1 and COL1A2, usually by massively parallel sequencing. It must be noted that most pathogenic variants in the type I collagen genes cause osteogenesis imperfecta, and the EDS phenotypes associated with variants in these genes may have some specific clinical features. In the case of a heterozygous VUS in one of the COL5A1/2 genes, segregation analysis in the family, skin electron microscopy, or functional/transcript studies, may provide additional diagnostic clues. Non-transcription of one copy of a collagen V gene may also be shown by loss of heterozygosity in transcript analyses in individuals with a heterozygous polymorphism on DNA level (also called null-allele test) [28, 39, 44].

Vascular EDS (vEDS)

Vascular or organ rupture is the hallmark of vEDS, the most life-threatening form of EDS [8]. It has an estimated incidence of 1:50 000–1:200 000 [27]. vEDS should be considered in all individuals with arterial rupture, dissection or aneurysm before age 40 years, otherwise unexplained bowel or uterine rupture, or carotid-cavernous sinus fistula without previous trauma. Hypermobility is often discrete and limited to the small joints. The skin is often thin and translucent with marked venous visibility and general acrogeric appearance; there may be bruising at unusual body sites and early onset varicose veins. Other features include spontaneous pneumothorax, congenital hip dislocation, and/or talipes equinovarus. However, with increased availability of genetic testing, diagnoses of vEDS are also made in individuals who have few or no specific clinical features of vEDS on examination [25].

vEDS is usually caused by heterozygous pathogenic variants in COL3A1 affecting the production of type III collagen, a major component of vessel walls and hollow organs. More than half of affected individuals have dominant negative substitutions for glycine in the repeated Gly-X-Y motif of the triple helical domain; the substituting amino acids are usually aspartate, arginine, valine or glutamate, whereas cysteine, serine, or alanine substitutions are underrepresented suggesting that these may potentially cause a milder phenotype. More than one third of individuals have splice-site and in-frame insertions–deletions in the triple-helical domain. Substitutions for glycine, splice site variants that result in in-frame deletion or insertion, and other in-frame variants are associated with a more severe clinical manifestation than less frequently observed null/LoF variants or non-glycine missense variants [17, 36]. Glutamate-to-lysine substitutions in triple helix domain Gly-Glu-Arg sequences have been reported in a number of families in which affected individuals showed prominent cEDS-type features in addition to vEDS complications [18]. Very occasionally, biallelic pathogenic variants in COL3A1 may be identified, leading to a severe childhood-onset phenotype with structural brain anomalies. Several Arg>Cys variants in COL1A1 (in particular p.Arg312Cys) have been reported to cause vascular fragility in combination with a (classical) vEDS phenotype [30]. Massive parallel sequencing of COL3A1 (and COL1A1) – as always with deletion/duplication analysis – is expected to identify the causative pathogenic variant in >98 % of affected individuals.

Cardiac-valvular EDS (cvEDS)

The term cvEDS is used for the very rare combination of an EDS phenotype with severe progressive cardiac-valvular disease, caused by biallelic null/LoF variants in COL1A2 [40]. In the very few (<10) reported patients with cvEDS, severe mitral and/or aortic valve regurgitation necessitated valve replacement. However, complete absence of the proα2(I) chain encoded by COL1A2 may cause variable clinical manifestations, including an osteogenesis imperfecta-EDS overlap syndrome associated with the production of a stable mutant proα2(I) chain that is degraded but may trigger an unfolded protein response (UPR) [7]. Genetic investigations in individuals with an EDS phenotype and severe cardiac valvular disease should entail thorough evaluation of the COL1A2 gene, including large deletion analysis.

Arthrochalasia EDS (aEDS)

This condition – originally described as “arthrochalasis multiplex congenita” [23] – has been reported as an EDS variant in approximately 50 individuals [7, 19]. aEDS usually presents at birth with extreme hypermobility and congenital bilateral hip dislocation, and is associated with recurrent subluxations and dislocations of both small and large joints. There may be various skeletal manifestations such as foot or spinal deformities, muscular hypotonia, and delayed motor development. Dysmorphic facial features include frontal bossing, hypertelorism, epicanthal folds, midfacial hypoplasia, depressed nasal bridge, and micrognathia. Most individuals also display the typical EDS features of soft, doughy, hyperextensible or redundant skin, often with easy bruising and atrophic scarring. aEDS is caused by complete or partial loss of exon 6 of either COL1A2 or COL1A1 that precludes trimming by procollagen N-proteinase and leads to retention of the N-propeptide in the collagen type I triple helix. Genetic investigations in individuals with suspected aEDS should focus on pathogenic COL1A2 or COL1A1 variants affecting exon 6 splicing and should include exon (5 and) 6 deletion analysis.

