Epiphyseal dysplasia and other skeletal anomalies in a patient with the 6p25 microdeletion syndrome

FOXC1 and FOXC2 regulate growth plate chondrocyte maturation towards hypertrophy in the embryonic mouse limb skeleton

The Forkhead box transcription factors FOXC1 and FOXC2 are expressed in condensing mesenchyme cells at the onset of endochondral ossification. We used the Prx1-cre mouse to ablate Foxc1 and Foxc2 in limb skeletal progenitor cells. Prx1-cre;Foxc1Δ/Δ;Foxc2Δ/Δ limbs were shorter than controls, with worsening phenotypes in distal structures. Cartilage formation and mineralization was severely disrupted in the paws. The radius and tibia were malformed, whereas the fibula and ulna remained unmineralized. Chondrocyte maturation was delayed, with fewer Indian hedgehog-expressing, prehypertrophic chondrocytes forming and a smaller hypertrophic chondrocyte zone. Later, progression out of chondrocyte hypertrophy was slowed, leading to an accumulation of COLX-expressing hypertrophic chondrocytes and formation of a smaller primary ossification center with fewer osteoblast progenitor cells populating this region. Targeting Foxc1 and Foxc2 in hypertrophic chondrocytes with Col10a1-cre also resulted in an expanded hypertrophic chondrocyte zone and smaller primary ossification center. Our findings suggest that FOXC1 and FOXC2 direct chondrocyte maturation towards hypertrophic chondrocyte formation. At later stages, FOXC1 and FOXC2 regulate function in hypertrophic chondrocyte remodeling to allow primary ossification center formation and osteoblast recruitment.

Case report: Extending the spectrum of clinical and molecular findings in FOXC1 haploinsufficiency syndrome

FOXC1 is a ubiquitously expressed forkhead transcription factor that plays a critical role during early development. Germline pathogenic variants in FOXC1 are associated with anterior segment dysgenesis and Axenfeld-Rieger syndrome (ARS, #602482), an autosomal dominant condition with ophthalmologic anterior segment abnormalities, high risk for glaucoma and extraocular findings including distinctive facial features, as well as dental, skeletal, audiologic, and cardiac anomalies. De Hauwere syndrome is an ultrarare condition previously associated with 6p microdeletions and characterized by anterior segment dysgenesis, joint instability, short stature, hydrocephalus, and skeletal abnormalities. Here, we report clinical findings of two unrelated adult females with FOXC1 haploinsufficiency who have ARS and skeletal abnormalities. Final molecular diagnoses of both patients were achieved using genome sequencing. Patient 1 had a complex rearrangement involving a 4.9 kB deletion including FOXC1 coding region (Hg19; chr6:1,609,721-1,614,709), as well as a 7 MB inversion (Hg19; chr6:1,614,710-8,676,899) and a second deletion of 7.1 kb (Hg19; chr6:8,676,900-8,684,071). Patient 2 had a heterozygous single nucleotide deletion, resulting in a frameshift and a premature stop codon in FOXC1 (NM_001453.3): c.467del, p.(Pro156Argfs*25). Both individuals had moderate short stature, skeletal abnormalities, anterior segment dysgenesis, glaucoma, joint laxity, pes planovalgus, dental anomalies, hydrocephalus, distinctive facial features, and normal intelligence. Skeletal surveys revealed dolichospondyly, epiphyseal hypoplasia of femoral and humeral heads, dolichocephaly with frontal bossin gand gracile long bones. We conclude that haploinsufficiency of FOXC1 causes ARS and a broad spectrum of symptoms with variable expressivity that at its most severe end also includes a phenotype overlapping with De Hauwere syndrome.

