<p><b>OBJECTIVE</b>To screen potential mutation of the CRELD1 gene in congenital atrioventricular septal defect (AVSD) and explore its functional implications.</p><p><b>METHODS</b>Fragments encompassing the 11 coding exons of CRELD1 gene, including at least 50 bp of flanking intronic regions, were amplified with PCR and subjected to DNA sequencing. Results of sequencing were compared with predicted sequence from the GenBank database. Eukaryotic expression vector pcDNA3.1CRELD1 containing the mutational sequence was constructed. Western blotting and real-time fluorescent quantitative reverse transcription polymerase chain reaction (FQ RT-PCR) was applied to examine the expression of CRELD1, Tenascin C and Aggrecan.</p><p><b>RESULTS</b>C857G was identified in a girl with an isolated partial AVSD. The mutation has resulted in a substitution of Alanine for Proline at amino acid 286 in the first cbEGF domain. Western blotting and FQ RT-PCR confirmed that the P286R missense mutation has been a gain-of-function mutation. Compared with the unloaded control, the Aggrecan mRNA expression was downregulated for both wild-type and mutant type samples (t=140.27 vs. 26.36, P < 0.01). The downregulation was more significant in mutant type (t=25.69, P=0.002). There was no significant difference of the Tenascin C expression between wild-type and the unload control (t=1.167, P> 0.05), whilst the Tenascin C expression was up-regulated in mutant type (t=6.66, P=0.022).</p><p><b>CONCLUSION</b>Mutation of the CRELD1 gene may increase the risk for AVSD rather than being directly causative. The P286R mutation of CRELD1 can downregulate the expression of Aggrecan and upregulates the expression of Tenascin C protein, both of which are crucial to extracellular matrix in the formation of the atrioventricular septum. The P286R mutation of CRELD1 may be correlated to the occurrence of AVSD.</p>