A novel frameshift variant in BHLHA9 underlies mesoaxial synostotic syndactyly associated with postaxial polydactyly

BHLHA9 homozygous duplication in a consanguineous family: A challenge for genetic counseling

Split-hand/foot malformation (SHFM) with long-bone deficiency (SHFLD) is a rare condition characterized by SHFM associated with long-bone malformation usually involving the tibia. It includes three different types; SHFLD1 (MIM % 119,100), SHFLD2 (MIM % 610,685) and SHFLD3 (MIM # 612576). The latter was shown to be the most commonly reported with a duplication in the 17p13.1p13.3 locus that was narrowed down to the BHLHA9 gene. Here, we report a consanguineous Lebanese family with three members presenting with limb abnormalities including tibial hemimelia. One of these patients presented with additional bowing fibula and another with bilateral split hand. CGH array analysis followed by RQ-PCR allowed us to detect the first homozygous duplication on the short arm of chromosome 17p13.3 including the BHLHA9 gene and involved in SHFLD3. Interestingly, one patient with the homozygous duplicated region, carrying thus four BHLHA9 copies presented with long bone deficiency but no SHFM. The incomplete penetrance and the variable expressivity of the disease in this family as well as the presence of the BHLHA9 homozygous duplication rendered genetic counseling extremely challenging and preimplantation genetic diagnosis almost impossible.

A novel homozygous variant in the GLI1 underlies postaxial polydactyly in a large consanguineous family with intra familial variable phenotypes

Polydactyly is a human inherited disorder caused by to anomalies in the genes involved in autopod development. The disorder segregates in both autosomal recessive and autosomal dominant form. Up till now, eleven genes causing non-syndromic polydactyly, have been identified. This includes ZNF141, GLI3, ZRS in LMBR1, MIPOL1, PITX1, IQCE, GLI1, FMA92A1, KIAA0825, STKLD1, and DACH1. In the present study, we have investigated a large consanguineous family of Pakistani origin segregating polydactyly in autosomal recessive pattern. Clinical examination of affected individuals revealed a non-syndromic form of the disorder. Genetic study based on homozygosity mapping and Sanger sequencing using DNA of the normal and affected individuals found a novel homozygous missense sequence variant [NM_005269.3: c.1133C > T, p.(Ser378Leu)] in the GLI1 located on human chromosome 12q13.3. In silico analysis of the identified variant showed a significant change in the secondary structure of the mutant protein that affects its function. Findings of the present study expand the mutation spectrum of the GLI1. In addition, the study will help in prevention of the disorder through carrier testing and bringing awareness among families affected with polydactyly.

Whole exome sequencing identified a novel frameshift variant in the BHLHA9 in an Iranian family with mesoaxial synostotic syndactyly

Mesoaxial synostotic syndactyly with phalangeal reduction (MSSD) represents a rare non-syndromic defect with an autosomal recessive pattern of inheritance. Sequence variants in the BHLHA9 gene cause MSSD and to date only a few mutations in this gene have been reported. In the present report, we have described a consanguineous Iranian family segregating MSSD in an autosomal recessive manner. The family had two affected siblings showing evidence of camptodactyly in some fingers, complete syndactyly of the 3rd and 4th fingers with synostoses of the corresponding metacarpals, and associated single phalanx in both right and left hand. Whole exome sequencing (WES) followed by segregation analysis using Sanger sequencing identified a novel homozygous frameshift variation [c.74_74delG p.(G25Afs*55)] in the BHLHA9 gene. This has expanded the spectrum of mutations in the BHLHA9 and will facilitate genetic counseling in Iranian families segregating MSSD-related phenotypes.

First case of compound heterozygous BHLHA9 variants in mesoaxial synostotic syndactyly with phalangeal reduction

Mesoaxial synostotic syndactyly with phalangeal reduction (MSSD) is an extremely rare autosomal recessive limb abnormality characterized by the fusion of third and fourth fingers. To date, only homozygous missense and frameshift mutations have been reported in BHLHA9 associated to MSSD. In this study, we report a patient who presented with clinical and radiological features of MSSD. A customized skeletal dysplasia NGS panel revealed the presence of two novel compounds heterozygous variants in BHLHA9: NM_001164405.1: c.[226A>T][269G>C]; p.[(Lys76*)][(Arg90Pro)]. Thus, this is the first case of MSSD in a nonconsanguineous family.