Unexpected molecular mechanism of Orc6-based Meier-Gorlin syndrome: insights from a humanized Drosophila model

Area of Science:

• Genetics and Molecular Biology
• Developmental Biology
• Rare Genetic Disorders

Background:

• Meier-Gorlin syndrome (MGS) is a rare autosomal recessive disorder.
• MGS is associated with mutations in origin recognition complex (ORC) genes, crucial for DNA replication.
• ORC6 is an essential subunit involved in DNA replication and cytokinesis.

Purpose of the Study:

• To investigate the functional impact of specific ORC6 mutations (c.2T > C and c.449 + 5G > A) found in MGS patients.
• To determine the mechanism by which these mutations lead to a functional ORC6 protein despite predicted truncation.
• To validate findings using a humanized ORC6-based Drosophila model.

Main Methods:

• Utilized a humanized ORC6-based Drosophila model.
• Analyzed the effect of patient-derived ORC6 variants on protein translation and function.
• Assessed the ability of truncated ORC6 proteins to rescue orc6 deletion phenotypes in vivo.

Main Results:

• Truncated ORC6 proteins produced by the c.2T > C/c.449 + 5G > A mutation combination failed to rescue orc6 deletion.
• A strong Kozak sequence in human ORC6 mRNA promotes translation initiation from a noncanonical ACG codon.
• The rescued Drosophila model exhibited phenotypes consistent with other MGS mutants.

Conclusions:

• MGS patients with the c.2T > C/c.449 + 5G > A mutation rely on full-size ORC6 protein initiated from an alternative ACG start codon.
• This alternative translation initiation mechanism explains the observed MGS phenotypes.
• The study highlights the importance of noncanonical translation initiation in genetic disorders.