Consensus Guidelines for the Use of Vosoritide in Children with Achondroplasia in Australia.

BACKGROUND: Achondroplasia, the most prevalent skeletal dysplasia, stems from a functional mutation in the fibroblast growth factor receptor 3 gene, leading to growth impairment. This condition presents multifaceted medical, functional and psychosocial challenges throughout childhood, adolescence and adulthood. Current management strategies aim to minimise medical complications, optimise functional capabilities and provide comprehensive supportive care. Vosoritide (trade name: VOXZOGO®, BioMarin Pharmaceuticals) is the first disease-modifying pharmaceutical treatment approved for the management of patients with achondroplasia and became available in Australia in May 2023. METHODS: Standardised clinical guidelines for its optimal use are not yet widely available. To address this gap, a multidisciplinary Australian Vosoritide Working Group, comprising 12 experts with experience in achondroplasia management from across Australia, developed recommendations to guide the use of vosoritide in clinical practice. RESULTS: The recommendations, which are expert opinions of the Australian Vosoritide Working Group, aim to (i) standardise the use of vosoritide across Australia, (ii) support the safe clinical rollout of vosoritide and (iii) support universal access. CONCLUSIONS: These recommendations have been developed for healthcare professionals and institutions that are engaged in using vosoritide in the management of achondroplasia and will be revised using a formal framework for clinical guideline development once more evidence is available.

Identification of potential non-invasive biomarkers in diastrophic dysplasia.

Diastrophic dysplasia (DTD) is a recessive chondrodysplasia caused by pathogenic variants in the SLC26A2 gene encoding for a cell membrane sulfate/chloride antiporter crucial for sulfate uptake and glycosaminoglycan (GAG) sulfation. Research on a DTD animal model has suggested possible pharmacological treatment approaches. In view of future clinical trials, the identification of non-invasive biomarkers is crucial to assess the efficacy of treatments. Urinary GAG composition has been analyzed in several metabolic disorders including mucopolysaccharidoses. Moreover, the N-terminal fragment of collagen X, known as collagen X marker (CXM), is considered a real-time marker of endochondral ossification and growth velocity and was studied in individuals with achondroplasia and osteogenesis imperfecta. In this work, urinary GAG sulfation and blood CXM levels were investigated as potential biomarkers for individuals affected by DTD. Chondroitin sulfate disaccharide analysis was performed on GAGs isolated from urine by HPLC after GAG digestion with chondroitinase ABC and ACII, while CXM was assessed in dried blood spots. Results from DTD patients were compared with an age-matched control population. Undersulfation of urinary GAGs was observed in DTD patients with some relationship to the clinical severity and underlying SLC26A2 variants. Lower than normal CXM levels were observed in most patients, even if the marker did not show a clear pattern in our small patient cohort because CXM values are highly dependent on age, gender and growth velocity. In summary, both non-invasive biomarkers are promising assays targeting various aspects of the disorder including overall metabolism of sulfated GAGs and endochondral ossification.

Investigation of a family affected by early-onset osteoarthritis - proposal of a clinical pathway and bioinformatics pipeline for the investigation of cases of familial OA.

BACKGROUND: Familial cases of early-onset osteoarthritis (OA) are rare although the exact prevalence is unknown. Early recognition of underlying OA-associated disorders is vital for targeted treatment, when available, and genetic counselling, in case of skeletal dysplasias. Currently, there is no clear guidance on how best to investigate families affected by early-onset OA. METHODS: We investigated a family with multiple members affected by early-onset OA (age at onset ≤ 40 years). Clinical and demographic characteristics were collected, followed by laboratory investigations screening for a range of potential OA-associated disorders, and whole genome sequencing in selected individuals. RESULTS: Seventeen members of the family were included (7 affected and 10 non-affected). There was an even split between the two sexes and two participants were under 18 years old. No pattern of abnormality was seen in the laboratory investigation that could explain the OA phenotype in the family. Whole-genome sequencing was perfomed in one participant and analysed for likely pathogenic variants in genes known to be associated with skeletal dysplasias. A heterozygous variant in the COL2A1 gene was identified (p.Arg519Cys). Confirmatory tests were performed in five additional participants (four affected and one unaffected). CONCLUSION: The methodology used in this study, including the clinical pathway and bioinformatics pipeline, could be applied to other families affected by early-onset OA.