COL1-related osteogenesis imperfecta/Ehlers-Danlos syndrome overlap disorder

This molecularly defined condition has not (yet) been classified as a specific EDS type. Affected individuals mainly present with severe joint hyperlaxity, soft and hyperextensible skin, abnormal wound healing, easy bruising, and sometimes signs of arterial fragility. In addition, they show subtle signs of osteogenesis imperfecta including blue sclerae, relatively short stature and osteopenia or fractures [31,35]. The diagnosis is based on the identification of pathogenic variants in COL1A1 or COL1A2, often glycine substitutions affecting residues within or near the procollagen N-proteinase cleavage sites. Some individuals carry pathogenic variants previously reported as causative for osteogenesis imperfecta [35], indicating variable expressivity. The full molecular basis of this condition is not well understood, and the classification is open for discussion.

Figure 1:

Genomic structure of the RCCX locus

1a: normal genomic structure; genes depicted in grey are pseudogenes.

1b: typical 32–33 kb deletion found in CAH-X syndrome

Tenascin X-associated classical-like EDS (TNXB-clEDS, clEDS type 1)

The 2017 classification uses the term clEDS for the phenotype caused by complete loss of tenascin X (TN-X) function. This autosomal recessive condition – subsequently also called clEDS type 1 – resembles cEDS with generalized joint hypermobility, hyperextensible skin, and easy bruising, but is not associated with atrophic scarring. Other distinguishing features include mild muscle weakness, axonal polyneuropathy, leg oedema, and various anomalies of hands and feet including sometimes debilitating foot deformities. Organ prolapse and fragility, particularly of the gastrointestinal tract, is more frequent than in cEDS; gastrointestinal complications also include diverticulitis, gastrointestinal bleeding, intestinal obstruction, and gallstones. More than half of affected individuals report excessive fatigue [14, 20,46].

TN-X-deficient clEDS is caused by biallelic null/LoF variants of TNXB and has been reported in more than 50 individuals [46], but the diagnosis is technically challenging. Tenascin X is a large extracellular matrix glycoprotein that regulates deposition, stability and mechanical properties of collagen fibres [33]. The TNXB gene is a component of the complex RCCX locus in the major histocompatibility complex (MHC) class III region on chromosome 6q21.3. It comprises genes for serine/threonine kinase type 19 (STK19, previously denoted RP, and its pseudogene STK19B), complement 4 (two expressed genes C4A and C4B), steroid 21-hydroxylase (CYP21A2 and its pseudogene CYP21A1P), and tenascin-X (TNXB and its pseudogene TNXA) [9] (Figure 1a). The homologous sequences of the locus are positioned in tandem in the same direction, leading to a propensity for non-allelic homologous recombination (NAHR). TNXB is a 68 kb gene with 44 exons, whereas the 4.5 kb non-transcribed TNXA pseudogene represents a 5’-truncated paralog and contains homologs to exons 32 to 44 only. NAHR-mediated deletion between TNXB and TNXA leads to loss of CYP21A2, the causal gene of autosomal recessive 21-hydroxylase deficiency (classical congenital adrenal hyperplasia, CAH), as well as C4B, absence of which may contribute to reduced complement 4 production. Furthermore, it results in distal TNXB-TNXA fusion that may have variable functional consequences (including possible abnormal protein-related effects in some heterozygotes) based on the exact recombination position and the extent of the inclusion of TNXA-specific variants [32].