The phenotypic spectrum of terminal and subterminal 6p deletions based on a social media-derived cohort and literature review

BACKGROUND

Terminal 6p deletions are rare, and information on their clinical consequences is scarce, which impedes optimal management and follow-up by clinicians. The parent-driven Chromosome 6 Project collaborates with families of affected children worldwide to better understand the clinical effects of chromosome 6 aberrations and to support clinical guidance. A microarray report is required for participation, and detailed phenotype information is collected directly from parents through a multilingual web-based questionnaire. Information collected from parents is then combined with case data from literature reports. Here, we present our findings on 13 newly identified patients and 46 literature cases with genotypically well-characterised terminal and subterminal 6p deletions. We provide phenotype descriptions for both the whole group and for subgroups based on deletion size and HI gene content.

RESULTS

The total group shared a common phenotype characterised by ocular anterior segment dysgenesis, vision problems, brain malformations, congenital defects of the cardiac septa and valves, mild to moderate hearing impairment, eye movement abnormalities, hypotonia, mild developmental delay and dysmorphic features. These characteristics were observed in all subgroups where FOXC1 was included in the deletion, confirming a dominant role for this gene. Additional characteristics were seen in individuals with terminal deletions exceeding 4.02 Mb, namely complex heart defects, corpus callosum abnormalities, kidney abnormalities and orofacial clefting. Some of these additional features may be related to the loss of other genes in the terminal 6p region, such as RREB1 for the cardiac phenotypes and TUBB2A and TUBB2B for the cerebral phenotypes. In the newly identified patients, we observed previously unreported features including gastrointestinal problems, neurological abnormalities, balance problems and sleep disturbances.

CONCLUSIONS

We present an overview of the phenotypic characteristics observed in terminal and subterminal 6p deletions. This reveals a common phenotype that can be highly attributable to haploinsufficiency of FOXC1, with a possible additional effect of other genes in the 6p25 region. We also delineate the developmental abilities of affected individuals and report on previously unrecognised features, showing the added benefit of collecting information directly from parents. Based on our overview, we provide recommendations for clinical surveillance to support clinicians, patients and families.

Chromosome 6p25 deletion syndrome: A case report and review of ophthalmic features

The 6p25 deletion syndrome is a rare genetic disorder characterized by a wide spectrum of congenital anomalies. Ophthalmic abnormalities appear to be highly associated with the syndrome, although this relationship has not been well characterized to date. We conducted a systematic literature review to highlight the ocular features in patients with this deletion syndrome and describe a 7-month-old female who has a 6.07 MB 6p25.1p25.3 deletion and a 4.25 MB 17q25.3 duplication. Our patient presented with multiple congenital anomalies, including macrocephaly, frontal bossing, low set ears, tent-shaped mouth, saddle nose, flat midface, and hearing impairment. Her ophthalmic features included proptosis, down-slanting palpebral fissures, hypertelorism, nystagmus, bilateral posterior embryotoxon, and decentered and abnormally shaped pupils. A systematic review of the published cases with sufficient clinical eye descriptions included 63 cases with a confirmed 6p25 deletion. The most common eye findings observed were posterior embryotoxon, iris hypoplasia, corectopia, cornea opacity, and glaucoma.

Ophthalmological Manifestations of Axenfeld-Rieger Syndrome: Current Perspectives

Axenfeld-Rieger syndrome (ARS) is a rare congenital disease that is primarily characterized by ocular anterior segment anomalies but is also associated with craniofacial, dental, cardiac, and neurologic abnormalities. Over half of cases are linked with autosomal dominant mutations in either FOXC1 or PITX2, which reflects the molecular role of these genes in regulating neural crest cell contributions to the eye, face, and heart. Within the eye, ARS is classically defined as the combination of posterior embryotoxon with iris bridging strands (Axenfeld anomaly) and iris hypoplasia causing corectopia and pseudopolycoria (Rieger anomaly). Glaucoma due to iridogoniodysgenesis is the main source of morbidity and is typically diagnosed during infancy or childhood in over half of affected individuals. Angle bypass surgery, such as glaucoma drainage devices and trabeculectomies, is often needed to obtain intraocular pressure control. A multi-disciplinary approach including glaucoma specialists and pediatric ophthalmologists produces optimal outcomes as vision is dependent on many factors including glaucoma, refractive error, amblyopia and strabismus. Further, since ophthalmologists often make the diagnosis, it is important to refer patients with ARS to other specialists including dentistry, cardiology, and neurology.

