Submission Details

The coiled-coil domain containing 65 gene (CCDC65) encodes a component of the nexin-dynein regulatory complex (N-DRC), a structure that links the outer doublet microtubules of the axoneme and regulates dynein motor activity in cilia and flagella (PMID: 25411337). CDCC65 has also been identified as a human sperm tail protein (PMID: 37975235, PMID: 17089017). CCDC65 was first reported in relation to PCD in 2013 by Austin-Tse et al. and Horani et al. (PMID: 24094744, PMID: 23991085). The two groups reported a homozygous 2-bp (c.877_878del) deletion in 4 probands of Ashkenazi-Jewish descent and a PCD-consistent phenotype without laterality defects. The variant causes a premature termination codon 2 amino acids downstream (p.Ile293Profs2Ter) that is predicted to result in nonsense-mediated decay, consistent with the decreased expression of CCDC65 revealed by immunofluorescent staining in nasal epithelial cells. On the other hand, the minor allele frequency of 0.004339 in the Ashkenazi Jewish population of the gnomAD database suggests a founder effect. CCDC65 deficiency was also shown to result in loss of GAS8, another component of the N-DRC (PMID: 23991085) that has similarly been linked to PCD (PMID: 26387594 27120127 27472056). Affected patients also had cilia with a reduced inner dynein arms and nexin link, and disorganization of the microtubules, as well as stiff and hyperkinetic cilia with a limited ciliary stroke (PMID: 24094744 and PMID: 23991085). Other studies have reported homozygous CCDC65 variants in probands with infertility due to aszthenospermia, exhibiting short, absent, and coiled flagella and near-absence of CDCC65 protein (PMID: 37975235), as well as in patients from PCD cohorts harboring apparent loss-of-function variants in CCDC65 in the homozygous state (PMID: 33479112, PMID: 39004944).

This gene-disease association is supported by experimental evidence including enriched gene expression in PCD-relevant human tissues (testis, fallopian tubes, and lung). CCDC65 has been detected during early ciliated cell differentiation, coinciding with the expression of the master ciliogenesis gene, FOXJ1 (PMID: 23991085). Expression of the CCDC65 ortholog DRC2 in the biflagellated alga Chlamydomonas reinhardtii increased significantly following deflagellation (PMID: 23991085). The Chlamydomonas ida6 motility mutant with inner arm dynein defects could be rescued by transformation with the CCDC65 ortholog Fap250. Morpholino-based knockdown of ccdc65 in zebrafish resulted in ciliopathy phenotypes and olfactory ciliary beat with either complete cilia paralysis or dyskinetic, faster beat rate (PMID: 24094744). A Ccdc65 knockout mouse showed laterality defects consistent with a PCD phenotype (cardiac transposition and dilated lateral ventricle) (PMID: 35844805).

In summary, there is definitive evidence to support the gene-disease relationship of CCDC65 with primary ciliary dyskinesia 27. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time without the emergence of contradictory evidence. This classification was approved by the ClinGen Motile Ciliopathy GCEP on August 8th, 2024 (SOP Version 10).