Australian guidelines for the management of children with achondroplasia.

Achondroplasia is the most common form of skeletal dysplasia. In addition to altered growth, children and young people with achondroplasia may experience medical complications, develop and function differently to others and require psychosocial support. International, European and American consensus guidelines have been developed for the management of achondroplasia. The Australian focused guidelines presented here are designed to complement those existing guidelines. They aim to provide core care recommendations for families and clinicians, consolidate key resources for the management of children with achondroplasia, facilitate communication between specialist, local teams and families and support delivery of high-quality care regardless of setting and geographical location. The guidelines include a series of consensus statements, developed using a modified Delphi process. These statements are supported by the best available evidence assessed using the National Health and Medicine Research Council's criteria for Level of Evidence and their Grading of Recommendations Assessment, Development and Evaluation (GRADE). Additionally, age specific guides are presented that focus on the key domains of growth, medical, development, psychosocial and community. The guidelines are intended for use by health professionals and children and young people with achondroplasia and their families living in Australia.

Bruck Syndrome: Beyond the Obvious.

INTRODUCTION: Bruck syndrome is a rare autosomal recessive disease characterized by multiple joint contractures, bone fragility, and fractures. Two genes have been associated with Bruck syndrome, FKBP10 and PLOD2, though they are phenotypically indistinguishable. CASE PRESENTATION: We present a prenatally diagnosed case of Bruck syndrome in a young multiparous woman, with no notable personal, family or obstetric history. A 12-week ultrasound raised the suspicion of short long bones, subsequently confirmed at 16 weeks. In addition, bilateral fixed flexion of the elbow, wrist, and knee joints as well as talipes was observed. Chromosomal SNP microarray analysis (0.2 Mb) detected a homozygous deletion at chromosome 3, band q24, involving a part of PLOD2 to a part of PLSCR4. At mid-trimester morphology, bilateral intrauterine fractures of the humerus and femur were evident. In the late third trimester, a fetal echocardiogram noted enlargement of the right heart with severe tricuspid regurgitation in combination with pulmonary insufficiency and a restrictive arterial duct. The potential risk of premature closure of the ductus arteriosus near term led to delivery by emergency caesarean section. CONCLUSION: To our knowledge, this is the first case of Bruck syndrome prenatally confirmed by chromosomal microarray analysis and the second reported case with an extra-skeletal abnormality. This case highlights the importance of comprehensive fetal morphological assessment during pregnancy as diagnosis of an additional abnormality has the potential to impact both management and prognosis.

Chondroitin sulfate disaccharide is a specific and sensitive biomarker for mucopolysaccharidosis type IVA.

Mucopolysaccharidosis type IVA (MPS IVA) is an inborn error of glycosaminoglycan (GAG) catabolism characterized by a deficiency of the lysosomal enzyme, N-acetylgalactosamine 6-sulphatase (GALNS). Consequently, partially degraded GAG, chondroitin 6-sulfate (CS) and keratan sulfate (KS), accumulate in the lysosomes of affected cells, primarily in cartilage resulting in skeletal disease. Excessive urinary excretion of these GAG is often used as the initial biochemical parameter to inform a laboratory diagnosis. Here we present the utility of a CS-disaccharide with a non-reducing 6-sulfated N-acetylgalactosamine residue (HNAc-UA (1S))-the enzyme's substrate-for the diagnosis and biochemical monitoring of MPS IVA patients. Following implementation of this method into the diagnostic laboratory, we identified one MPS IVA patient over 3 years of MPS urine screening, with no false positive results from 2050 urines tested. Uniquely, urinary concentrations of HNAc-UA (1S) are independent of age meaning that age-related reference ranges are not required. Urinary HNAc-UA (1S) was also able to identify two MPS IVA siblings who remained misdiagnosed with spondyloepiphyseal dysplasia for 5 years because of normal urinary GAG. HNAc-UA (1S) could also be used as a biomarker for monitoring response to enzyme replacement therapy (ERT) as there was a drop in urinary concentration following the administration of ERT in all 12 patients and concentrations correlated with urinary KS (R 2 = 0.92). In conclusion, HNAc-UA (1S) is a reliable, sensitive and specific biomarker for the diagnosis of MPS IVA and can be used to biochemically monitor the response to ERT.

