ACF Lam, KKS Lai, DHC Chan, TMF Tong, STS Lam

Abstract

Desbuquois syndrome is characterised by craniofacial dysmorphism, carpotarsal, vertebral and joint abnormalities; with associated developmental delay. It is genetically heterogeneous and has been subdivided into two groups, the "typical hands" and "atypical hands" groups. We hypothesised that the "atypical hands" subgroup may be due to Filamin B (FLNB) mutations, and the objective of this study is to evaluate if mutation in FLNB gene is associated with this clinical subgroup. After sequencing the whole FLNB gene, we report that no mutation was found in our case of Desbuquois syndrome.

Introduction

Desbuquois syndrome (DS) is a rare autosomal recessive osteochondrodysplasia that is characterised by prenatal onset of short stature, facial dysmorphism of round flat midface and prominent bulging eyes, joint laxity or dislocation, advanced carpal and tarsal bone age, swedish key appearance of the proximal femur with flat proximal femoral metaphysis and exaggerated lesser trochanter, and variable degree of mental retardation.

The condition was initially reported by Desbuquois et al in 1966,¹ and subsequent reports of cases divided Desbuquois syndrome into two subgroups based on presence (46%) of a supernumerary ossification centre distal to second metacarpal, and variable thumb changes, being regarded as typical hand changes; or the slightly more common group (54%) with absence of above hand changes, regarded as atypical subtype changes.²

Spondylocarpotarsal synostosis, Larsen syndrome and Desbuquois syndrome share in common similar clinical features with variable degree of severity consisted of advancement of carpotarsal ossification, joint formation, vertebral and bones developmental abnormalities. Krakow et al reported Filamin B (FLNB) gene mutation in Spondylocarpotarsal synostosis and dominant form of Larsen syndrome.³ Since Desbuquois syndrome with typical hand changes was linked to 17q25.3 and the same group reported exclusion of 17q25.3 locus in Desbuquois syndrome with atypical hand changes.^2,4 We hypothesised that Desbuquois syndrome with atypical hand changes are due to mutations in FLNB gene at chromosomal locus 3p14.3, and the objective of this study is to evaluate if mutation in FLNB gene is associated with this clinical subgroup.

Subject and Methods

The case was previously reported by Lam et al,⁵ and was initially referred to our Clinical Genetic Service on day 2 of life for suspected skeletal dysplasia. The proband and her sister were born to consanguineous parents; and both siblings were developmentally delayed and similarly affected with DS.

Genomic DNA was extracted from peripheral blood of Proband and her parents using QIAamp DNA blood mini kit (Qiagen). DNA was amplified in a final reaction volume of 50 ul by using 100 ng genomic DNA, 1 x FX buffer, 0.2 mM of each dNTPs, 20 pmole primers and 1 unit AmpliTaq Gold polymerase. Primers and the annealing temperatures were self-designed and are available on request. PCR cycling conditions consisted of an initial denaturation step at 95°C for 10 min following by 35 cycles of 94°C for 1 min, the specified annealing temperature for 1 min, 72°C for 1 min and ending with a final elongation step at 72°C for 7 min.

PCR products were purified using QIAquick PCR purification kit (Qiagen) and then subjected to cycle sequencing in a 20 ul reaction with the ABI BigDye Terminator V1.1 cycle sequencing kit. Each reaction contained 3.2 pmole forward or reverse primer, 8 ng template DNA, 2 ul BigDye Terminator ready reaction mix, 3 ul of BigDye sequencing buffer and using water to make up to 20 ul. Cycle sequencing conditions consisted of an initial denaturation step at 96°C for 1 min following by 25 cycles of 96°C for 10 sec, 50°C for 5 sec, and 60°C for 4 min. Unincorporated dye and other contaminants were removed using the CentriSep columns (Applied Biosystems). The purified extension products were then sequenced on an ABI 3100 Genetic Analyzer (Applied Biosystems).

Parental consent for publication was kindly given and signed.

Results

After sequencing the whole FLNB gene (exon 1 to 46), we found no mutation in either of our proband or her parents.

Discussion

DS shares in common with the recessively inherited Spondylocarpotarsal synostosis (SCT) and the dominant form of Larsen syndrome (LS) in certain phenotypic features namely craniofacial dysmorphism; carpotarsal, vertebral and joint abnormalities. Faivre et al (2003) demonstrated genetic heterogeneity by mapping Desbuquois syndrome with "typical hands" to 17q25.3 and excluded the same locus in the syndrome with "atypical hands".^2,4 Our case as reported by Lam et al (2003) falls in the atypical hands group. As Krakow et al (2004) reported FLNB mutations in SCT and LS, FLNB should be considered a likely candidate gene for DS.

Sheen et al (2002) reported that both Filamin A (FLNA) and Filamin B (FLNB) proteins, apart from forming FLNA homodimers and FLNB homodimers, they also closely interact by forming heterodimers.⁶ While FLNA was widely expressed in all cortical layers and FLNB was highly expressed in ventricular and subventricular regions during brain development, it would not be surprising for patients with FLNB mutations to have variable degree of developmental delay. Since FLNA gene is X-linked therefore it is unlikely to be responsible for DS, which is an autosomal recessive condition. Therefore we hypothesised that DS with atypical hands may be caused by FLNB mutations. After sequencing the whole FLNB gene, we found no mutation in this case. The reasons may be that it is due to some other forms of mutation not detected by our current methods for example, mutations in the gene promoter region, deep intronic mutations, exon deletions or duplication; or it may not be due to FLNB mutations at all. This is significant because Larsen syndrome is a major differential diagnosis of Desbuquois syndrome. Since FLNB mutations have been found in Larsen syndrome patients, mutations in FLNB must be strongly suspected in Desbuquois syndrome. Our results suggested that Desbuquois syndrome is not part of the recently defined Filamin B "Filaminopathy" spectrum; and a gene other than FLNB that is involved in craniofacial, carpotarsal, vertebral and joint development, and at the same time may also be expressed in the brain, causes Desbuquois syndrome.

References

1. Shohat M, Lachman R, Gruber HE, et al. Desbuquois syndrome: clinical, radiographic, and morphologic characterization. Am J Med Genet 1994;52:9-18.

2. Faivre L, Le Merrer M, Al-Gazali LI, et al. Homozygosity mapping of a Desbuquois dysplasia locus to chromosome 17q25.3. J Med Genet 2003;40:282-4.

3. Krakow D, Robertson SP, King LM, et al. Mutations in the gene encoding filamin B disrupt vertebral segmentation, joint formation and skeletogenesis. Nat Genet 2004;36:405-10.

4. Faivre L, Le Merrer M, Zerres K, et al. Clinical and genetic heterogeneity in Desbuquois dysplasia. Am J Med Genet A 2004;128:29-32.

5. Lam W, Chan H, Sillence D. Desbuquois syndrome: Clinical and radiological report of the first two Chinese cases from a consanguineous family. J Paediatr Child Health 2003;39:707-12.

6. Sheen VL, Feng Y, Graham D, Takafuta T, Shapiro SS, Walsh CA. Filamin A and Filamin B are co-expressed within neurons during periods of neuronal migration and can physically interact. Hum Mol Genet 2002;11:2845-54.