Identification of three novel homozygous variants in COL9A3 causing autosomal recessive Stickler syndrome

Background Stickler syndrome (STL) is a rare, clinically and molecularly heterogeneous connective tissue disorder. Pathogenic variants occurring in a variety of genes cause STL, mainly inherited in an autosomal dominant fashion. Autosomal recessive STL is ultra-rare with only four families with biallelic COL9A3 variants reported to date. Results Here, we report three unrelated families clinically diagnosed with STL carrying different novel biallelic loss of function variants in COL9A3. Further, we have collected COL9A3 genotype–phenotype associations from the literature. Conclusion Our report substantially expands the molecular genetics and clinical basis of autosomal recessive STL and provides an overview about allelic COL9A3 disorders. Supplementary Information The online version contains supplementary material available at 10.1186/s13023-022-02244-6.

Collagen IX proteins are encoded by COL9A1, COL9A2 and COL9A3 that together form fibril heterotrimer associated collagens and have been recently linked to autosomal recessive STL [13]. Very recently, heterozygous COL9A3 variants have been identified as causing peripheral vitreoretinal degeneration and retinal detachment [14]. COL9A1 and COL9A2 are causally associated with autosomal recessive STL type IV (OMIM #614134) and V (OMIM #61484), respectively. The main clinical characteristics of individuals affected with biallelic COL9A1 variants include moderate-to-severe sensorineural hearing loss, moderate-to-high myopia with vitreoretinopathy, and epiphyseal dysplasia, whereas COL9A2 variants are associated with high myopia, vitreoretinal degeneration, retinal detachment, hearing loss, and short stature. Only very recently, biallelic mutations in COL9A3 have been described to cause autosomal recessive STL in four unrelated families with seven patients. The main phenotypes that are common in all these patients consisted of high myopia, moderate to severe sensorineural hearing loss, and spondylo/epiphyseal dysplasia. Here, we report three additional unrelated consanguineous STL families with five affected individuals in total who each present three novel biallelic COL9A3 variants.

Clinical assessments
Three unrelated consanguineous families of Iranian descent were referred for genetic testing due to hearing and vision impairment (Fig. 1), as well as skeletal dysplasia that resulted in a clinical diagnosis of STL (Fig. 2).
The female proband (II1) from Family 1 is the oldest and only affected individual out of three children from first cousin parents. She had a normal delivery and birth, with a birth weight of 3.2 kg (− 0.43 SD). She was 28 years old at last examination with a weight of 64 kg (+ 0.41 SD), height of 157 cm (− 0.8 SD) and occipitofrontal circumference (OFC) of 55 cm (+ 0.62 SD). She suffers from high myopia in both eyes, in addition to vitreoretinal degeneration with empty vitreous, multiple lattice degenerations and retinal pigmentary changes. There was unilateral absence of the frontal sinus in her skull X-ray. She has severe and progressive sensorineural hearing loss. X-ray and detailed examination of her joints and bones, including mobility testing and examination for signs of osteoarthritis were normal, however she complained of pain in her knee joints. Typical STL craniofacial features such as midface hypoplasia, cleft palate, micrognathia, depressed nasal bridge and anteverted nares are absent.
Family 2 presented with two affected individuals out of four children who were born from a first cousin marriage. The proband (II1) and his affected sibling (II2) both had a normal delivery around term, measurements at birth could not be obtained. Weight, height and OFC at last clinical assessment (at 65 and 57 years-old) were 68 kg (− 0.16 SD) and 66 kg (− 0.39 SD), 166 cm (− 1.4 SD) and 163 cm (− 1.8 SD), 56 cm (+ 0.62 SD) and 57 cm (+ 1.32 SD), respectively. Both had a history of multiple vitreoretinal surgeries due to recurrent rhegmatogenous retinal detachments resulting from advanced vitreoretinal degeneration. Despite vitreoretinal surgeries, the older patient is considered blind without light perception (NLP) in either eye while his sibling has counting finger vision for one eye while NLP was noted for the other eye.
Both suffer from severe and progressive sensorineural hearing loss. Likewise, both show a herniated cervical disc and muscular atrophy was noted in the older sibling. No radiologic documentation was available for review.
Family 3 presented with two affected and two healthy children from first cousin parents. Both affected individuals had normal delivery with a birth weight of 3.4 kg (− 0.26 SD) and 3.75 kg (+ 0.71 SD), length of 49 cm (− 0.6 SD) and 49.5 cm (0.1 SD), and OFC of 35 cm (− 0.40 SD) and 36 cm (+ 0.62 SD). The most current weight, height and OFC measurements for the proband (II1) at age 11.8 years and his sister (II4) at age 3.1 years are 32 kg (-1.43 SD), 137 cm (0.9 SD), and 53 cm (− 0.53 SD) and 12 kg (− 0.05 SD), 84 cm (− 0.6 SD), and 48 cm (+ 0.39 SD), respectively. Both affected individuals have myopia and congenital moderate to severe progressive sensorineural hearing impairment. The affected male complains of knee joint pain, especially when he runs. X-ray and detailed examination demonstrated spondyloepiphyseal dysplasia in both children. Both individuals  Detailed clinical features of all affected individuals are described in Table 1 and Additional file 1: Table S1. None of the individuals showed signs of intellectual disability.