An Activating Variant in CTNNB1 is Associated with a Sclerosing Bone Dysplasia and Adrenocortical Neoplasia.

CONTEXT: The WNT/ß-catenin pathway is central to the pathogenesis of various human diseases including those affecting bone development and tumor progression. OBJECTIVE: To evaluate the role of a gain-of-function variant in CTNNB1 in a child with a sclerosing bone dysplasia and an adrenocortical adenoma. DESIGN: Whole exome sequencing with corroborative biochemical analyses. PATIENTS: We recruited a child with a sclerosing bone dysplasia and an adrenocortical adenoma together with her unaffected parents. INTERVENTION: Whole exome sequencing and performance of immunoblotting and luciferase-based assays to assess the cellular consequences of a de novo variant in CTNNB1. MAIN OUTCOME MEASURE(S)/RESULT: A de novo variant in CTNNB1 (c.131C>T; p.[Pro44Leu]) was identified in a patient with a sclerosing bone dysplasia and an adrenocortical adenoma. A luciferase-based transcriptional assay of WNT signaling activity verified that the activity of ß-catenin was increased in the cells transfected with a CTNNB1p.Pro44Leu construct (P = 4.00 × 10-5). The ß-catenin p.Pro44Leu variant was also associated with a decrease in phosphorylation at Ser45 and Ser33/Ser37/Thr41 in comparison to a wild-type (WT) CTNNB1 construct (P = 2.16 × 10-3, P = 9.34 × 10-8 respectively). CONCLUSION: Increased ß-catenin activity associated with a de novo gain-of-function CTNNB1 variant is associated with osteosclerotic phenotype and adrenocortical neoplasia.

Defective DNA Polymerase α-Primase Leads to X-Linked Intellectual Disability Associated with Severe Growth Retardation, Microcephaly, and Hypogonadism.

Replicating the human genome efficiently and accurately is a daunting challenge involving the duplication of upward of three billion base pairs. At the core of the complex machinery that achieves this task are three members of the B family of DNA polymerases: DNA polymerases α, δ, and ε. Collectively these multimeric polymerases ensure DNA replication proceeds at optimal rates approaching 2 × 103 nucleotides/min with an error rate of less than one per million nucleotides polymerized. The majority of DNA replication of undamaged DNA is conducted by DNA polymerases δ and ε. The DNA polymerase α-primase complex performs limited synthesis to initiate the replication process, along with Okazaki-fragment synthesis on the discontinuous lagging strand. An increasing number of human disorders caused by defects in different components of the DNA-replication apparatus have been described to date. These are clinically diverse and involve a wide range of features, including variable combinations of growth delay, immunodeficiency, endocrine insufficiencies, lipodystrophy, and cancer predisposition. Here, by using various complementary approaches, including classical linkage analysis, targeted next-generation sequencing, and whole-exome sequencing, we describe distinct missense and splice-impacting mutations in POLA1 in five unrelated families presenting with an X-linked syndrome involving intellectual disability, proportionate short stature, microcephaly, and hypogonadism. POLA1 encodes the p180 catalytic subunit of DNA polymerase α-primase. A range of replicative impairments could be demonstrated in lymphoblastoid cell lines derived from affected individuals. Our findings describe the presentation of pathogenic mutations in a catalytic component of a B family DNA polymerase member, DNA polymerase α.

