Expanding the Phenotypic and Genotypic Spectrum of Postaxial Polydactyly: A Study of Four Consanguineous Pakistani Families.

Background

Postaxial polydactyly (PAP) is characterized by the development of extra digits at the fifth finger. It can occur as an isolated disease or a part of a syndrome. The genetic basis of nonsyndromic PAP has been linked to sequence variants in different genes. The aim of the present study was to identify the causative genetic variants in four Pakistani families demonstrating PAP.

Methods

Causative genetic variants were identified using whole-exome sequencing and microsatellite mapping, followed by validation through Sanger sequencing. Further analysis was carried out through conservation, structural modeling, and 100-ns molecular dynamic simulations of GLI1.

Results

Whole-exome and targeted sequencing in four families (A, B, C, and D) identified novel variants in the GLI1 (c.1013G>T; p.Cys338Phe), and GLI3 (c.2003C>T; p.Pro668Leu and c.4564delG; p.Ala1522ProfsTer2), and a recurrent variant in IQCE genes (c.895_904del; p.Val301SerfsTer8) linked to 7p22.3. All the variants were highly conserved across different species. Comparative analysis of the GLI1WT and GLI1Cys338Phe proteins revealed domain fluctuations leading to the loss of structurally and functionally important inter- and intramolecular interactions. The three-dimensional structural analysis of the GLI3 protein showed that the missense variant p. (Pro668Leu) disturbed the protein folding and intra-residue interaction, while the frameshift variant p. (Ala1522ProfsTer2) led to the loss of the C-terminus of the protein. Similarly, the IQCE structural analysis confirmed the loss of protein function due to frameshift and C-terminal deletion.

Conclusion

This study has broadened the phenotypic and allelic spectrum of the genes associated with isolated PAP and strengthened the role of these genes in regulating limb development.