Case Report of Two Siblings Diagnosed with Osteogenesis Imperfecta Type XV with a New Mutation in the WNT1 Gene and Review of the Literature

Abstract

Introduction

Osteogenesis imperfecta (OI) is a heritable disorder characterized by bone fractures and low bone mass. Recently, mutations of the WNT1 gene have been reported to be causative in OI. The mutation in WNT1 causes autosomal-recessive OI due to its critical role in bone formation. WNT1 mutations cause varying degrees of clinical severity, ranging from moderate to progressively deforming forms. In addition to the OI phenotype, our cases also had extra-skeletal findings.

Case Presentation

We describe two siblings with multiple fractures and developmental delay. A novel homozygous frameshift WNT1 mutation was detected in this family, and we reviewed the literature for WNT1-related OI cases.

Discussion

We report a novel variant with a clinical diagnosis of severe OI, and this review will provide a comprehensive overview of previously published cases of OI type XV. With a better understanding of disorders associated with WNT1 mutations, therapies targeting Wnt1 signaling pathway may contribute therapeutic benefits.

Established Facts

• Osteoporosis is a common skeletal disorder characterized by low bone mineral density and bone fragility.

• The majority of patients with osteogenesis imperfecta (OI) have mutations in genes encoding type 1 collagen, COL1A1 and COL1A2.

• Recently, mutations of the WNT1 gene have been reported to be causative in OI or osteoporosis.

Novel Insights

• The novel Cys315Leufs*78 frameshift variant in the WNT1 gene that we identified in two siblings is expected to result in a truncated protein.

• Carrier detection of parents by segregation analysis is important for their follow-up in terms of osteoporosis.

Introduction

Osteogenesis imperfecta (OI) is an inherited connective tissue disease characterized by bone fragility and susceptibility to fractures. OI usually progresses with blue sclera, hearing loss, deformity in the arms and legs, and growth retardation. More than 90% of the disease is caused by dominant mutations of COL1A1 and COL1A2 genes, which encode type 1 collagen [Marini et al., 2007]. Approximately 5–10% of all cases are inherited autosomal recessively [Marini et al., 2017]. The genes responsible for this inheritance are the genes encoding type 1 collagen folding, modification, and matrix mineralization proteins. While mutations in type 1 collagen genes help suggest the diagnosis of OI, the absence of mutation does not exclude the diagnosis of OI. It has a wide clinical spectrum, ranging from very mild forms to intrauterine fractures and death.

OI constitutes a phenotypic and molecular heterogeneous group of inherited connective tissue diseases. With the discovery of new genes in recent years, it has been possible to understand the pathophysiology of the disease and normal bone development.

Type XV OI was first described by Keupp et al. [2013] and is caused by homozygous mutations in the wingless-type MMTV integration site family 1 (WNT1) gene. Studies show that the WNT1 gene is critical in bone development and contributes to osteoblast proliferation, differentiation, and survival [Baron and Kneissel, 2013]. In WNT1-related OI, neurological findings may accompany bone fractures as well [Aldinger et al., 2016; Kantaputra et al., 2019].

In recent years, the role of the WNT signaling pathway in bone biology and diseases has been tried to be demonstrated by mouse and human studies. The discovery of proteins that play an essential role in this pathway such as LRP5, SOST, and DKK1 has been defined [Gong et al., 2001; Balemans et al., 2008]. It was previously reported that WNT1 homozygous mutations cause OI, while WNT1 heterozygous mutations cause early-onset osteoporosis. To support the hypothesis that WNT1 is an important WNT ligand regulating bone formation and bone homeostasis, Joeng et al. [2014] examined the mechanism of WNT1 function in bone in an in vivo animal model. The Swaying mouse carrying the Wnt1 mutant allele was first described in 1967 [Lane, 1967]. The Wnt1^sw/sw mice exhibit severe cerebellar defect that is also observed in some OI patients with the WNT1 mutation. Joeng et al. [2014] demonstrated the Swaying (Wnt1^sw/sw) mice share main features of OI, including fracture tendency and severe osteopenia.

In this report, we present two siblings with novel homozygous WNT1 mutation and diagnosed with OI. In addition, rare coexistence of resistant epilepsy in one of the siblings makes our presentation valuable. We aimed to present clinical findings and molecular genetic data in two siblings with autosomal recessive OI and to review the literature.