Homozygous variant in C21orf2 in a case of Jeune syndrome with severe thoracic involvement: Extending the phenotypic spectrum.

We previously reported exome sequencing in a short-rib thoracic dystrophy (SRTD) cohort, in whom recessive mutations were identified in SRTD-associated genes in 10 of 11 cases. A heterozygous stop mutation in the known SRTD gene WDR60 was identified in the remaining case; no novel candidate gene/s were suggested by homozygous/compound heterozygous analysis. This case was thus considered unsolved. Re-analysis following an analysis pipeline update identified a homozygous mutation in C21orf2 (c.218G > C; p.Arg73Pro). This homozygous variant was previously removed at the quality control stage by the default GATK parameter "in-breeding co-efficient." C21orf2 was recently associated with both Jeune asphyxiating thoracic dystrophy (JATD) and axial spondylometaphyseal dysplasia (axial SMD); this particular mutation was reported in homozygous and compound heterozygous state in both conditions. Our case has phenotypic features of both JATD and axial SMD; and the extent of thoracic involvement appears more severe than in other C21orf2-positive cases. Identification of a homozygous C21orf2 mutation in this case emphasizes the value of exome sequencing for simultaneously screening known genes and identifying novel genes. Additionally, it highlights the importance of re-interrogating data both as novel gene associations are identified and as analysis pipelines are refined. Finally, the severity of thoracic restriction in this case adds to the phenotypic spectrum attributable to C21orf2 mutations.

Mutations in LTBP3 cause acromicric dysplasia and geleophysic dysplasia.

BACKGROUND: Acromelic dysplasias are a group of disorders characterised by short stature, brachydactyly, limited joint extension and thickened skin and comprises acromicric dysplasia (AD), geleophysic dysplasia (GD), Myhre syndrome and Weill-Marchesani syndrome. Mutations in several genes have been identified for these disorders (including latent transforming growth factor ß (TGF-ß)-binding protein-2 (LTBP2), ADAMTS10, ADAMSTS17 and fibrillin-1 (FBN1) for Weill-Marchesani syndrome, ADAMTSL2 for recessive GD and FBN1 for AD and dominant GD), encoding proteins involved in the microfibrillar network. However, not all cases have mutations in these genes. METHODS: Individuals negative for mutations in known acromelic dysplasia genes underwent whole exome sequencing. RESULTS: A heterozygous missense mutation (exon 14: c.2087C>G: p.Ser696Cys) in latent transforming growth factor ß (TGF-ß)-binding protein-3 (LTBP3) was identified in a dominant AD family. Two distinct de novo heterozygous LTPB3 mutations were also identified in two unrelated GD individuals who had died in early childhood from respiratory failure-a donor splice site mutation (exon 12 c.1846+5G>A) and a stop-loss mutation (exon 28: c.3912A>T: p.1304*Cysext*12). CONCLUSIONS: The constellation of features in these AD and GD cases, including postnatal growth retardation of long bones and lung involvement, is reminiscent of the null ltbp3 mice phenotype. We conclude that LTBP3 is a novel component of the microfibrillar network involved in the acromelic dysplasia spectrum.

Mutations in human C2CD3 cause skeletal dysplasia and provide new insights into phenotypic and cellular consequences of altered C2CD3 function.