Comparison of Anterior Segment Abnormalities in Individuals With FOXC1 and PITX2 Variants

PURPOSE

Axenfeld-Rieger syndrome encompasses a group of developmental disorders affecting the anterior chamber structures of the eye, with associated systemic features in some cases. This study aims to compare the difference in anterior segment phenotypes such as those involving the cornea, iris, lens, and anterior chamber angle between cases with disease-causing sequence variations in FOXC1 and PITX2 .

METHODS

This cross-sectional study involved 61 individuals, from 32 families with pathogenic FOXC1 or PITX2 variants, who were registered with the Australian and New Zealand Registry of Advanced Glaucoma.

RESULTS

The median age of the cohort was 39 years at the time of last assessment (range 3-85 years; females, 54%). Thirty-two patients had pathogenic variants in the FOXC1 gene, and 29 patients had pathogenic variants in the PITX2 gene. Corneal abnormalities were more common in individuals with FOXC1 variants (18/36, 50%) than those with PITX2 variants (4/25, 16%; P = 0.007). Iris abnormalities such as hypoplasia ( P = 0.008) and pseudopolycoria ( P = 0.001) were more common in individuals with PITX2 variants than those with FOXC1 variants. Glaucoma was present in 72% of participants. Corneal decompensation was positively associated with corneal abnormalities ( P < 0.001), glaucoma surgery ( P = 0.025), and cataract surgery ( P = 0.002).

CONCLUSIONS

Corneal abnormalities were more common in individuals with FOXC1 than in those with PITX2 variants and were often associated with early onset glaucoma. These findings highlight that patients with FOXC1 variations require close follow-up and monitoring throughout infancy and into adulthood.

A case of childhood glaucoma with a combined partial monosomy 6p25 and partial trisomy 18p11 due to an unbalanced translocation

Background: Chromosomal deletion involving the 6p25 region results in a clinically recognizable syndrome characterized by anterior eye chamber anomalies with risk of glaucoma and non-ocular malformations (6p25 deletion syndrome). We report a newborn infant case of childhood glaucoma with a combination of partial monosomy 6p25 and partial trisomy 18p11 due to an unbalanced translocation.Materials and methods: The patient was a 0-year-old girl. Both eyes showed aniridia and left eye Peters anomaly with multiple malformations. To identify the chromosomal aberrations in the patient with clinically suspected 6p25 deletion syndrome, we performed cytogenetic analysis (G-banding and multicolor fluorescent in-situ hybridization) and array-based comparative genomic hybridization (array-CGH) analysis.Results: Cytogenetic analyses revealed a derivative chromosome 6 with its distal short arm replaced by an extra copy of the short arm of chromosome 18. Array-CGH analysis detected a 4.6-Mb deletion at 6pter to 6p25.1 and 8.9-Mb duplication at 18pter to 18p11.22. To determine the breakpoint of the unbalanced rearrangement at the single-base level, we performed a long-range PCR for amplifying the junctional fragment of the translocation breakpoint. By sequencing the junctional fragment, we defined the unbalanced translocation as g.chr6:pter_4594783delinschr18:pter_8911541.Conclusions: A phenotype corresponding to combined monosomy 6p25 and trisomy 18p11 presented as childhood glaucoma associated with non-acquired (congenital) ocular anomalies consist of aniridia and Peters anomaly and other systemic malformations. To the best of our knowledge, this is the first report which demonstrated the breakpoint sequence of an unbalanced translocation in a Japanese infant with childhood glaucoma.

