Achondrogenesis: a review with special consideration of achondrogenesis type II (Langer-Saldino)

A Novel Splicing Variant of COL2A1 in a Fetus with Achondrogenesis Type II: Interpretation of Pathogenicity of In-Frame Deletions

Achondrogenesis type II (ACG2) is a lethal skeletal dysplasia caused by dominant pathogenic variants in COL2A1. Most of the variants found in patients with ACG2 affect the glycine residue included in the Gly-X-Y tripeptide repeat that characterizes the type II collagen helix. In this study, we reported a case of a novel splicing variant of COL2A1 in a fetus with ACG2. An NGS analysis of fetal DNA revealed a heterozygous variant c.1267-2_1269del located in intron 20/exon 21. The variant occurred de novo since it was not detected in DNA from the blood samples of parents. We generated an appropriate minigene construct to study the effect of the variant detected. The minigene expression resulted in the synthesis of a COL2A1 messenger RNA lacking exon 21, which generated a predicted in-frame deleted protein. Usually, in-frame deletion variants of COL2A1 cause a phenotype such as Kniest dysplasia, which is milder than ACG2. Therefore, we propose that the size and position of an in-frame deletion in COL2A1 may be relevant in determining the phenotype of skeletal dysplasia.

Germline mutation within COL2A1 associated with lethal chondrodysplasia in a polled Holstein family

Background

The bulldog calf syndrome is a lethal form of the inherited congenital chondrodysplasias. Among the progeny of the polled Holstein bull Energy P cases of lethal chondrodysplasia were observed. Pedigrees of the cases and the frequency of 3/8 cases among the offspring of Energy P at our teaching and experimental farm Ruthe (LuFG Ruthe) supported the assumption of a germline mutation with a mosaic of normal and defective sperm.

Results

All three malformed calves were examined using necropsy, histopathology and computed tomography scanning. The phenotypic appearance of the affected calves was highly similar; they presented with severe disproportionate dwarfism and reduced body weight. The syndrome was characterized by brachygnathia superior, bilateral palatoschisis, shortening and compression of the body due to malformed vertebrae, in their size reduced and malformed ribs and reduced length of the long bones of the limbs. The bones had small irregular diaphyses and enlarged epiphyses. Whole genome sequencing of one bulldog calf, sperm of its sire Energy P and a normal progeny of Energy P identified a deleterious missense mutation (g.32476082G > A, c.2986G > A, ss2019324576) within COL2A1 on bovine chromosome (BTA) 5. Sanger sequencing confirmed the ss2019324576 variant in the affected calves and sperm of Energy P. This mutation is located within the collagen triple helix repeat and causes an exchange of glycine to serine (p.996G > S) in COL2A1. This private single nucleotide variant (SNV) was present as a gonadal mosaic in sperm of the bull. All affected calves were in a heterozygous state whereas normal half-siblings and all dams of the progeny from Energy P were missing this SNV. Validation in polled Holstein bulls and normal Holstein calves randomly sampled from several herds and from the LuFG Ruthe confirmed this SNV as private.

Conclusions

The identified spontaneous missense mutation within COL2A1 is most likely the cause of lethal chondrodysplasia in the progeny of Energy P through a dominant negative effect. This example suggests that it would be beneficial to conduct whole genome sequencing of sperm from bulls widely used in artificial insemination in order to detect germline mosaicism.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4153-0) contains supplementary material, which is available to authorized users.

Meckel's and condylar cartilages anomalies in achondroplasia result in defective development and growth of the mandible

Activating FGFR3 mutations in human result in achondroplasia (ACH), the most frequent form of dwarfism, where cartilages are severely disturbed causing long bones, cranial base and vertebrae defects. Because mandibular development and growth rely on cartilages that guide or directly participate to the ossification process, we investigated the impact of FGFR3 mutations on mandibular shape, size and position. By using CT scan imaging of ACH children and by analyzing Fgfr3^Y367C/+ mice, a model of ACH, we show that FGFR3 gain-of-function mutations lead to structural anomalies of primary (Meckel’s) and secondary (condylar) cartilages of the mandible, resulting in mandibular hypoplasia and dysmorphogenesis. These defects are likely related to a defective chondrocyte proliferation and differentiation and pan-FGFR tyrosine kinase inhibitor NVP-BGJ398 corrects Meckel’s and condylar cartilages defects ex vivo. Moreover, we show that low dose of NVP-BGJ398 improves in vivo condyle growth and corrects dysmorphologies in Fgfr3^Y367C/+ mice, suggesting that postnatal treatment with NVP-BGJ398 mice might offer a new therapeutic strategy to improve mandible anomalies in ACH and others FGFR3-related disorders.