Biallelic loss of functional CYP21A2 in CAH is frequently caused by a large NAHR-mediated deletion on at least one allele. If the deletion results in loss of functional TNXB because of TNXB-TNXA fusion, the resulting phenotype combines CAH with Ehlers-Danlos syndrome, denoted CAH-X syndrome. Severity of the connective tissue manifestation may be linked to heterozygous or biallelic presence of the deletion, and potentially to specific variant TNX protein effects. Heterozygous loss of TNXB was claimed as a cause of hypermobile Ehlers-Danlos-Syndrome [48] but this association has not been confirmed. Population data indicate that TNXB is haplosufficient (pLI = 0, LOEUF = 0,674, data from https://gnomad.broadinstitute.org). Due to the high sequence homologies, standard short-read sequencing methods do not allow determination of the exact genomic structure of the RCCX locus. A method combining long-range PCR with short-read sequencing for accurate detection of variants and TNXA-derived sequences in TNXB has been described [46]; long-read sequencing is also promising to resolve these issues. In consequence, the exact functional effects of genetic-genomic variants in the TNXB gene region and its relevance for hypermobility and different types of Ehlers-Danlos syndromes beyond clEDS remain to be clarified.

AEBP1-associated classical-like EDS (AEBP1-clEDS, clEDS type 2)

Pathogenic variants in AEBP1 as cause of clEDS were fully described only after the 2017 classification [1, 5], and the condition has also been called clEDS type 2. Affected individuals have the typical EDS manifestations of hypermobility, skin hyperextensibility, atrophic scarring and easy bruising, occasionally with other cardiovascular manifestations and/or hernias; a majority also has osteopenia sometimes with fractures, various skeletal abnormalities, and (mild) myopathy [3, 46]. AEBP1 encodes two protein isoforms with various ECM and collagen-related functions. AEBP1-clEDS is an autosomal recessive condition caused mostly by biallelic null/LoF variants, and has been reported in >10 individuals. It is easily identifiable by standard (massively parallel) sequencing.

Dermatosparaxis EDS (dEDS)

The clinical hallmark of dEDS is extreme fragility of generally redundant, lax and/or hyperextensible skin. Additional features include unusual craniofacial features (prominent eyes with puffy eyelids and excessive periorbital skin, postnatally large fontanels, a hypoplastic chin and bluish or greyish discoloration of the sclerae), easy bruising, postnatal growth retardation, and recurrent fractures. Joint hypermobility becomes more prominent with age. Prematurity and perinatal complications are common [7]. dEDS is an autosomal recessive condition repeatedly observed in animals (cattle, sheep, dogs, etc.) but only reported in <20 human individuals. It is caused by biallelic null/LoF variants in ADAMTS2, which encodes a disintegrin and metalloproteinase that cleaves the propeptides of type I and II collagens prior to fibril assembly. dEDS is easily identifiable by standard sequencing; there is one relatively frequent recurrent pathogenic variant c.673C>T (p.Gln225Ter) that has an allele frequency of 0.3 % in persons of Ashkenzi Jewish descent (gnomAD v4.1.0) [10].

Kyphoscoliotic EDS (kEDS)

Children with kEDS often present at birth with marked muscular hypotonia and kyphoscoliosis without other neuromuscular abnormalities. Hypotonia tends to improve with age, whereas kyphoscoliosis is usually progressive and severe. There is generalized joint hypermobility, often associated with dislocations or subluxations of large joints (hips, shoulder, knees, wrist). The skin is hyperextensible, often with fragility and bruising. Additional skeletal abnormalities include deformities of hands and/or feet, as well as unusual craniofacial features. There are variable ocular manifestations such as bluish sclerae, microcornea or myopia; loss of an eye following trauma is a known risk in these individuals. Rupture of medium-sized arteries has been reported in several cases [7]. kEDS has been diagnosed in >100 individuals and is due to the autosomal recessive deficiency of either lysyl hydroxylase 1 encoded by PLOD1, which catalyzes the formation of hydroxylysine in collagens, or the peptidyl-prolyl cis-trans isomerase encoded by FKBP14, which assists processing of type III collagen in the endoplasmic reticulum and also interacts with types VI and X collagens. Individuals with FKBP14-kEDS often have impaired hearing, which is not usually observed in PLOD1-kEDS. Genetic testing for kEDS beyond standard sequencing should target a common 8.9 kb duplication of PLOD1 exons 10–16 (c.1067_1846dup) that has a relative allele frequency of 30 %, i.e., represents 30 % of reported pathogenic alleles [7]. There is one relatively common FKBP14 frameshift variant c.362dup (allele frequency 0,086 % in Europeans, gnomAD v.4.1.0) with a relative allele frequency of up to 70 % in European patients.