Case Presentation

Case 1

A 7-year-old Iraqi girl was born as the second child of healthy parents who were first-degree cousins (Fig. 1), weighing 2,900 g by normal spontaneous vaginal delivery. It was learned that the first fracture of the patient was noticed when she was 5 months old, and then multiple fractures developed in both arms and legs. In the examination of the patient with neuromotor developmental delay, it was observed that her mental status was low; she had blue sclera, exotropia in the left eye, ptosis in the right eye, microcephaly (relative macrocephalic appearance), oligodontia, hypotonicity, hyperlaxity, pectus carinatum, and diffuse pseudoarthrosis in the extremities (Fig. 2). Height and body weight were below the 3rd percentile. Hearing was normal. Laboratory tests for etiology are shown in Table 1. Abdominal ultrasonography, electrocardiography, and echocardiography were evaluated as normal. Pamidronate treatment was started in the patient who was found to be osteoporotic in bone densitometry. During the follow-ups, it was learned that the patient did not go to regular check-ups and interrupted the medication.

Fig. 1.

Pedigree of the family.

Fig. 2.

Images of the proband, showing the severe extremity deformities, kyphoscoliosis, and pectus carinatum.

Table 1.

Laboratory test results of case 1 and case 2

	Case 1	Case 2	Reference
First fracture	5 months old	4 months old
Height, cm	101	92
Weight, kg	13	9
CBC	Normal	Normal
Vitamin B12, pg/mL	403	636	126.5–505
25-OHD, ng/mL	37.58	31.86	10–80
TSH, mU/L	3.24	2.75	0.77–5.64
tTG-IgA	Negative	Negative
tTG-IgG	Negative	Negative
Ferritin, ng/mL	11.2	27.9	23.9–336.2
Somatomedin C, ng/mL	34.3	19.3	15–129
PTH, pg/mL	11.1	9.9	15–68
ALP, U/L	161	222	30–128
Ca++, mg/dL	10.3	9.2	8.4–10.8
Pi, mg/dL	4.5	4.0	2.6–5.5
IGFBP-3, ng/mL	1,790	994	700–3,600

CBC, complete blood count; TSH, thyroid-stimulating hormone; tTG-IgA, tissue transglutaminase IgA; tTG-IgG, tissue transglutaminase IgG; PTH, parathyroid hormone; ALP, alkaline phosphatase; IGFBP-3, insulin-like growth factor-binding protein-3.

Case 2

The 5-year-old brother of the proband was also born by normal spontaneous vaginal delivery at term, 3,100 g. He was examined in many centers due to his dysmorphic facial appearance, hypotonicity, a history of frequent lung infections, and a similar sibling history. The patient had a history of fractures in the long bones that started when he was 4 months old. It was learned that the patient, whose first seizure started when he was 9 months old, was first treated with levetiracetam, and during the follow-up, phenobarbital and topiramate treatment was added as his seizures continued. In the examination of the patient with severe developmental and mental retardation, it was observed that there was blue sclera, ptosis in the left eye, oligodontia, brachycephaly, hypertelorism, prominent forehead, triangular face, diffuse hypotonicity, hyperlaxity, narrow (chest) thoracic structure, and diffuse pseudoarthrosis all over the body (Fig. 3). Height, body weight, and head circumference were below the 3rd percentile. The patient, who was hypotonic, was fed with a nasogastric tube because he did not have a sucking reflex. His hearing was normal. Laboratory tests with reference values are shown in Table 1. Abdominal ultrasonography, electrocardiography, and echocardiography were normal. He had a history of frequent intensive care admissions due to bronchopneumonia. The patient had rotoscoliosis in the thorax on computed tomography. In the brain computed tomography examination, both lateral ventricles were larger than normal, and there was moderate hydrocephalic dilatation and brachiocephalic type craniosynostosis in the calvarium (Fig. 4). Focal interictal EEG findings were observed. Pamidronate treatment was started in the patient whose bone mineral density was found to be osteoporosis. It was learned that new bone fractures continued despite both of our cases receiving pamidronate treatment for 3 years. The X-ray images of the proband and his brother are shown in Figure 5.

Fig. 3.

Images of the proband's brother, showing the severe extremity deformities, narrow thorax, and dysmorphic face (high forehead, low-set posteriorly rotated ears, hypertelorism).

Fig. 4.

Brain CT of the proband's brother. a Moderate enlargement of both lateral ventricles with decreased cerebral volume, especially in the frontal lobes. b Bone window evaluation from the same level shows brachiocephalic type craniosynostosis. CT, computed tomography.

Fig. 5.

X-rays of the patients. a−c Radiographs of case 1 at age of 4 years show scoliosis, bowing of the upper and lower extremity bones, depressed vertebrae. d−f Radiographs of case 2 at the age of 1 year show narrow chest, upper and lower extremity deformities, anterior and lateral view of depressed vertebrae.