Molecular genetics of the COL2A1-related disorders

Type II collagen, comprised of three identical alpha-1(II) chains, is the major collagen synthesized by chondrocytes, and is found in articular cartilage, vitreous humour, inner ear and nucleus pulposus. Mutations in the collagen type II alpha-1 gene (COL2A1) have been reported to be responsible for a series of abnormalities, known as type II collagenopathies. To date, 16 definite disorders, inherited in an autosomal dominant or recessive pattern, have been described to be associated with the COL2A1 mutations, and at least 405 mutations ranging from point mutations to complex rearrangements have been reported, though the underlying pathogenesis remains unclear. Significant clinical heterogeneity has been reported in COL2A1-associated type II collagenopathies. In this review, we highlight current knowledge of known mutations in the COL2A1 gene for these disorders, as well as genetic animal models related to the COL2A1 gene, which may help us understand the nature of complex phenotypes and underlying pathogenesis of these conditions.

Length of life and treatment intensity in infants diagnosed prenatally or postnatally with congenital anomalies considered to be lethal

OBJECTIVES

The objectives of this study were (1) to compare age at death and the intensity and cost of medical treatment for infants diagnosed prenatally or postnatally with congenital anomalies considered to be lethal. (2) To determine whether greater treatment intensity is associated with longer life.

STUDY DESIGN

This is a retrospective cohort study of all fetuses and neonates with congenital anomalies classified as lethal who were diagnosed or treated at the University of North Carolina Hospitals from January 1998 to December 2003.

RESULT

The cohort consisted of 192 fetuses and infants: 160 were diagnosed prenatally, 2 were diagnosed perinatally, and 30 were diagnosed postnatally. In all, 115 (72%) pregnancies were terminated. Of the liveborn infants, 75% died before 10 days of age and 90% before 4 months of age. Compared with postnatally diagnosed infants, prenatally diagnosed infants received less intense treatment (median average daily Neonatal Therapeutic Intervention Scoring System score 8.3 versus 14.0; P=0.02), at less cost (median direct cost of hospitalization $1550 versus $8474; P=0.03) and died sooner (median age at death <1 day versus 4 days; P=0.01). Greater treatment intensity did not correlate with longer survival (r=-0.04; P=0.66).

CONCLUSION

Although some kinds of medical therapy may be appropriate for newborns with lethal congenital anomalies, highly aggressive interventions did not prolong survival and should not be offered. Even when pregnancy termination is not elected, infants diagnosed prenatally receive less intense care.

Hereditary and familial syndromes of bone and blood. Genetic pathways, diagnostic pitfalls

Numerous metabolic disorders, teratogenic agents, and in utero infections lead to congenital bone disease and malformation. This review focuses on hereditary and familial disorders of bone with particular emphasis on impaired hematopoiesis, myelofibrosis, pathologic fractures, and dysmorphology of the forearm and craniofacial structures. The severity of bone disease and marrow dysfunction of any given disorder may vary considerably from one affected individual to the next, and intrapersonal variability over time may be substantial as well. Both can impart difficulty to the appropriate evaluation and delay the correct diagnosis. Many of these disorders are phenotypically quite similar but require very different therapeutic intervention.

Recurrence of achondrogenesis type II within the same family: evidence for germline mosaicism

Achondrogenesis type II is a lethal skeletal dysplasia caused by new dominant mutations within the type II collagen gene (COL2A1). Here we report on two pregnancies of a healthy, nonconsanguineous young couple. In the first pregnancy, severe micromelia and generalized edema were noted on ultrasound at 21 weeks' gestation. Clinical, radiological, and histological evaluation of the fetus after termination of the pregnancy showed typical findings of achondrogenesis type II. In the second pregnancy, fetal hygroma was noted at 11 weeks' gestation. Similar clinical, radiographic, and histologic findings were observed in the second fetus, suggesting the recurrence of achondrogenesis II within this family. Molecular analysis of genomic DNA extracted from amniotic cells of the second fetus revealed heterozygosity for a 1340G > A missense mutation (G316D) in the COL2A1 gene. This mutation was not found in the parents. Although, we could not evaluate the presence of this mutation in the first fetus, we strongly believe that our data are in favor of germline mosaicism as the most likely explanation for the recurrence of type II achondrogenesis in both sibs.

