We present a comprehensive review dealing with rare genetic skeletal disorders. More than 400 entities are included in the latest classification. The most severe or lethal phenotypes are identifiable in the prenatal period and the pregnancy can be terminated. Perinatal autopsy and posmortem X-rays are crucial in providing a definitive diagnosis. The number of cases confirmed by genetic testing is increasing. We report our own experience with genetic skeletal disorders based on 41 illustrative fetal and neonatal cases which we encountered over a 10-year period. Thanatophoric dysplasia and osteogenesis imperfecta represent approximately half of the cases coming to autopsy. Achondrogenesis type 2 and hypochondrogenesis, short-rib dysplasia, chondrodysplasia punctata, campomelic dysplasia and achondroplasia are less common. Skeletal dysplasias with autosomal recessive inheritance are the least frequent, e.g. perinatally lethal hypophophatasia, achondrogenesis type 1A, diastrophic dysplasia/atelosteogenesis type 2 or mucolipidosis type 2 (I cell disease).

Odontochondrodysplasia (ODCD, OMIM #184260) is a rare, non-lethal skeletal dysplasia characterized by involvement of the spine and metaphyseal regions of the long bones, pulmonary hypoplasia, short stature, joint hypermobility, and dentinogenesis imperfecta. ODCD is inherited in an autosomal recessive fashion with an unknown frequency caused by mutations of the thyroid hormone receptor interactor 11 gene (TRIP11; OMIM *604505). The TRIP11 gene encodes the Golgi microtubule-associated protein 210 (GMAP-210), which is an indispensable protein for the function of the Golgi apparatus. Mutations in TRIP11 also cause achondrogenesis type 1A (ACG1A). Null mutations of TRIP11 lead to ACG1A, also known as a lethal skeletal dysplasia, while hypomorphic mutations cause ODCD. Here we report a male child diagnosed as ODCD with a novel compound heterozygous mutation who presented with skeletal changes, short stature, dentinogenesis imperfecta, and facial dysmorphism resembling achondroplasia and hypochondroplasia.

Biallelic loss of function variants in TRIP11 encoding for the Golgi microtubule-associated protein 210 (GMAP-210) causes the lethal chondrodysplasia achondrogenesis type 1A (ACG1A). Loss of TRIP11 activity has been shown to impair Golgi structure, vesicular transport, and results in loss of IFT20 anchorage to the Golgi that is vital for ciliary trafficking and ciliogenesis. Here, we report four fetuses, two each from two families, who were ascertained antenatally with ACG1A. Affected fetuses in both families are homozygous for the deep intronic TRIP11 variant, c.5457+81T>A, which was found in a shared region of homozygosity. This variant was found to cause aberrant transcript splicing and the retention of 77 base pairs of intron 18. The TRIP11 messenger RNA and protein levels were drastically reduced in fibroblast cells derived from one of the affected fetuses. Using immunofluorescence we also detected highly compacted Golgi apparatus in affected fibroblasts. Further, we observed a significant reduction in the frequency of ciliated cells and in the length of primary cilia in subject-derived cell lines, not reported so far in patient cells with TRIP11 null or hypomorphic variants. Our findings illustrate how pathogenic variants in intronic regions of TRIP11 can impact transcript splicing, expression, and activity, resulting in ACG1A.

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.

Achondrogenesis is a group of rare and fatal disorders occurring in approximately one in every 40,000-60,000 newborns. Achondrogenesis is classified in three groups, as Achondrogenesis type 1A (Houston-Harris type or AC-G1A), Achondrogenesis type 1B (Parenti-Fraccaro type or ACG1B) and Achondrogenesis type 2 (Langer-Saldino type or ACG2), depending on clinical and radiological findings. Achondrogenesis Type 2 is a lethal skeletal dysplasia that is typically characterized by short arms and legs, a small chest with short ribs, lung hypoplasia, a prominent forehead, a small chin, and an enlarged abdomen that may accompanied by polydramnios and hydrops. This study contributes to the literature by presenting a patient who was admitted to the Level ΙΙΙ Neonatal Intensive Care Unit (NICU), Bursa, Turkey), with extremely short extremities, a small chest, abdominal distention and respiratory distress, who was diagnosed with ACG2. On the COL2A1 gene, genetic analysis with next generation sequencing (NGS), was revealed to have a heterozygous missense variation, c.2546G>A, p.Gly849Asp mutation, which is a different genetic variant that has not been previously described in the literature.