Genetic Analysis

First, COL1A1 and COL1A2 sequence analyses were studied for the present findings. OI panel (WES) was studied because of its normal results and accompanying neurological findings. Homozygous c.944delG (p.Cys315Leufs*78) mutation was detected in the WNT1 gene. According to the American College of Medical Genetics and Genomics sequence variant classification guideline, the variant was classified as pathogenic. The pathogenicity index of the variant was calculated using different online analysis tools including MutationTaster, SIFT, VarSome, PolyPhen-2 and was predicted as disease causing. This variant was not annotated in dbSNP, 1000 Genomes Project, Exome Aggregation Consortium Browser, NHLBI Exome Sequencing Project Exome Variant Server, and Human Gene Mutation Database. As a result of the 1-bp deletion in the WNT1 gene, it was predicted to be functionally harmful as it causes the premature termination of the protein by causing a frameshift. When the parents were evaluated in terms of this change, they were found to be heterozygous carriers.

Discussion

The WNT1-mediated signaling pathway induces the expression of genes required for skeletal development and bone homeostasis; thus, OI and osteoporosis develop as a result of WNT1 mutation [Monroe et al., 2012; Fahiminiya et al., 2013; Aldinger et al., 2016]. In addition to skeletal findings, non-skeletal findings such as teeth, skin, ligaments, sclera, and vessels can also be observed in OI. Both of our cases had a dysmorphic triangular face, blue sclera, brownish discoloration of the teeth, hypermobility in the joints, and subcutaneous hemorrhages in the skin.

It has been previously shown that the WNT1 gene plays an important role in brain development as well as normal skeletal development and homeostasis. It has also been shown that WNT1 affects signaling pathways in critical pathways in the development of the central nervous system [McMahon and Bradley, 1990]. There are studies showing that WNT1 gene-related mutations affect brain development at an early stage. This supports mental retardation, abnormalities in brain imaging, and neurological findings in our patients. Brachiocephalic-type craniosynostosis with bilateral ventricular enlargement was found in the brain imaging of case 2 who was diagnosed with epilepsy. Rare cases with co-occurrence of OI and neurological involvement have been reported previously.

In 2016, Aldinger et al. reported a case of severe autosomal recessive OI accompanied by a brain malformation with a compound heterozygous mutation in the WNT1 gene [Aldinger et al., 2016]. In addition, a case with accompanying central nervous system malformations was presented by Kantaputra et al. [2019].

Ptosis, one of the most common features of OI due to WNT1 mutation, was present in both siblings presented here. Online supplementary Table (for all online suppl. material, see www.karger.com/doi/10.1159/000528201) shows that 40% (25/62) of the cases presented in the literature have unilateral/bilateral ptosis. This finding is not seen in other OI types. In online supplementary Table neurological findings and brain anomalies were detected in 30% (19/62) of the cases previously presented in the literature. The presence of neurological involvement is a finding that enables differential diagnosis between OI type XV and other types. It was observed that the two siblings in the family presented in this report were affected differently from a neurological point of view. In the literature, it has been observed that although one of the family members with the same mutation has severe brain anomalies, the other has no symptoms and has normal mental development. In this case, some significant differences in clinical features due to WNT1 mutation can be explained by the difference in expression of this gene, modifying genes, epigenetic and non-genetic factors.

It is known that bisphosphonates used in the routine treatment of OI act by suppressing osteoclastic bone resorption [Dwan et al., 2016]. However, as previously reported in the literature, there was no response to these agents in our cases [Fahiminiya et al., 2013; Keupp et al., 2013]. This is likely because the WNT signaling pathway influences bone development through osteoblast function. With further understanding of WNT1-related bone diseases, it will be possible to develop efficacious treatments targeting this signaling pathway.

In such cases, identifying parents as carriers is important for clinical follow-up due to the risk of osteoporosis reported in heterozygotes, as well as providing benefits for future pregnancies. We referred parents who did not have any existing complaints to the relevant clinic to be examined for osteoporosis.

Consanguineous marriage is a social condition that causes rare genes to come together and cause recessive diseases. The widespread use of comprehensive genetic analyses such as whole-exome sequencing, which enables the diagnosis of rare diseases in this way, has become important, especially in societies such as Turkey where consanguineous marriages are common.

A clear genotype-phenotype correlation has also not been established in WNT1-related OI. The variants will further expand the spectrum of mutations in the genes causing OI. In this respect, the presentation of new cases to the literature, as in this report, will also contribute to the establishment of this relationship.

Statement of Ethics

This study was performed in accordance with the Declaration of Helsinki principles. Ethical approval was not required for this study in accordance with local/national guidelines. Written informed consent was obtained from the patients' parents for the publication of this case report and accompanying images.

Conflict of Interest Statement

The authors have no conflict of interest to declare.

Funding Sources

There are no funding sources to report.

Author Contributions

The current case report was designed and written by Büşra Eser Çavdartepe. The data were collected by Büşra Eser Çavdartepe and Rojan İpek.

Data Availability Statement

All data generated and analyzed during this study are included in this article and its online supplementary material. Further inquiries can be directed to the corresponding author.

Supplementary Material

Supplementary data

Supplementary data

Funding Statement

There are no funding sources to report.