HOXD13 and Limb Patterning

• During limb development, HOXD13 is expressed in the autopod, providing molecular instructions necessary for normal digit formation and spatial organization.

• HOXD13 controls the growth and separation of digits by modulating downstream target genes related to digit identity and morphology.

• Mutations in HOXD13 disrupt these processes, leading to abnormalities in digit number, shape, and separation.

Phenotypic Effects of HOXD13 Mutations in Humans

• Syndactyly (Fusion of Digits):
  The most characteristic consequence of HOXD13 mutations in humans is synpolydactyly, a condition featuring fused and sometimes extra digits. The digits fail to separate properly due to impaired regulation of apoptosis and morphogenesis.

• Digit Malformations:
  Besides fusion, affected individuals may show malformations such as clinodactyly, broad or short digits, joint stiffness, and other digital anomalies. These defects reflect partial or complete loss of distal limb patterning cues.

• Molecular Mechanisms:
  Mutations alter the DNA-binding homeodomain or disrupt regulatory regions of HOXD13, affecting gene function and expression of digit-specific genes. This impairs normal transcriptional cascades required for digit identity.

Why Other Phenotypes Are Less Relevant

• HOXD13 mutations do not typically affect zeugopod or stylopod development (forearm or upper arm), ruling out options related to proximal limb segment malformations such as absence of zeugopod (option 1) or deformities in stylopod (option 3).

• They also do not impact the formation of femur or patella (option 4), which are unrelated to HOXD13’s main autopod specification role.

Summary

Mutations in the HOXD13 gene lead to developmental defects primarily affecting the distal limb, causing fusion and malformations of digits—a condition known as syndactyly. These findings underscore HOXD13’s essential role in digit specification and separation during human limb development.

Final Answer:
(2) Abnormalities of the hands and feet wherein the digits fuse