Phenotype of a Belgian Family With 6p25 Deletion Syndrome

BACKGROUND

The 6p25 deletion syndrome is one of the many syndromes with both hearing impairment as well as vision impairment. However, the audiometric characteristics and radiological findings of patients with 6p25 deletions are only scarcely described in literature. This study focused on characterizing the audiometric and radiological features of a Belgian family with a chromosome 6p25 deletion.

OBJECTIVE

To evaluate the hearing impairment, audiometric testing and radiological examination of the temporal bones in 3 family members with a 3.4 Mb deletion in chromosome band 6p25.

RESULTS

All 3 family members demonstrated slowly progressive sensorineural or mixed hearing impairment. Radiologic examination revealed thickened and sclerotic stapes in all patients and a minor internal partition type II of the cochlea in 2 patients.

CONCLUSION

There is a significant phenotypic variability within and among families with the 6p25 deletion syndrome. A thorough genotype-phenotype correlation is difficult because of the small number of affected patients and the limited clinical data available. More clinical data of families with 6p25 deletions need to be published in order to create a reliable and precise phenotypic characterization. However, our findings can facilitate counseling of hearing impairment caused by 6p25 deletions.

Use of Targeted Exome Sequencing for Molecular Diagnosis of Skeletal Disorders

Genetic disorders of the skeleton comprise a large group of more than 450 clinically distinct and genetically heterogeneous diseases associated with mutations in more than 300 genes. Achieving a definitive diagnosis is complicated due to the genetic heterogeneity of these disorders, their individual rarity and their diverse radiographic presentations. We used targeted exome sequencing and designed a 1.4Mb panel for simultaneous testing of more than 4,800 exons in 309 genes involved in skeletal disorders. DNA from 69 individuals from 66 families with a known or suspected clinical diagnosis of a skeletal disorder was analyzed. Of 36 cases with a specific clinical hypothesis with a known genetic basis, mutations were identified for eight cases (22%). Of 20 cases with a suspected skeletal disorder but without a specific diagnosis, four causative mutations were identified. Also included were 11 cases with a specific skeletal disorder but for which there was at the time no known associated gene. For these cases, one mutation was identified in a known skeletal disease genes, and re-evaluation of the clinical phenotype in this case changed the diagnoses from osteodysplasia syndrome to Apert syndrome. These results suggest that the NGS panel provides a fast, accurate and cost-effective molecular diagnostic tool for identifying mutations in a highly genetically heterogeneous set of disorders such as genetic skeletal disorders. The data also stress the importance of a thorough clinical evaluation before DNA sequencing. The strategy should be applicable to other groups of disorders in which the molecular basis is largely known.

Indications for a systemic work-up in glaucoma

Most glaucomas are primary in nature. However, many adult and childhood glaucomas are secondary, and they require systemic evaluation to pick up associated systemic disease. Conditions such as nocturnal hypotension and sleep apnea may contribute to glaucomatous progression, whereas neurologic diseases may mimic normal tension glaucoma based on disc appearance. This review highlights those conditions in which a focused systemic work-up can improve glaucoma management and potentially discover life-threatening disease.

A genome-wide copy number association study of osteoporotic fractures points to the 6p25.1 locus

BACKGROUND

Osteoporosis is a systemic skeletal disease characterised by reduced bone mineral density and increased susceptibility to fracture; these traits are highly heritable. Both common and rare copy number variants (CNVs) potentially affect the function of genes and may influence disease risk.

AIM

To identify CNVs associated with osteoporotic bone fracture risk.

METHOD

We performed a genome-wide CNV association study in 5178 individuals from a prospective cohort in the Netherlands, including 809 osteoporotic fracture cases, and performed in silico lookups and de novo genotyping to replicate in several independent studies.

