Achondrogenesis Type IA (Houston-Harris): a still-unresolved molecular phenotype

Pathogenic variants in the TRIP11 gene cause a skeletal dysplasia spectrum from odontochondrodysplasia to achondrogenesis 1A

The thyroid hormone receptor interactor 11 (TRIP11) gene encodes the Golgi microtubule-associated protein 210 (GMAP-210), a protein essential for the operation of the Golgi apparatus. It is known that null mutations in TRIP11 disrupt Golgi function and cause a lethal skeletal dysplasia known as achondrogenesis type 1A (ACG1A), however recently, hypomorphic mutations in that gene have been linked to odontochondrodysplasia (ODCD), a nonlethal skeletal dysplasia characterized by skeletal changes in the spine and in the metaphyseal regions, associated with dentinogenesis imperfecta. Here we present two patients reflecting the phenotypic spectrum related to different TRIP11 variants. The first is a female child with ODCD, for whom a homozygous in-frame splicing mutation in intron 9 of TRIP11 was identified. The mutation appears to lead to the expression of an alternative TRIP11 transcript, that may explain the less severe radiological alterations in ODCD. The second is a fetus with classical form of ACG1A, associated with typical molecular findings (frameshift) in exon 11 of TRIP11, both novel mutations. The two patients reported here represent the TRIP11 spectrum of skeletal dysplasia ranging from mild to lethal phenotypes, thereby enabling one to suggest a genotype-phenotype correlation in these diseases.

Hypomorphic mutations of TRIP11 cause odontochondrodysplasia

Odontochondrodysplasia (ODCD) is an unresolved genetic disorder of skeletal and dental development. Here, we show that ODCD is caused by hypomorphic TRIP11 mutations, and we identify ODCD as the nonlethal counterpart to achondrogenesis 1A (ACG1A), the known null phenotype in humans. TRIP11 encodes Golgi-associated microtubule-binding protein 210 (GMAP-210), an essential tether protein of the Golgi apparatus that physically interacts with intraflagellar transport 20 (IFT20), a component of the ciliary intraflagellar transport complex B. This association and extraskeletal disease manifestations in ODCD point to a cilium-dependent pathogenesis. However, our functional studies in patient-derived primary cells clearly support a Golgi-based disease mechanism. In spite of reduced abundance, residual GMAP variants maintain partial Golgi integrity, normal global protein secretion, and subcellular distribution of IFT20 in ODCD. These functions are lost when GMAP-210 is completely abrogated in ACG1A. However, a similar defect in chondrocyte maturation is observed in both disorders, which produces a cellular achondrogenesis phenotype of different severity, ensuing from aberrant glycan processing and impaired extracellular matrix proteoglycan secretion by the Golgi apparatus.

Bi-allelic mutations of TRIP11 cause a spectrum of skeletal phenotypes whose severity is primarily based on impaired secretory trafficking and aberrant glycan processing.

Achondrogenesis type 1A: clinical, histologic, molecular, and prenatal ultrasound diagnosis

Background

Achondrogenesis type IA (ACG1A) is a rare, lethal autosomal recessive chondrodysplasia affecting endochondral bone ossification and differentiation, causing intrauterine growth restriction, narrow thorax, and short limbs. Mutations in TRIP11, which encodes Golgi microtubule-binding protein 210 in the Golgi apparatus, alter protein transport in tissues.

Case presentation

A 28-week gestation male fetus was diagnosed with ACG1A by clinical, radiological, histologic, and molecular findings.

Results

Whole exome sequencing was performed on fetal DNA and parental blood. Two fetal heterozygous novel variants of TRIP11, c.2304_2307delTCAA (p.Asn768Lysfs*7) and c.2128_2129delAT (p.lle710Cysfs*19), were inherited from the mother and father, respectively. Both variants created a reading frameshift leading to a premature stop codon and loss of protein function.

