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).

The human inherited cartilage disease, Hypochondrogenesis, is caused by mutations in the collagen type II gene, COL2A1. To produce an in vitro disease model, we generated a heterozygous patient mutation, COL2A1 p.G1113C, in an established control human induced pluripotent stem cell (iPSC) line, MCRIi019-A, using CRISPR-Cas9 gene editing. The gene-edited mutant line, MCRIi019-A-7, exhibited normal iPSC characteristics, including normal cell morphology, expression of pluripotency markers, the ability to differentiate into three embryonic germ layers, and normal karyotype. Together with its parental isogenic control, this cell line will be useful for Hypochondrogenesis disease modelling and drug testing.

Type II collagen is a major component of the cartilage matrix. Pathogenic variants (ie, disease-causing aberrations) in the type II collagen gene (COL2A1) lead to an abnormal structure of type II collagen, causing a large group of skeletal dysplasias termed type II collagenopathies. Because type II collagen is also located in the vitreous body of the eyes and inner ears, type II collagenopathies are commonly associated with vitreoretinal degeneration and hearing impairment. Type II collagenopathies can be radiologically divided into two major groups: the spondyloepiphyseal dysplasia congenita (SEDC) group and the Kniest-Stickler group. The SEDC group is characterized by delayed ossification of the juxtatruncal bones, including pear-shaped vertebrae. These collagenopathies comprise achondrogenesis type 2, hypochondrogenesis, SEDC, and other uncommon subtypes. The Kniest-Stickler group is characterized by disordered tubular bone growth that leads to "dumbbell" deformities. It comprises Kniest dysplasia and Stickler dysplasia type 1, whose radiographic manifestations overlap with those of type XI collagenopathies (a group of disorders due to abnormal type XI collagen) such as Stickler dysplasia types 2 and 3. This phenotypic overlap is caused by type II and type XI collagen molecules sharing part of the same connective tissues. The authors describe the diagnostic pathways to type II and type XI collagenopathies and the associated differential diagnoses. In addition, they review the clinical features and genetic bases of these conditions, which radiologists should know to participate in multidisciplinary care and translational research. Online supplemental material is available for this article. ©RSNA, 2020.

We present a case of hypochondrogenesis, a rare autosomal dominant skeletal dysplasia that often results in infant death shortly after birth. Hypochondrogenesis can present similarly to other skeletal dysplasia diseases, notably achondrogenesis type II. The diagnosis of hypochondrogenesis was given during the prenatal stage after fetal imaging was performed using ultrasound, magnetic resonance imaging (MRI), and low-dose computerized tomography (CT). To the best of our knowledge, this is the first known case that reported the use of low-dose CT to assist in the prenatal diagnosis of hypochondrogenesis.