Chst3-Related Skeletal Dysplasia

Clinical characteristics.

CHST3-related skeletal dysplasia is characterized by short stature of prenatal onset, joint dislocations (knees, hips, radial heads), clubfeet, and limitation of range of motion that can involve all large joints. Kyphosis and occasionally scoliosis with slight shortening of the trunk develop in childhood. Minor heart valve dysplasia has been described in several persons. Intellect and vision are normal.

Diagnosis/testing.

The diagnosis is based on the radiographic features of progressive spondyloepiphyseal dysplasia with joint anomalies, spinal abnormalities, normal thumbs (not spatulate), and normal bone age. CHST3 is the only gene in which pathogenic variants are known to cause CHST3-related skeletal dysplasia.

Management.

Treatment of manifestations: Surgical correction of the abnormal joints is the only treatment modality; however, surgical correction is often only partially successful and multiple procedures are needed. Physical therapy has not been effective.

Surveillance: If normal at the time of diagnosis, echocardiogram should probably be repeated every five years.

Agents/circumstances to avoid: Activities with a high impact on joints (e.g., jogging) and obesity.

Genetic counseling.

CHST3-related skeletal dysplasia is inherited in an autosomal recessive manner. 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. Carrier testing for at-risk family members and prenatal diagnosis for pregnancies at increased risk are possible if the pathogenic variants in the family have been identified.

Suggestive Findings

CHST3-related skeletal dysplasia should be suspected in individuals with the following clinical and radiographic features.

Clinical features

• Joint dislocations at birth (knees, hips, radial heads) with short stature (Figure 1)
• Clubfeet
• Limitation of range of motion that can involve all large joints
• Development of kyphosis and occasionally scoliosis with slight shortening of the trunk in childhood

Figure 1.

A newborn with molecularly confirmed CHST3-related skeletal dysplasia. Note the bilateral dislocation of the knees and radial heads.

Radiographic features

• Progressive spondyloepiphyseal dysplasia with joint anomalies
  • Generalized mild epiphyseal dysplasia (small epiphyses)
  • Delayed ossification of the capital femoral epiphyses and femoral necks
  • Coxa valga (increase in the angle formed between the head and neck of the femur and the shaft of the femur)
• Spinal abnormalities
  • Conspicuous increase in interpediculate distance from T12 to L1 or L2 (Figure 2)
  • Notching of the vertebral bodies, similar in appearance to coronal clefts (Figure 3)
• Normal thumbs (not spatulate)
• Normal or (more rarely) slightly advanced bone age (especially carpal)

Figure 2.

Note the conspicuous increase in interpediculate distance from T12 to L1. Also appreciable is the bilateral hip subluxation.

Figure 3.

Mild platyspondyly is observed. Note also the coronal clefts throughout the lumbar region.

Establishing the Diagnosis

The diagnosis of CHST3-related skeletal dysplasia is established in a proband by identification of biallelic pathogenic variants in CHST3 by molecular genetic testing (see Table 1).

Molecular genetic testing approaches can include single-gene testing, use of a multigene panel, and more comprehensive genomic testing.

• Single-gene testing. Sequence analysis of CHST3 is performed first. If no pathogenic variants or only a single pathogenic variant have been identified and the clinical-radiographic index of suspicion is high for a CHST3-related skeletal dysplasia, consider doing gene-targeted deletion/duplication analysis. However, no individuals with a deletion/duplication have been reported.
• A multigene panel that includes CHST3 and other genes of interest (see Differential Diagnosis) may also be considered. Laboratories may offer a general "skeletal dysplasia" panel or a more targeted "skeletal dysplasia with multiple dislocations" panel. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change over time. (2) Some multigene panels may include genes not associated with the condition discussed in this GeneReview; thus, clinicians need to determine which multigene panel is most likely to identify the genetic cause of the condition at the most reasonable cost while limiting identification of variants of uncertain significance and pathogenic variants in genes that do not explain the underlying phenotype. (3) Methods used in a panel may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.
  For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.
• More comprehensive genomic testing (when available) including exome sequencing and genome sequencing may be considered. Such testing may provide or suggest a diagnosis not previously considered (e.g., mutation of a different gene or genes that results in a similar clinical presentation).
  For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Biochemical testing. Cultured fibroblasts can be used to determine proteoglycan sulfation (i.e., sulfotransferase activity). The fibroblasts are incubated with radioactive sulfate [³⁵S] and chondroitin. In all patients with CHST3 deficiency studied thus far, the incorporation of sulfate at the C6 position was dramatically decreased while incorporation at the C4 position was within normal levels [Hermanns et al 2008, van Roij et al 2008]. This test requires a skin biopsy and thus is more invasive and less widely available than molecular genetic testing. Biochemical testing can be useful in those cases in which pathogenic variants are not identified or variants of uncertain significance have been detected. The diminished sulfation at carbon 6 of proteoglycans offers unambiguous evidence of CHST3 deficiency.

Clinical Description

Most children with CHST3-related skeletal dysplasia are identified at birth as having a generalized skeletal disorder. The features of this disorder are generally limited to the skeleton and joints and are progressive in nature.

Occasionally, short stature and knee dislocations are seen on prenatal ultrasound examination [Unger et al 2010]. The prenatal presentation may be that of arthrogryposis [Muys et al 2017].

At birth, affected infants are noted to have short stature (birth length: 39 to 44 cm) and joint dislocations: the large majority have bilateral knee luxation or subluxation. The radial heads and hips are the next most commonly affected joints. Clubfeet are also frequently seen. Despite the congenital joint dislocations, the overall phenotype is one of restricted movement and many children undergo multiple corrective procedures with only limited success [Rajab et al 2004, Unger et al 2010, Searle et al 2014].