Ciliopathies are a group of genetic disorders caused by defective assembly or dysfunction of the primary cilium, a microtubule-based cellular organelle that plays a key role in developmental signalling. Ciliopathies are clinically grouped in a large number of overlapping disorders, including the orofaciodigital syndromes (OFDS), the short rib polydactyly syndromes and Jeune asphyxiating thoracic dystrophy. Recently, mutations in the gene encoding the centriolar protein C2CD3 have been described in two families with a new sub-type of OFDS (OFD14), with microcephaly and cerebral malformations. Here we describe a third family with novel compound heterozygous C2CD3 mutations in two fetuses with a different clinical presentation, dominated by skeletal dysplasia with no microcephaly. Analysis of fibroblast cultures derived from one of these fetuses revealed a reduced ability to form cilia, consistent with previous studies in C2cd3-mutant mouse and chicken cells. More detailed analyses support a role for C2CD3 in basal body maturation; but in contrast to previous mouse studies the normal recruitment of the distal appendage protein CEP164 suggests that this protein is not sufficient for efficient basal body maturation and subsequent axonemal extension in a C2CD3-defective background.

A study of the clinical and radiological features in a cohort of 93 patients with a COL2A1 mutation causing spondyloepiphyseal dysplasia congenita or a related phenotype.

Type 2 collagen disorders encompass a diverse group of skeletal dysplasias that are commonly associated with orthopedic, ocular, and hearing problems. However, the frequency of many clinical features has never been determined. We retrospectively investigated the clinical, radiological, and genotypic data in a group of 93 patients with molecularly confirmed SEDC or a related disorder. The majority of the patients (80/93) had short stature, with radiological features of SEDC (n = 64), others having SEMD (n = 5), Kniest dysplasia (n = 7), spondyloperipheral dysplasia (n = 2), or Torrance-like dysplasia (n = 2). The remaining 13 patients had normal stature with mild SED, Stickler-like syndrome or multiple epiphyseal dysplasia. Over 50% of the patients had undergone orthopedic surgery, usually for scoliosis, femoral osteotomy or hip replacement. Odontoid hypoplasia was present in 56% (95% CI 38-74) and a correlation between odontoid hypoplasia and short stature was observed. Atlanto-axial instability, was observed in 5 of the 18 patients (28%, 95% CI 10-54) in whom flexion-extension films of the cervical spine were available; however, it was rarely accompanied by myelopathy. Myopia was found in 45% (95% CI 35-56), and retinal detachment had occurred in 12% (95% CI 6-21; median age 14 years; youngest age 3.5 years). Thirty-two patients complained of hearing loss (37%, 95% CI 27-48) of whom 17 required hearing aids. The ophthalmological features and possibly also hearing loss are often relatively frequent and severe in patients with splicing mutations. Based on clinical findings, age at onset and genotype-phenotype correlations in this cohort, we propose guidelines for the management and follow-up in this group of disorders.

Compound heterozygous mutations in RIPPLY2 associated with vertebral segmentation defects.

Segmentation defects of the vertebrae (SDV) are caused by aberrant somite formation during embryogenesis and result in irregular formation of the vertebrae and ribs. The Notch signal transduction pathway plays a critical role in somite formation and patterning in model vertebrates. In humans, mutations in several genes involved in the Notch pathway are associated with SDV, with both autosomal recessive (MESP2, DLL3, LFNG, HES7) and autosomal dominant (TBX6) inheritance. However, many individuals with SDV do not carry mutations in these genes. Using whole-exome capture and massive parallel sequencing, we identified compound heterozygous mutations in RIPPLY2 in two brothers with multiple regional SDV, with appropriate familial segregation. One novel mutation (c.A238T:p.Arg80*) introduces a premature stop codon. In transiently transfected C2C12 mouse myoblasts, the RIPPLY2 mutant protein demonstrated impaired transcriptional repression activity compared with wild-type RIPPLY2 despite similar levels of expression. The other mutation (c.240-4T>G), with minor allele frequency <0.002, lies in the highly conserved splice site consensus sequence 5' to the terminal exon. Ripply2 has a well-established role in somitogenesis and vertebral column formation, interacting at both gene and protein levels with SDV-associated Mesp2 and Tbx6. We conclude that compound heterozygous mutations in RIPPLY2 are associated with SDV, a new gene for this condition.

Optimal management of complications associated with achondroplasia.

