Identification of a de novo heterozygous missense FLNB mutation in lethal atelosteogenesis type I by exome sequencing

Disruption of FLNB leads to skeletal malformation by interfering with skeletal segmentation through the HOX gene

Filamin B (FLNB) plays an important role in skeletal development. Mutations in FLNB can lead to skeletal malformation such as an abnormal number of ossification centers, indicating that the skeletal segmentation in the embryonic period may be interfered with. We established a mouse model with the pathogenic point mutation FLNB NM_001081427.1: c.4756G > A (p.Gly1586Arg) using CRISPR-Cas9 technology. Micro-CT, HE staining and whole skeletal preparation were performed to examine the skeletal malformation. In situ hybridization of embryos was performed to examine the transcription of HOX genes during embryonic development. The expression of FLNB was downregulated in FLNBG1586R/G1586R and FLNBWT/G1586R mice, compared to FLNBWT/WT mice. Fusions in tarsal bones were found in FLNBG1586R/G1586R and FLNBWT/G1586R mice, indicating that the skeletal segmentation was interfered with. In the embryo of FLNBG1586R/G1586R mice (E12.5), the transcription levels of HOXD10 and HOXB2 were downregulated in the carpal region and cervical spine region, respectively. This study indicated that the loss-of-function mutation G1586R in FLNB may lead to abnormal skeletal segmentation, and the mechanism was possibly associated with the downregulation of HOX gene transcription during the embryonic period.

A Stop-gain Variant c.220C>T (p.(Gln74*)) in FLNB Segregates with Spondylocarpotarsal Synostosis Syndrome in a Consanguineous Family

Spondylocarpotarsal synostosis (SCT) syndrome is a very rare and severe form of skeletal dysplasia. The hallmark features of SCT are disproportionate short stature, scoliosis, fusion of carpal and tarsal bones, and clubfoot. Other common manifestations are cleft palate, conductive and sensorineural hearing loss, joint stiffness, and dental enamel hypoplasia. Homozygous variants in FLNB are known to cause SCT. This study was aimed to investigate the phenotypic and genetic basis of unique presentation of SCT syndrome segregating in a consanguineous Pakistani family. Three of the four affected siblings evaluated had severe short stature, short trunk, short neck, kyphoscoliosis, pectus carinatum, and winged scapula. The subjects had difficulty in walking and gait problems and complained of knee pain and backache. Roentgenographic examination of the eldest patient revealed gross anomalies in the axial skeleton including thoracolumbar and cervical fusion of ribs, severe kyphoscoliosis, thoracic and lumbar lordosis, coxa valga, fusion of certain carpals and tarsals, and clinodactyly. The patients had normal faces and lacked other typical features of SCT like cleft palate, conductive and sensorineural hearing loss, joint stiffness, and dental enamel hypoplasia. Whole exome sequencing (WES) of two affected siblings led to the discovery of a rare stop-gain variant c.220C>T (p.(Gln74*)) in exon 1 of the FLNB gene. The variant was homozygous and segregated with the malformation in this family. This study reports extensive phenotypic variability in SCT and expands the mutation spectrum of FLNB.

Whole-genome sequencing analysis in families with recurrent pregnancy loss: A pilot study

One to two percent of couples suffer recurrent pregnancy loss and over 50% of the cases are unexplained. Whole genome sequencing (WGS) analysis has the potential to identify previously unrecognized causes of pregnancy loss, but few studies have been performed, and none have included DNA from families including parents, losses, and live births. We conducted a pilot WGS study in three families with unexplained recurrent pregnancy loss, including parents, healthy live births, and losses, which included an embryonic loss (<10 weeks' gestation), fetal deaths (10-20 weeks' gestation) and stillbirths (≥ 20 weeks' gestation). We used the Illumina platform for WGS and state-of-the-art protocols to identify single nucleotide variants (SNVs) following various modes of inheritance. We identified 87 SNVs involving 75 genes in embryonic loss (n = 1), 370 SNVs involving 228 genes in fetal death (n = 3), and 122 SNVs involving 122 genes in stillbirth (n = 2). Of these, 22 de novo, 6 inherited autosomal dominant and an X-linked recessive SNVs were pathogenic (probability of being loss-of-function intolerant >0.9), impacting known genes (e.g., DICER1, FBN2, FLT4, HERC1, and TAOK1) involved in embryonic/fetal development and congenital abnormalities. Further, we identified inherited missense compound heterozygous SNVs impacting genes (e.g., VWA5B2) in two fetal death samples. The variants were not identified as compound heterozygous SNVs in live births and population controls, providing evidence for haplosufficient genes relevant to pregnancy loss. In this pilot study, we provide evidence for de novo and inherited SNVs relevant to pregnancy loss. Our findings provide justification for conducting WGS using larger numbers of families and warrant validation by targeted sequencing to ascertain causal variants. Elucidating genes causing pregnancy loss may facilitate the development of risk stratification strategies and novel therapeutics.