Skeletal anomalies

It is possible to identify many types of skeletal dysplasias and conditions involving limb deformities prenatally using ultrasound. It is likely that in the future, with the advancing technology and discoveries in molecular genetics, specific mutation analysis will become available for many of these conditions. This will make first trimester diagnosis an option in many cases. Because of the complex nature of many of these cases, it may be helpful to use a multidisciplinary approach involving a radiologist and a geneticist at times. In utero radiographs may help clarify a diagnosis. In lethal cases where a specific diagnosis has not been confirmed, it may be helpful postpartum to obtain an autopsy; photographs; complete body radiographs; karyotypic analysis; and specimens of bone, cartilage, and fetal blood for further analysis.

Widely distributed mutations in the COL2A1 gene produce achondrogenesis type II/hypochondrogenesis

The COL2A1 gene was assayed for mutations in genomic DNA from 12 patients with achondrogenesis type II/hypochondrogenesis. The exons and flanking sequences of the 54 exons in the COL2A1 gene were amplified by a series of specific primers using PCR. The PCR products were scanned for mutations by conformation sensitive gel electrophoresis, and PCR products that generated heteroduplex bands were then sequenced. Mutations in the COL2A1 gene were found in all 12 patients. Ten of the mutations were single base substitutions that converted a codon for an obligate glycine to a codon for an amino acid with a bulkier side chain. One of the mutations was a change in a consensus RNA splice site. Another was an 18-base pair deletion of coding sequences. The results confirmed previous indications that conformation sensitive gel electrophoresis is highly sensitive for detection of mutations in large and complex genes. They also demonstrate that most, if not all, patients with achondrogenesis type II/hypochondrogenesis have mutations in the COL2A1 gene.

Achondrogenesis type 2 diagnosed by transvaginal ultrasound at 12 weeks' gestation

Ultrasound examination at 12 weeks' gestation revealed severe generalised subcutaneous oedema in a pregnancy at risk for achondrogenesis type II. Transvaginal scanning confirmed the oedema and suggested abnormal limb development. The prenatal diagnosis was confirmed by X-ray examination after transvaginal termination.

Respiratory complications in children with spondyloepiphyseal dysplasia congenita

Mild respiratory distress at birth is a common occurrence in infants with spondyloepiphyseal dysplasia congenita (SEDC) while severe respiratory insufficiency and death have been reported only rarely. We describe three infants with SEDC who experienced severe respiratory complications and required tracheostomies. Two of these infants have also needed long-term continuous positive airway pressure to maintain adequate ventilation. The features and clinical course of these children are described, previous reports of respiratory complications in SEDC are summarized, mechanisms resulting in respiratory problems in SEDC are postulated, and comparison is made with mechanisms giving rise to respiratory risks in other skeletal dysplasias.

Achondrogenesis type II (Langer-Saldino) in association with jugular lymphatic obstruction sequence

The prenatal diagnosis of achondrogenesis in association with cystic hygroma is described. Ultrasound findings of severe short-limbed dwarfism, decreased vertebral ossification, and normal ossification of the calvarium were all consistent with achondrogenesis type II. Although the unusual finding of associated cystic hygroma raised the suspicion of a concurrent chromosome abnormality, the karyotype of both fetal lymphocytes and fetal fibroblasts was normal. Autopsy confirmed dilated lymphatic channels in the basal endothelial layer of the skin, cystic hygroma, and coarctation of the aorta. Although previously unreported, we suggest that the features of this case of achondrogenesis indicate an association with lymphatic stasis and jugular lymphatic obstruction sequence in this syndrome.