Achondroplasia is the most common form of skeletal dysplasia, resulting in disproportionate short stature, and affects over 250,000 people worldwide. Individuals with achondroplasia demonstrate a number of well-recognized anatomical features that impact on growth and development, with a complex array of medical issues that are best managed through a multidisciplinary team approach. The complexity of this presentation, whereby individual impairments may impact upon multiple activity and participation areas, requires consideration and discussion under a broad framework to gain a more thorough understanding of the experience of this condition for individuals with achondroplasia. This paper examines the general literature and research evidence on the medical and health aspects of individuals with achondroplasia and presents a pictorial model of achondroplasia based on The International Classification of Functioning, Disability, and Health (ICF). An expanded model of the ICF will be used to review and present the current literature pertaining to the musculoskeletal, neurological, cardiorespiratory, and ear, nose, and throat impairments and complications across the lifespan, with discussion on the impact of these impairments upon activity and participation performance. Further research is required to fully identify factors influencing participation and to help develop strategies to address these factors.

The IFITM5 mutation c.-14C > T results in an elongated transcript expressed in human bone; and causes varying phenotypic severity of osteogenesis imperfecta type V.

BACKGROUND: The genetic mutation resulting in osteogenesis imperfecta (OI) type V was recently characterised as a single point mutation (c.-14C > T) in the 5' untranslated region (UTR) of IFITM5, a gene encoding a transmembrane protein with expression restricted to skeletal tissue. This mutation creates an alternative start codon and has been shown in a eukaryotic cell line to result in a longer variant of IFITM5, but its expression has not previously been demonstrated in bone from a patient with OI type V. METHODS: Sanger sequencing of the IFITM5 5' UTR was performed in our cohort of subjects with a clinical diagnosis of OI type V. Clinical data was collated from referring clinicians. RNA was extracted from a bone sample from one patient and Sanger sequenced to determine expression of wild-type and mutant IFITM5. RESULTS: All nine subjects with OI type V were heterozygous for the c.-14C > T IFITM5 mutation. Clinically, there was heterogeneity in phenotype, particularly in the manifestation of bone fragility amongst subjects. Both wild-type and mutant IFITM5 mRNA transcripts were present in bone. CONCLUSIONS: The c.-14C > T IFITM5 mutation does not result in an RNA-null allele but is expressed in bone. Individuals with identical mutations in IFITM5 have highly variable phenotypic expression, even within the same family.

Semantic interestingness measures for discovering association rules in the skeletal dysplasia domain.

BACKGROUND: Lately, ontologies have become a fundamental building block in the process of formalising and storing complex biomedical information. With the currently existing wealth of formalised knowledge, the ability to discover implicit relationships between different ontological concepts becomes particularly important. One of the most widely used methods to achieve this is association rule mining. However, while previous research exists on applying traditional association rule mining on ontologies, no approach has, to date, exploited the advantages brought by using the structure of these ontologies in computing rule interestingness measures. RESULTS: We introduce a method that combines concept similarity metrics, formulated using the intrinsic structure of a given ontology, with traditional interestingness measures to compute semantic interestingness measures in the process of association rule mining. We apply the method in our domain of interest - bone dysplasias - using the core ontologies characterising it and an annotated dataset of patient clinical summaries, with the goal of discovering implicit relationships between clinical features and disorders. Experimental results show that, using the above mentioned dataset and a voting strategy classification evaluation, the best scoring traditional interestingness measure achieves an accuracy of 57.33%, while the best scoring semantic interestingness measure achieves an accuracy of 64.38%, both at the recall cut-off point 5. CONCLUSIONS: Semantic interestingness measures outperform the traditional ones, and hence show that they are able to exploit the semantic similarities inherently present between ontological concepts. Nevertheless, this is dependent on the domain, and implicitly, on the semantic similarity metric chosen to model it.

Inferring characteristic phenotypes via class association rule mining in the bone dysplasia domain.

