Novel COL1A1 Mutation c.3290G>T Associated With Severe Form of Osteogenesis Imperfecta in a Fetus

Introduction

Osteogenesis imperfecta (OI) is characterized by fractures with minimal or absent trauma, variable dentinogenesis imperfecta and adult-onset hearing loss. Its severity ranges from perinatal lethality to a mild tendency to fractures diagnosed much later in life. OI is genetically heterogenous and at present, causative mutations in 19 different genes have been reported in the Osteogenesis Imperfecta Variant Database. ¹ However, mutations in COL1A1 and COL1A2 genes account for the majority of cases. ² The COL1A1 gene (NM_000088.3) is 18 kb in size and comprises 52 exons. It encodes the pro-alpha chains of type I collagen, whose triple helix comprises two alpha1 chains and one alpha2 chain coded by the COL1A2 gene. Type I collagen is a fibril-forming collagen found in most connective tissues, especially in bone, cornea, dermis, and tendon. In addition to OI, mutations in COL1A1 have also been associated with classical type of Ehlers–Danlos syndrome, Caffey disease, and idiopathic osteoporosis. OI has been classified into several subtypes, of which the perinatally lethal type (previously classified as type II) is the most severe. ³ Fetuses with this type of OI have short crumpled long bones, bowing or angulation deformities of long bones, and deficient ossification of the skull. Progressive fracturing leads to the beaded appearance of the ribs. Mean birth length and weight are less than the 50th centile, and the chest is small. ⁴ The prognosis is extremely poor: more than half of the affected infants die of respiratory insufficiency on the first day, and survival beyond 1 year is rare. ⁵ We describe here a novel COL1A1 mutation associated with severe form of prenatally diagnosed OI.

Case Report

A 39-year-old woman who was 21 weeks 4 days pregnant was referred to the department of clinical genetics because of abnormal ultrasound findings including short limbs, small thorax, and abnormally shaped head of the fetus. Early pregnancy had been uneventful. Both parents were of Turkish origin but unrelated and they have one healthy child. The parents were of normal height and neither of them had any family history of fractures, short stature, or skeletal dysplasia.

A control ultrasound showed extremely short limbs corresponding roughly to gestational age of 12 weeks. The thorax was so small that it would have been extremely unlikely for the child to survive after birth and the parents applied for termination of pregnancy which was performed at 22 weeks. The mother also needed evacuation of retained placenta.

Computed tomography was performed to the fetus after termination of the pregnancy. The fetus was badly severed, making the radiological examination difficult. However, abnormally short and poorly ossified long bones and absent ossification of the skull were evident (Figure 1).

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Figure 1.

Computed tomography image of the fetus.

The fetus weighed 99.6 g (mean weight at 21 weeks 301–400 g), crown-rump length was 15 cm (mean 18.3 cm, SD 1.1 cm), and crown-heel length 18 cm (mean 25.6 cm, SD 1.4 cm). Several fractures were detected in long bones. All the limbs were short and abnormally shaped and clinodactyly of both hands was noted. The bones of the skull were almost totally missing, only a small amount of bone at the posterior cranial fossa could be identified. It was therefore not possible to study the facial features. However, mildly dysmorphic right earlobe was noted. The ribs were brittle and had a beaded appearance (Figure 2) suggesting repeated fractures. The left lung could not be identified at all but the right lung was of normal size and morphology. The tongue was large but normally shaped, hard palate was very thin but no cleft was present. Heart and large blood vessels were normal as were other internal organs.

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Figure 2.

The beaded ribs of the fetus as seen at the autopsy.

Histopathological examination of the long bones showed multiple abnormalities. The cortical bone was totally absent and the metaphyseal and diaphyseal trabeculae were disorganized, sparse, narrow, and poorly ossificated (Figure 3(a) to (c)). Fractured areas with distortion of the shaft were detected (Figure 3(d)). In addition, islets of metaplastic cartilage were seen in the fracture areas. However, the growth plate and resting cartilage appeared normal and the metaphyseal border was regular.

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Figure 3.

Bone histopathology in a section from the lower limb. (a), The diaphysis with missing cortical bone and immature and sparse woven bone. Hematoxylin–eosin ×100. (b), Disorganized trabeculae. Hematoxylin–eosin ×50. (c), Detail of the immature woven bone. Hematoxylin–eosin ×200. (d), Fractured and distorted shaft. The epiphyses are indicated with *. Hematoxylin–eosin ×25.

A bone dysplasia gene panel (Fulgent Diagnostics, 161 genes) was then performed and it revealed a novel variant in exon 45 of the COL1A1 gene (c.3290G>T, p.Gly1097Val, NM_000088.3) not present in the parental samples, suggesting a De novo mutation. Four different computational tools predicted the variant in question to be damaging (Align GVGD = Deleterious, SIFT = Deleterious, Mutation Taster = disease causing, MAPP = bad). The variant disrupts a glycine residue critical to function in the collagen triple helix repeat domain of the encoded protein (PubMed: 17078022). Amino acid Gly1097 is completely conserved in 99 vertebrate species, increasing the likelihood that a change would not be tolerated.

Discussion

To date, 912 unique variants of the COL1A1 have been reported. The total number of reported variants of this gene is 1730 (Osteogenesis Imperfecta Variant Database October 14 2016). ¹ The majority of mutations causative of perinatal lethal form of OI result in substitutions for glycine within the triple helical domain of the pro-alpha chain. The pro-alpha chains consist of an amino-terminal propeptide, a triple-helical segment of 1014 amino acids with glycine in every third position. The correct positioning of glycine, which is the smallest amino acid, is critical for the proper chain folding. If the glycine is substituted, the propagation of triple helix is delayed, additional post-translational modification occurs and some of the assembled trimers are never secreted. ⁶ Consequently, a diminished amount of procollagen is secreted, resulting in the abnormal structure of the protein in the matrix. This type of mutation may result in perinatal lethal, progressively deforming or common variable type of OI, depending on the position of the substituted glycine, the chain in which the substitution occurs and the nature of the substituting amino acid. Glycine substitutions in the triple helical domain of the alpha-1 chain may be non-lethal if the position is smaller or equal to 688 but substitutions occurring terminally to 688 are lethal.^7,8 The number of identified mutations is constantly growing. For example, Wang and colleagues have described a cohort of 10 fetuses with severe short-limb dwarfism; COL1A1/2 mutations were identified in all of the cases and six of these were new mutations. ⁹

The mutation found in our patient has not been previously described in the literature, but as it substitutes a glycine residue critical to the formation of the collagen triple helix, it is the likely cause of the symptoms observed in the fetus.