In the large family reported from Oman, the adult heights ranged from 110 cm to 130 cm [Rajab et al 2004], while in a large Pakistani family the mean adult height was 84 cm [Waryah et al 2016]. A review article included information on three adults with heights of 117 cm, 121 cm, and 134.5 cm [Unger et al 2010]. Adult height appears to be severely affected in all individuals with CHST3-related skeletal dysplasia but with some intrafamilial variability and large interfamilial variability.

Many adults develop arthritic-type changes. They also develop spinal kyphosis, frequently in the cervical spine, and (rarely) scoliosis.

Other findings include:

• Minor heart valve dysplasia with valvular insufficiency in several persons, including one in the kindred from Oman [Hall 1997, Rajab et al 2004, Tuysuz et al 2009];
• Non-skeletal complications: inguinal hernia and gastric volvulus;
• Tooth anomalies (microdontia, delayed eruption), reported in the large Omani kindred though not observed in others [Rajab et al 2004];
• Normal intellect, vision, and hearing [Unger et al 2010]. However, in a large Pakistani kindred, at least six affected individuals also had mixed hearing loss; thus, this may be an associated feature [Waryah et al 2016];
• Sagittal craniosynostosis has been reported in a single individual [Searle et al 2014] while "sclerosis of sutures" was reported in three unrelated patients [Srivastava et al 2017].

Genotype-Phenotype Correlations

No genotype-phenotype correlations have been observed. The phenotype reported thus far has been strikingly homogeneous regardless of type of CHST3 variant [Unger et al 2010]. Persons with homozygous pathogenic missense variants are no less severely affected than those with nonsense variants.

Nomenclature

In 1950, Dr LJ Larsen described autosomal dominant Larsen syndrome, now known to be caused by pathogenic variants in FLNB, the gene encoding filamin B [Bicknell et al 2007]. Larsen syndrome is characterized by multiple joint dislocations, dysmorphic facial features, spatulate thumbs, and accelerated carpal ossification (see FLNB-Related Disorders).

Following the delineation of autosomal dominant Larsen syndrome, several reports of "autosomal recessive Larsen syndrome" and other similar disorders were published. By careful reevaluation of patients and through recruitment of cases with autosomal recessive Larsen syndrome, several investigators showed that, in fact, all the various reports of autosomal recessive Larsen syndrome, humerospinal dysostosis, and spondyloepiphyseal dysplasia (SED), Omani type could be attributed to CHST3 deficiency and that the different names had arisen from the part of the phenotype the various authors had emphasized [Hermanns et al 2008, Unger et al 2010]; that is, they were describing the same condition from different viewpoints:

• Humerospinal dysostosis was described by Kozlowski et al [1974] in two brothers with joint dislocations and radiographic abnormalities (bifid humeri and coronal clefts). Because the brothers were reported to be half-sibs, autosomal dominant inheritance was suspected and no link was made to autosomal recessive Larsen syndrome.
• Mégarbané and Ghanem [2004] described "a newly recognized chondrodysplasia with joint dislocations." Although they made the link to humero-spinal dysostosis, they rejected that diagnosis because the evidence strongly suggested autosomal recessive inheritance.
• Rajab et al [2004] described a large family originating from Oman with what they termed "a new recessive type of SED with progressive spinal involvement." The same group went on to demonstrate that the disorder was caused by CHST3 deficiency and renamed the disorder SED, Omani type [Thiele et al 2004].

The name "CHST3-related skeletal dysplasia" has been proposed as an unbiased and inclusive designation for this disorder. However, an argument could also be made for retaining the name "autosomal recessive Larsen syndrome," as the joint dislocations are the presenting feature and "Larsen syndrome" is usually the first diagnosis considered; thus, the continued use of this designation is open to debate. The term "recessive Larsen syndrome" is more appropriate for CHST3-related skeletal dysplasia than for the B4GALT7-associated linkeropathy, as bilateral knee dislocation at birth is much more common in CHST3 deficiency than in B4GALT7 deficiency.

Prevalence

No firm data regarding the prevalence of CHST3-related skeletal dysplasia are available. More than 40 cases (including familial recurrences) have been reported.

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with CHST3-related skeletal dysplasia, the following evaluations are recommended if they have not already been completed:

• Orthopedic referral
• Referral to a specialized skeletal dysplasia clinic if available
• Echocardiogram
• Consultation with a clinical geneticist and/or genetic counselor

Treatment of Manifestations

The focus of treatment has thus far been surgical correction of the abnormal joints. Presumably because of the basic defect present in CHST3-related skeletal dysplasia, surgical correction is often only partially successful and most patients have had multiple procedures by adulthood [Unger et al 2010].

Of note, physical therapy has not been demonstrated to be effective in this disorder.

Surveillance

Thus far, heart valve dysplasia has not required correction; thus, no firm guidelines for appropriate surveillance have been developed. Echocardiogram is suggested at time of diagnosis and, if normal, should probably be repeated at intervals of five years.

Agents/Circumstances to Avoid

Activities with a high impact on joints (e.g., jogging) should be avoided.

Obesity, which places an excessive load on the large weight-bearing joints, should be avoided.

Evaluation of Relatives at Risk

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

Therapies Under Investigation

Search ClinicalTrials.gov in the US and www.ClinicalTrialsRegister.eu 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.