Filamin B: The next hotspot in skeletal research?

Filamin B (FLNB) is a large dimeric actin-binding protein which crosslinks actin cytoskeleton filaments into a dynamic structure. Up to present, pathogenic mutations in FLNB are solely found to cause skeletal deformities, indicating the important role of FLNB in skeletal development. FLNB-related disorders are classified as spondylocarpotarsal synostosis (SCT), Larsen syndrome (LS), atelosteogenesis (AO), boomerang dysplasia (BD), and isolated congenital talipes equinovarus, presenting with scoliosis, short-limbed dwarfism, clubfoot, joint dislocation and other unique skeletal abnormalities. Several mechanisms of FLNB mutations causing skeletal malformations have been proposed, including delay of ossification in long bone growth plate, reduction of bone mineral density (BMD), dysregulation of muscle differentiation, ossification of intervertebral disc (IVD), disturbance of proliferation, differentiation and apoptosis in chondrocytes, impairment of angiogenesis, and hypomotility of osteoblast, chondrocyte and fibroblast. Interventions on FLNB-related diseases require prenatal surveillance by sonography, gene testing in high-risk carriers, and proper orthosis or orthopedic surgeries to correct malformations including scoliosis, cervical spine instability, large joint dislocation, and clubfoot. Gene and cell therapies for FLNB-related diseases are also promising but require further studies.

Differential expression of filamin B splice variants in giant cell tumor cells

Giant cell tumor of bone (GCT) is the most commonly reported non-malignant bone tumor in Hong Kong. This kind of tumor usually affects people aged 20–40 years. Also, it is well known for recurrence locally, especially when the tumor cannot be removed completely. Filamins are actin-binding proteins which contain three family members, filamin A, B and C. They are the products of three different genes, FLNA, FLNB and FLNC, which can generate various transcript variants in different cell types. In this study, we focused on the effects of FLNBv2 and FLNBv4 toward GCT cells. The only difference between FLNBv2 and FLNBv4 is that FLNBv4 does not contain hinge 1 region. We found that the relative abundance of FLNBv4 varies among different GCT cell lines while the expression level of FLNBv4 in normal osteoblasts was only marginally detectable. In the functional aspect, overexpression of FLNBv4 led to upregulation of RANKL, OCN, OPG and RUNX2, which are closely related to GCT cell survival and differentiation. Moreover, FLNBv4 can have a negative effect on cell viability of GCT cells when compare with FLNBv2. In conclusion, splicing variants of FLNB are differentially expressed in GCT cells and may play a role in the proliferation and differentiation of tumor cells.

A case of hereditary novel mutation in SLC26A2 gene (c.1796 A.> C) identified in a couple with a fetus affected with atelosteogenesis type 2 phenotype in an antecedent pregnancy

Atelosteogenesis type 2 also known as atelosteogenesis de la Chapelle type, De la Chapelle dysplasia, McAlister dysplasia or neonatal osseous dysplasia 1 is an extremely lethal osteochondrodysplasia with unknown incidence. Very few cases have been reported in the literature. We present a case with findings of prenatal ultrasound, autopsy and cytogenetic work up with report of a novel familial mutation.

The impact of sequencing on diagnosis and treatment of malignant melanoma

Melanoma is one of the clinically most important cancer types considering its high mortality rate and that it is commonly diagnosed in relatively young people. With the advent of targeted therapies and, more recently, immune checkpoint inhibitors, more treatment options are available resulting in higher patient survival rates. However, the successful application of these targeted therapies critically depends on the reliable detection of molecular aberrations. Today, massively parallel sequencing techniques enable us to analyze large sets of genes in a relatively short time. It has allowed increased knowledge of acquired somatic mutations in melanoma and has helped to identify new targets for personalized therapy, and potentially may help to predict response to immune therapies. Described here are the development of sequencing techniques, how their improvement has changed diagnosis, prognosis and management of malignant melanoma and the future perspectives of melanoma diagnostics in the routine clinical setting.