Finding, capturing and describing characteristic features represents a key aspect in disorder definition, diagnosis and management. This process is particularly challenging in the case of rare disorders, due to the sparse nature of data and expertise. From a computational perspective, finding characteristic features is associated with some additional major challenges, such as formulating a computationally tractable definition, devising appropriate inference algorithms or defining sound validation mechanisms. In this paper we aim to deal with each of these problems in the context provided by the skeletal dysplasia domain. We propose a clear definition for characteristic phenotypes, we experiment with a novel, class association rule mining algorithm and we discuss our lessons learned from both an automatic and human-based validation of our approach.

Mutations in the gene encoding IFT dynein complex component WDR34 cause Jeune asphyxiating thoracic dystrophy.

Bidirectional (anterograde and retrograde) motor-based intraflagellar transport (IFT) governs cargo transport and delivery processes that are essential for primary cilia growth and maintenance and for hedgehog signaling functions. The IFT dynein-2 motor complex that regulates ciliary retrograde protein transport contains a heavy chain dynein ATPase/motor subunit, DYNC2H1, along with other less well functionally defined subunits. Deficiency of IFT proteins, including DYNC2H1, underlies a spectrum of skeletal ciliopathies. Here, by using exome sequencing and a targeted next-generation sequencing panel, we identified a total of 11 mutations in WDR34 in 9 families with the clinical diagnosis of Jeune syndrome (asphyxiating thoracic dystrophy). WDR34 encodes a WD40 repeat-containing protein orthologous to Chlamydomonas FAP133, a dynein intermediate chain associated with the retrograde intraflagellar transport motor. Three-dimensional protein modeling suggests that the identified mutations all affect residues critical for WDR34 protein-protein interactions. We find that WDR34 concentrates around the centrioles and basal bodies in mammalian cells, also showing axonemal staining. WDR34 coimmunoprecipitates with the dynein-1 light chain DYNLL1 in vitro, and mining of proteomics data suggests that WDR34 could represent a previously unrecognized link between the cytoplasmic dynein-1 and IFT dynein-2 motors. Together, these data show that WDR34 is critical for ciliary functions essential to normal development and survival, most probably as a previously unrecognized component of the mammalian dynein-IFT machinery.

Defects in the IFT-B component IFT172 cause Jeune and Mainzer-Saldino syndromes in humans.

Intraflagellar transport (IFT) depends on two evolutionarily conserved modules, subcomplexes A (IFT-A) and B (IFT-B), to drive ciliary assembly and maintenance. All six IFT-A components and their motor protein, DYNC2H1, have been linked to human skeletal ciliopathies, including asphyxiating thoracic dystrophy (ATD; also known as Jeune syndrome), Sensenbrenner syndrome, and Mainzer-Saldino syndrome (MZSDS). Conversely, the 14 subunits in the IFT-B module, with the exception of IFT80, have unknown roles in human disease. To identify additional IFT-B components defective in ciliopathies, we independently performed different mutation analyses: candidate-based sequencing of all IFT-B-encoding genes in 1,467 individuals with a nephronophthisis-related ciliopathy or whole-exome resequencing in 63 individuals with ATD. We thereby detected biallelic mutations in the IFT-B-encoding gene IFT172 in 12 families. All affected individuals displayed abnormalities of the thorax and/or long bones, as well as renal, hepatic, or retinal involvement, consistent with the diagnosis of ATD or MZSDS. Additionally, cerebellar aplasia or hypoplasia characteristic of Joubert syndrome was present in 2 out of 12 families. Fibroblasts from affected individuals showed disturbed ciliary composition, suggesting alteration of ciliary transport and signaling. Knockdown of ift172 in zebrafish recapitulated the human phenotype and demonstrated a genetic interaction between ift172 and ift80. In summary, we have identified defects in IFT172 as a cause of complex ATD and MZSDS. Our findings link the group of skeletal ciliopathies to an additional IFT-B component, IFT172, similar to what has been shown for IFT-A.