Recent Advances in Syndactyly: Basis, Current Status and Future Perspectives

A novel homozygous missense variant in LRP4 causing Cenani-Lenz syndactyly syndrome and literature review

BACKGROUND

Cenani-Lenzsyndactyly syndrome (CLSS; OMIM 212780) is a rare autosomal recessive acral deformity, which is mainly manifested in the fusion of fingers or toes, disordered phalangeal structure, shortening or fusion of the radius and ulna, and renal hypoplasia.

CASE PRESENTATION

Our report described an individual with mild phenotypes from China. His parents were not consanguineous. The affected individual was non-dysmorphic. Standard X-ray showed that the both hands have only four metacarpal bones. The distal end of the first metacarpal bone on the right was relatively slender, and the distal phalanx was absent. Multiple phalanges and some soft tissues of both hands were fused. Exome sequencing revealed a novel biallelic c.282C⟩Avariant in low-density lipoprotein receptor-related protein 4 (LRP4; OMIM604270; NM_002334.4) causing p. (Asn94Lys) change in the encoded protein. This variant is predicted to be potentially pathogenic, affecting protein structure and function.

CONCLUSION

We report a novel missense variant present in homozygosity in LRP4 to broaden the pathogenic spectrum of LRP4 in syndactyly, and exome sequencing technology is a powerful tool for genetic analysis in prenatal diagnosis and medical research, as a preferred method for the diagnosis of syndactyly and related phenotypes.

A Complex Case of Clino-Syndactyly with Fourth Metacarpal Aplasia

Syndactyly is the most common congenital malformation of the hand, leading to the fusion of the digits and frequently affecting the ring and middle fingers. The incidence is 1 out of 2500 children, predominantly occurring in boys and Caucasians. Clinically, the malformation may present as a soft tissue or bony fusion, resulting in the union of the fingers characterised as complete or incomplete. This fusion may involve the phalanges but may also extend to the carpal/tarsal bones, even to the metacarpal or metatarsal level, rarely to the distal end of the forearm and lower leg. The malformation is mostly isolated but may occur together with other disorders or malformations such as synostosis, acro-syndactyly, cleft hand, clinodactyly, or polydactyly. Syndromic syndactyly can be observed in cases of Apert syndrome, Poland's syndrome, Pfeiffer syndrome, and many others. A girl born in June of 2019 was diagnosed with congenital malformation of the right hand at birth-affecting the right middle, ring, and little fingers, respectively. After X-ray imaging, the fusion of the third and fourth proximal phalanges to a common metacarpal was identified, forming a unique diagnosis of clino-syndactyly with metacarpal aplasia. Surgical intervention was advocated for, including a wedge osteotomy to correct the synchondrosis at the phalangeal base and a dorsal flap to close the interdigital space created during the correction of the III and IV. fingers. A trapezoid flap for the release of the syndactyly of the IV and V. fingers was applied. The paper aims to present this surgical correction and its results regarding an atypical case of syndactyly with clinodactyly and metacarpal aplasia.

Syndactyly in the Pediatric Population: A Review of the Literature

Syndactyly is one of the most common congenital upper extremity deformities. Syndactyly can be described as either simple, involving just the skin and soft tissue, or complex, involving the phalanges. Additionally, syndactyly can be categorized as complete, involving the entire digit (including the nail fold), or incomplete, which does not involve the nail fold. Multiple familial or spontaneous genetic abnormalities can cause syndactyly, and these mutations typically involve the canonical wingless-type (WNT) pathway. Surgical repair of syndactyly is typically done between six to 18 months of age, depending on the type of syndactyly. Regardless of the classification of the syndactyly, the repair is performed before school-going age (except in the case of extremely mild or rare, extremely complex syndactyly). One or more imaging modalities are used to aid the surgeon in deciding the surgical approach for the syndactyly repair. The surgical plan must be clearly communicated with parents to manage expectations of aesthetics and function of the digits post-surgery. In brief, a syndactyly release surgery involves the creation of the web space using a geometrical design of the surgeon's choice, defatting of finger flaps, separation of the digits, and closure with absorbable sutures. However, the approach may vary depending on the patient. A "best" approach for rectifying the difference in surface area of separated versus fused digits has not yet been determined. While this was typically done using a skin graft, the use of alternative methods (most notably, using a synthetic dermal substitute or not using a graft at all and allowing the skin to heal with secondary intention) has been on the rise given the undesirable side effects of a graft. Less commonly, an external fixator can be used to expand soft tissue and skin. In the case of complete syndactyly, the Buck-Gramcko technique is most commonly used for nail flap reconstruction. Complications of the surgery include contracture, web creep, and the need for a second surgery. Thus, parents must be counseled in recognizing signs of complications.

Identification of a HOXD13 variant in a Mongolian family with incomplete penetrance syndactyly by exon sequencing

Background

Syndactyly (SD) refers to a deformity caused by the fusion and limb differentiation disorder of soft tissues and/or skeletons to varying extents between adjacent fingers (toes). The main features of this disease are phenotypic heterogeneity and genetic heterogeneity. In this study, we examined four generations of a Chinese Mongolian with different phenotypes of syndactylia and analysed and identified the pathogenic genetic variants of SD by exon sequencing.

Methods

The clinical phenotypes of patients were analysed, and the hands and feet were examined by X-ray. The pedigree was drawn, and the family data were analysed. Peripheral blood was collected from the family members, and genomic DNA was extracted. The candidate genes of SD were identified by exon sequencing, and the mutation sites of the captured candidate genes were amplified by PCR and verified by Sanger sequencing.

Results

The family has congenital syndactyly, which is an autosomal dominant disease. At present, this condition has been passed down for 4 generations and was identified in 9 patients, including 4 males and 5 females. Five patients, I₂, II₄, III₅, III_,7 and III₁₀, had unilateral syndactyly, and four patients, III₁₆, IV₃, IV₆ and IV₇, had bilateral finger syndactyly. All of their toes were unaffected. The proband and the other patients in this family had a c.917G > A (p.R306Q) mutation, which is located at position 917 of the second exon of the HOXD13 gene. This mutation results in a change in the amino acid at position 306, in which arginine is changed to glutamine. This mutation cosegregates in unaffected individuals and affected patients in this family. Moreover, 201 Mongolian genome databases and a thousand human genome databases were referenced to further confirm that the pathogenic genetic variant that causes syndactyly in this family is found in HOXD13.

Conclusion

This study found that the mutation site of the pathogenic gene in this family was HOXD13, c.917G > A (p.R306Q). The phenotype of the family member III₁₂ was normal, but this member was also a carrier of the pathogenic genetic variant. This indicates that the disease of this family has incomplete penetrance characteristics. Our results further enrich the expression profile of the HOXD13 gene.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01360-3.