Conclusion

To our knowledge, this is the first Latin American report with clinical, radiographic, and molecular diagnosis of ACG1A. Clinical and molecular diagnosis in utero is essential for genotype–phenotype correlation and is useful for providing better genetic counseling.

Lethal skeletal dysplasia in mice and humans lacking the golgin GMAP-210

BACKGROUND

Establishing the genetic basis of phenotypes such as skeletal dysplasia in model organisms can provide insights into biologic processes and their role in human disease.

METHODS

We screened mutagenized mice and observed a neonatal lethal skeletal dysplasia with an autosomal recessive pattern of inheritance. Through genetic mapping and positional cloning, we identified the causative mutation.

RESULTS

Affected mice had a nonsense mutation in the thyroid hormone receptor interactor 11 gene (Trip11), which encodes the Golgi microtubule-associated protein 210 (GMAP-210); the affected mice lacked this protein. Golgi architecture was disturbed in multiple tissues, including cartilage. Skeletal development was severely impaired, with chondrocytes showing swelling and stress in the endoplasmic reticulum, abnormal cellular differentiation, and increased cell death. Golgi-mediated glycosylation events were altered in fibroblasts and chondrocytes lacking GMAP-210, and these chondrocytes had intracellular accumulation of perlecan, an extracellular matrix protein, but not of type II collagen or aggrecan, two other extracellular matrix proteins. The similarities between the skeletal and cellular phenotypes in these mice and those in patients with achondrogenesis type 1A, a neonatal lethal form of skeletal dysplasia in humans, suggested that achondrogenesis type 1A may be caused by GMAP-210 deficiency. Sequence analysis revealed loss-of-function mutations in the 10 unrelated patients with achondrogenesis type 1A whom we studied.

CONCLUSIONS

GMAP-210 is required for the efficient glycosylation and cellular transport of multiple proteins. The identification of a mutation affecting GMAP-210 in mice, and then in humans, as the cause of a lethal skeletal dysplasia underscores the value of screening for abnormal phenotypes in model organisms and identifying the causative mutations.

Temperature alters solute transport in growth plate cartilage measured by in vivo multiphoton microscopy

Solute delivery to avascular cartilaginous plates is critical to bone elongation, and impaired transport of nutrients and growth factors in cartilage matrix could underlie many skeletal abnormalities. Advances in imaging technology have revolutionized our ability to visualize growth plates in vivo, but quantitative methods are still needed. We developed analytical standards for measuring solute delivery, defined by amount and rate of intravenous tracer entry, in murine growth plates using multiphoton microscopy. We employed an acute temperature model because of its well-established impact on bone circulation and tested the hypothesis that solute delivery changes positively with limb temperature when body core and respiration are held constant (36 degrees C, 120 breaths/min). Tibial growth plates were surgically exposed in anesthetized 5-wk-old mice, and their hindlimbs were immersed in warm (36 degrees C) or cool (23 degrees C) saline (n = 6/group). After 30 min of thermal equilibration, we administered an intracardiac injection of fluorescein (50 microl, 0.5%) and captured sequentially timed growth plate images spanning 10 min at standardized depth. Absolute growth plate fluorescence was normalized to vascular concentrations for interanimal comparisons. As predicted, more fluorescein infiltrated growth plates at 36 degrees C, with standardized values nearly double those at 23 degrees C. Changing initial limb temperature did not alter baseline values, suggesting a sustained response period. These data validate the sensitivity of our system and have relevance to strategies for enhancing localized delivery of therapeutic agents to growth plates of children. Applications of this technique include assessment of solute transport in models of growth plate dysfunction, particularly chondrodysplasias with matrix irregularities.

Achondrogenesis

In this issue of Pediatric and Developmental Pathology, Aigner and colleagues report a detailed investigation of cartilage matrix changes in a 14-week fetus with achondrogenesis type IA. The changes reported differ from matrix alterations observed in achondrogenesis types IB or II and provide insight into the phenotypic and genotypic differences within this group of skeletal dysplasias.