Brittle cornea syndrome (BCS)

Marked corneal thinning with a high risk of corneal perforation in childhood – spontaneously or after minor trauma – is the leading manifestation of BCS. Typical ocular features prior to rupture include blue sclerae, keratoconus/keratoglobus, and high myopia. Most affected individuals have hypermobility predominantly of small joints; other skeletal anomalies such as kyphoscoliosis, foot deformities, developmental dysplasia of the hip or mild craniofacial abnormalities are common [7]. Skin abnormalities are milder than in other EDS types and may be absent. Hearing impairment is frequent. BCS has been reported in >60 individuals and is caused by biallelic (autosomal recessive) pathogenic (mostly null/LoF) variants in either ZNF469 or PRDM5, which code for proteins involved in transcription regulation of collagens and/or other ECM proteins. BCS is easily identifiable by standard sequencing.

Spondylodysplastic EDS (spEDS)

Major clinical features of spEDS are progressive growth delay and short stature with bowing of limbs, other skeletal anomalies including osteopenia and recurrent fractures, and muscular hypotonia ranging from congenital severe to mild [7]. Affected children often show delayed cognitive and motor development, a characteristic facial gestalt, and joint hypermobility or contractures. The skin is often hyperextensible or loose, soft and translucent, but atrophic scars are rare, and there is usually no generalized tissue fragility. spEDS has been reported in >100 individuals and is inherited in an autosomal recessive fashion. It is mostly caused by biallelic null/LoF variants in enzymes required for tetrasaccharide linker assembly in GAG biosynthesis; known causative genes are B4GALT7, B3GALT6, and B3GAT3, encoding galactosyltransferase I, galactosyltransferase II, and glucuronyltransferase, respectively. The clinical manifestation shows minor differences between genes, and marked overlap with other defined phenotypes. The disorders of GAG linker assembly are often summarized as linkeropathies [37]. There is a prevalent B4GALT7 founder variant c.808C>T (p.Arg270Cys) in the La Réunion island population, cause of the Larsen of La Réunion Island syndrome. Another form of spEDS, clinically somewhat different and originally denoted spondylocheirodysplastic EDS, is caused by biallelic (mostly null/LoF) variants in SLC39A13. This gene codes for the zinc transporter (importer) ZIP13, which is predominantly expressed in bone, teeth, and connective tissues [22].

Musculocontractural EDS (mcEDS)

Congenital multiple contractures, characteristic craniofacial features, and subsequently EDS typical skin manifestations (hyperextensibility, fragility, atrophic scars, easy bruising) are the hallmarks of mcEDS. Joint dislocations, other skeletal changes (including spinal and thorax deformities), and various organ manifestations (myopia, hearing loss, heart defects, constipation, etc.) have been described [34]. This autosomal recessive condition has been independently recorded under different names such as “adducted thumb-clubfoot syndrome” or “Kosho type EDS”, and has been reported in >70 individuals. mcEDS is caused by the deficiency of enzymes required for the formation of dermatan sulphate, due to biallelic null/LoF variants in CHST14 encoding dermatan 4-O-sulfotransferase 1, or less frequently in DSE encoding dermatan sulphate epimerase.

Myopathic EDS (mEDS)

The characteristic presentation of mEDS is neonatal muscular hypotonia associated with generalized joint hypermobility and (sometimes progressive) contractures [7, 13]. Severe cases may have fetal akinesia and require postnatal tube feeding; there are often various other skeletal anomalies. Motor development is usually delayed, muscle weakness tends to improve during childhood but may deteriorate again in adulthood. The skin tends to be soft and is sometimes hyperextensible; abnormal bruising or scarring is infrequent. mEDS has been reported in >20 individuals and is due to pathogenic variants in COL12A1 that codes for the nonfibrillar homotrimeric type XII collagen. The severity of mEDS and the inheritance pattern in families depends on the functional effects of the individual variant(s): stable structural missense variants with a dominant negative effect are associated with autosomal dominant inheritance, whereas recessive inheritance has been reported for homozygous null/LoF variants. A large in-frame multi-exon deletion with severe heterozygous manifestation – suggesting a dominant negative effect – has been reported [12].