Genetic analysis
The

Discussion
Here, we report three families with five affected individuals clinically diagnosed with autosomal recessive STL due to biallelic LOF variants in COL9A3. Our report reaffirms previous studies that have described four families with biallelic LOF causing autosomal recessive STL, increasing the total number of families reported to date to seven [13,15,16]. These COL9A3 variants as well a other disease causing COL9A3 variants submitted to HGMD are visualized in Fig. 3 for localization on cDNA as well as on protein level. COL9A3, along with two other heterodimers (COL9A1 and COL9A2), belongs to the collagen IX complex, forming a fibril-associated collagen with interrupted triple (FACIT) helices and connecting with collagen II and XI fibrils. A Col9a1 knockout mouse study previously demonstrated that absence of this protein in mice results in the loss of the entire collagen IX heterotrimer complex [17]. Recent reports on the clinical phenotype of STL and MED syndromes that are caused by variants affecting NA not ascertained, OFC occipitofrontal circumference, SD standard deviation different members of collagen IX have supported the hypothesis that each of the three proteins is essential for collagen IX function [13,18]. While a variety of disorders have been described to result from heterozygous pathogenic variants in COL9A3, only four unrelated STL families and one family with nonsyndromic hearing loss have been reported to date carrying biallelic variants ( Table 2). Allelic disorders resulting from COL9A3 variants include nonsyndromic hearing loss, MED, pseudoachondroplasia, cerebral palsy, and lumbar disc disease and severe peripheral vitreoretinal degeneration and retinal detachment ( Table 2).
Consistent clinical features among STL patients with biallelic COL9A3 LOF alleles comprise moderate-toprofound progressive sensorineural hearing loss and moderate high myopia with vitreoretinal degeneration. Retinal detachment and cataract occur occasionally. In contrast, skeletal involvement seems to be more variable. For instance, Nixon et al. [13] reported a family with two affected siblings where the oldest affected sibling had severe arthropathy in the shoulders and hip, requiring a wheelchair. The X-ray of this patient showed spinal scoliosis and narrowing of the articular space in both knees, while the younger affected sibling did not show any of these signs. In line with this report, we also observed that the affected individuals in family 2, at the ages of 65 and 57 years-old, suffer only from myopia, hearing loss and each have a herniated cervical disc while the two much younger affected individuals in family 3, at ages 3 and 11 years-old, have more prominent skeletal findings that include radiological signs of spondyloepiphyseal dysplasia as well as craniofacial abnormalities including depressed nasal bridge and anteverted nares (Table 1). Moreover, Nixon et al. [13] observed that carrier parents

Conclusion
In summary, our report consolidates that homozygous loss of function variants in COL9A3 cause STL (type VI). We find high myopia and moderate-severe hearing loss to be consistent features amongst all cases while skeletal findings seem more variable.

Subjects
Three unrelated Iranian families with syndromic phenotypes including hearing loss, vision impairment and skeletal dysplasia were referred for clinical genetic diagnostics. Blood samples were collected after obtaining informed consent from patients or their parents. Molecular genetic diagnostic testing was performed in Nijmegen via the Radboud innovative diagnostics programme and at the University of Tuebingen (197/2019BO01). Informed consent from the parents or legal guardians of the patients/participants was obtained for the publication of their data.

Exome and Sanger sequencing
After extraction of DNAs from whole blood by standard protocol, proband DNA samples were subjected to exome capture using the Agilent SureSelect Human All Exon V6 Kit and exome sequencing (ES) was performed on an Illumina HiSeq 2500 sequencer for an average 50 × sequencing depth, resulting in sequences of greater than 100 bases from each end of the fragments [Cambridge (Novogene UK)]. Exome data were processed for analysis using a GATK-based pipeline [20] that uses Burrows-Wheeler alignment [21] to the GRCh37/UCSC hg19 (Families 1 and 2) and GRCh38/UCSC hg38 (Family 3). VarScan version 2.2.5, MuTec and GATK Somatic Indel Detector were used to detect SNV and InDels, respectively. The protocol to interpret potential pathogenic variants was previously described [22]. For population-specific filtering, gnomAD [23], Iranome [24] and Greater Middle East (GME) Variome Project [25] databases were used.
Segregation analysis using Sanger sequencing was performed in available family members to confirm variant segregation after PCR amplification. Primers are available upon request.