RESULTS

A rare (population prevalence 0.14%, 95% CI 0.03% to 0.24%) 210 kb deletion located on chromosome 6p25.1 was associated with the risk of fracture (OR 32.58, 95% CI 3.95 to 1488.89; p = 8.69 × 10(-5)). We performed an in silico meta-analysis in four studies with CNV microarray data and the association with fracture risk was replicated (OR 3.11, 95% CI 1.01 to 8.22; p = 0.02). The prevalence of this deletion showed geographic diversity, being absent in additional samples from Australia, Canada, Poland, Iceland, Denmark, and Sweden, but present in the Netherlands (0.34%), Spain (0.33%), USA (0.23%), England (0.15%), Scotland (0.10%), and Ireland (0.06%), with insufficient evidence for association with fracture risk.

CONCLUSIONS

These results suggest that deletions in the 6p25.1 locus may predispose to higher risk of fracture in a subset of populations of European origin; larger and geographically restricted studies will be needed to confirm this regional association. This is a first step towards the evaluation of the role of rare CNVs in osteoporosis.

PITX2 and FOXC1 spectrum of mutations in ocular syndromes

Anterior segment dysgenesis (ASD) encompasses a broad spectrum of developmental conditions affecting anterior ocular structures and associated with an increased risk for glaucoma. Various systemic anomalies are often observed in ASD conditions such as Axenfeld-Rieger syndrome (ARS) and De Hauwere syndrome. We report DNA sequencing and copy number analysis of PITX2 and FOXC1 in 76 patients with syndromic or isolated ASD and related conditions. PITX2 mutations and deletions were found in 24 patients with dental and/or umbilical anomalies seen in all. Seven PITX2-mutant alleles were novel including c.708_730del, the most C-terminal mutation reported to date. A second case of deletion of the distant upstream but not coding region of PITX2 was identified, highlighting the importance of this recently discovered mechanism for ARS. FOXC1 deletions were observed in four cases, three of which demonstrated hearing and/or heart defects, including a patient with De Hauwere syndrome; no nucleotide mutations in FOXC1 were identified. Review of the literature identified several other patients with 6p25 deletions and features of De Hauwere syndrome. The 1.3-Mb deletion of 6p25 presented here defines the critical region for this phenotype and includes the FOXC1, FOXF2, and FOXQ1 genes. In summary, PITX2 or FOXC1 disruptions explained 63% of ARS and 6% of other ASD in our cohort; all affected patients demonstrated additional systemic defects with PITX2 mutations showing a strong association with dental and/or umbilical anomalies and FOXC1 with heart and hearing defects. FOXC1 deletion was also found to be associated with De Hauwere syndrome.

Pre- and postnatal phenotype of 6p25 deletions involving the FOXC1 gene

FOXC1 deletion, duplication, and mutations are associated with Axenfeld-Rieger anomaly, and Dandy-Walker malformation spectrum. We describe the clinical history, physical findings, and available brain imaging studies in three fetuses, two children, and one adult with 6p25 deletions encompassing FOXC1. Various combinations of ocular and cerebellar malformations were found. In all three fetuses, necropsy including detailed microscopic assessments of the eyes and brains showed ocular anterior segment dysgenesis suggestive of Axenfeld-Rieger anomaly. Five 6p25 deletions were terminal, including two derived from inherited reciprocal translocations; the remaining 6p25 deletion was interstitial. The size and breakpoints of these deletions were characterized using comparative genomic hybridization arrays. All six deletions included FOXC1. Our data confirm that FOXC1 haploinsufficiency plays a major role in the phenotype of patients with 6p25 deletions. Histopathological features of Axenfeld-Rieger anomaly were clearly identifiable before the beginning of the third-trimester of gestation.