The absence of type II collagen and changes in proteoglycan structure of hyaline cartilage in a case of Langer-Saldino achondrogenesis

Structural analysis of hyaline cartilage extracellular matrix components from the ribs and knee joint of a stillborn female with type II achondrogenesis was carried out. The absence of type II collagen, a decrease in the amount of proteoglycans (PG), and structural changes in PG, namely, increased electrophoretic mobility of PG, lower relative content of chondroitin 4-sulfate (Ch4-S), lower molecular weight and decreased total chondroitin sulfate (ChS) sulfation, were detected. Increased amounts of type I and type III collagens, atypical for hyaline cartilage, were revealed. Among the link proteins (LPs), a large protein with a mol. wt. of 48 kDa was predominant. Molecular and cellular mechanisms of the pathogenesis of achondrogenesis ("chondrogenesis imperfecta") are discussed. The data obtained suggest that the primary defect in type II achondrogenesis involves ChS or type II collagen synthesis.

Variant of atelosteogenesis? Report of a 20-week fetus

We report on a 20-week fetus with manifestations similar, but not identical, to those of atelosteogenesis. The present fetus had rhizomelic micromelia with absence of ossification in the humerus, radius, ulna, and cervical and upper thoracic vertebral bodies; coronal clefts in the ossified thoracic vertebral bodies; and talipes equinovarus. The physes were relatively normal on histologic examination.

Achondrogenesis-hypochondrogenesis: the spectrum of chondrogenesis imperfecta. A radiological, ultrasonographic, and histopathologic study of 23 cases

In the classification of lethal osteochondrodysplasias, achondrogenesis and hypochondrogenesis have recently received special attention. We describe 23 cases representing the different subtypes. Within the classical type I (Parenti-Fraccaro) two distinct disorders can be recognized: type IA (Houston-Harris) and type IB (Fraccaro). The classical type II (Langer-Saldino) and hypochondrogenesis represent phenotypic variants of one disorder in which type II is the most severe form and hypochondrogenesis the mildest form, while transitional forms exist. It is likely that a basic defect in cellular function of the chondrocyte results in a deficient cartilage matrix and in disorganized enchondral ossification.

Achondrogenesis II-hypochondrogenesis: variability versus heterogeneity

Recently hypochondrogenesis was described as a form of neonatally lethal dwarfism said to resemble spondyloepiphyseal dysplasia congenita radiographically and achondrogenesis II morphologically. Because of the difficulty in distinguishing radiographically between mild achondrogenesis II and severe hypochondrogenesis, we performed a clinical, radiographic, and morphologic study of 24 cases originally classified as either achondrogenesis II or hypochondrogenesis, in an attempt to distinguish between heterogeneity and clinical variability. Review of the radiographic findings in these cases show a fairly continuous spectrum of bony defects, rather than two distinct radiographic syndromes. Chondro-osseous histology and ultrastructure was similar in all cases regardless of severity and was characterized by hypervascularity and hypercellularity of the cartilage with multiple small, round dilated cysternae of rough endoplasmic reticulum. These findings suggest that hypochondrogenesis and achondrogenesis type II represent a spectrum with marked phenotypic variability.

Hypochondrogenesis

Three clinicopathological observations of a mild form of type II achondrogenesis are presented. The cases were selected from a group of 21 similar cases to illustrate the various degrees of clinical and roentgenological signs that can be found. The cases had various survival periods after birth but not exceeding several months. The roentgenological signs were less severe than those of type II achondrogenesis. Some cases similar to case no. 3 have roentgenological signs very close to spondylo-epiphyseal dysplasia congenita and probably were confused previously with the latter. The name of hypochondrogenesis was proposed for these cases because the lesions of the growth plate are similar although less marked to those found in type II achondrogenesis: high cellularity with poor matrix development; irregular columnization and vascular penetration; large chondrocytes and even more enlarged lacunae; large sclerotic cartilage canals. The clinical and roentgenological diagnosis of hypochondrogenesis could be difficult especially in the less severe forms. The delay in vertebral ossification, the absence of all the epiphyseal nuclei and of the tarsal bones might suggest the diagnosis of hypochondrogenesis, rather than that of spondyloepiphyseal dysplasia. The evolution which seems to be always lethal in a period of several weeks or months would make the diagnosis still more likely and it could be confirmed by histopathological examination. Cases of spondylo-epiphyseal dysplasia congenita might have at birth, roentgenological signs indistinguishable from those of hypochondrogenesis, as was illustrated by case no. 4.(ABSTRACT TRUNCATED AT 250 WORDS)