Clinical phenotypes associated with type II collagen mutations

COL2A1 mutations give rise to a spectrum of phenotypes predominantly affecting cartilage and bone from the severe disorders that are perinatally lethal to the milder conditions that are recognised in the post-natal period and childhood. The milder chondrodysplasias are characterised by disproportionate short stature, eye abnormalities, cleft palate and hearing loss. It remains poorly recognised that there is significant variability in the disease presentation, with early onset short stature conditions and later onset milder phenotypes. Similarly, it is under-acknowledged that COL2A1 mutations may solely cause joint disease in the absence of the other mentioned phenotypic clues. The underlying hypothesis is that there are novel phenotypes caused by mutations in type II collagen that extend from premature arthritis through to more severe bone dysplasias. The importance of finding a COL2A1 mutation lies in the subsequent ability to accurately assess recurrence risks and offer information regarding disease natural history. Most importantly, it enables at-risk individuals to be identified for implementation of preventative strategies and early ameliorative management of their condition. Such interventions potentially translate into a reduction in health costs associated with musculoskeletal disease.

Two novel COL2A1 mutations associated with a Legg-Calvé-Perthes disease-like presentation

BACKGROUND

Abnormal development and growth of the capital femoral epiphysis and acetabulum are associated with a wide variety of underlying etiologies, one of which is Legg-Calvé-Perthes disease.

CASE DESCRIPTION

We report the cases of two children who presented with abnormal development of both hips and in whom novel mutations in the COL2A1 gene were found. These cases illustrate the importance of identifying individuals with a type II collagen abnormality, as it informs management, allows investigation for other complications, and provides the opportunity for accurate genetic counseling and consideration of other family members who might be at risk.

LITERATURE REVIEW

The literature documents numerous private mutations in COL2A1 associated with diverse clinical phenotypes including bilateral hip dysplasia and premature osteoarthritis. Some of these mutations are associated with a joint-specific phenotype but few other skeletal or extraskeletal manifestations. Only careful clinical examination of children presenting with hip anomalies therefore will reveal additional findings that warrant an evaluation by a clinical geneticist. DNA mutation analysis may be useful for making a specific diagnosis and identifying other at-risk family members.

PURPOSES AND CLINICAL RELEVANCE

The purpose of our report is to alert clinicians to the possibility that children who present with bilateral Perthes-like disease of the hip might have an underlying mutation in the gene encoding type II collagen. It is important to consider this in the differential diagnosis and workup of such children as it has specific prognostic, clinical, genetic counseling, and reproductive sequelae.

A complex 6p25 rearrangement in a child with multiple epiphyseal dysplasia

Genomic rearrangements are increasingly recognized as important contributors to human disease. Here we report on an 11½-year-old child with myopia, Duane retraction syndrome, bilateral mixed hearing loss, skeletal anomalies including multiple epiphyseal dysplasia, and global developmental delay, and a complex 6p25 genomic rearrangement. We have employed oligonucleotide-based comparative genomic hybridization arrays (aCGH) of different resolutions (44 and 244K) as well as a 1 M single nucleotide polymorphism (SNP) array to analyze this complex rearrangement. Our analyses reveal a complex rearrangement involving a ∼2.21 Mb interstitial deletion, a ∼240 kb terminal deletion, and a 70-80 kb region in between these two deletions that shows maintenance of genomic copy number. The interstitial deletion contains eight known genes, including three Forkhead box containing (FOX) transcription factors (FOXQ1, FOXF2, and FOXC1). The region maintaining genomic copy number partly overlaps the dual specificity protein phosphatase 22 (DUSP22) gene. Array analyses suggest a homozygous loss of genomic material at the 5' end of DUSP22, which was corroborated using TaqMan® copy number analysis. It is possible that this homozygous genomic loss may render both copies of DUSP22 or its products non-functional. Our analysis suggests a rearrangement mechanism distinct from a previously reported replication-based error-prone mechanism without template switching for a specific 6p25 rearrangement with a 1.22 Mb interstitial deletion. Our study demonstrates the utility and limitations of using oligonucleotide-based aCGH and SNP array technologies of increasing resolutions in order to identify complex DNA rearrangements and gene disruptions.