The cytoskeletal calyx of mammalian sperm heads surrounding the basolateral part of the nucleus contains 2 kinds of basic proteins: cylicins, or multiple-band polypeptides, and calicin (603960). See cylicin-1 (CYLC1; 300768). Hess et al. (1995) isolated bovine cDNAs encoding cylicin-2, a second form of cylicin. Like cylicin-1, the deduced cylicin-2 protein has a high content of charged amino acids, an abundance of lys-lys-asp tripeptides, and repetitive units that are predicted to form alpha helices. Overall, bovine cylicin-1 and -2 share approximately 50% homology. Although cylicin-2 has a calculated molecular mass of approximately 53 kD, its apparent molecular mass is 63 to 69 kD by SDS-PAGE. The authors attributed this discrepancy to the protein's unusually high concentration of charged amino acids. Both bovine cylicin genes are expressed specifically in testis. By screening a human testis library with a bovine cylicin cDNA, Hess et al. (1995) isolated cDNAs encoding the human homolog. The predicted 348-amino acid protein contains 52% charged amino acids and has a calculated pI of 10.53. The N- and C-terminal regions of bovine and human cylicin-2 are highly homologous, but the human protein has several large deletions in the central region.

Schneider et al. (2023) used polyclonal antibodies to visualize the localization of cylicins during spermiogenesis in mice. Immunofluorescence staining showed the presence of both Cylc1 and Cylc2 from the round spermatid stage onward, first as a cap-like structure lining the developing acrosome in the subacrosomal region, then moving toward the caudal part of the cell as the spermatids elongated. As localization intensified in the postacrosomal calyx region, staining in the subacrosome faded.

The CYLC2 sequence reported by Hess et al. (1995), Z46788, maps to 9q31.1 (build 36.3) (Amberger, 2009).

Associations Pending Confirmation

Schneider et al. (2023) reported a 40-year-old German man with infertility due to morphologic abnormalities of the sperm (see SPGFX8, 301119) who was hemizygous for a missense mutation the CYLC1 gene and heterozygous for a missense mutation in exon 5 of the CYLC2 gene (c.551G-A; G184D). Both variants were present at low minor allele frequency in the gnomAD database, and both were classified as variants of unknown significance (VUS) by ACMG criteria. The proband's father, who reported difficulties with fertility, was also heterozygous for the G184D variant in CYLC2. No other potentially pathogenic variants in genes associated with sperm morphologic defects were identified by analysis of the proband's exome data.

Using CRISPR/Cas9 gene editing, Schneider et al. (2023) generated mice deficient in Cylc1 and/or Cylc2. Cylc1 -/y male hemizygotes showed significantly reduced pregnancy rates and litter sizes. Cylc2 -/- males were infertile, whereas Cylc2 +/- males showed no significant differences in fertility parameters compared to wildtype mice. Intercrossing mouse lines generated Cylc1 -/y;Cylc2 +/- and Cylc1 -/y;Cylc2 -/- mice; males with either genotype were infertile. A strong decline in sperm counts was observed in all cylicin-deficient males, with the greatest reduction (to only 15% of wildtype) in Cylc1 -/y;Cylc2 -/- mice. Sperm morphology was severely altered in cylicin-deficient mice, with coiling of sperm tails, kinked sperm heads, and acrosomal malformations. Mislocalization of other calyx-specific proteins, such as Ccin (603960) and Capza3 (608722), was also observed. Transmission electron microscopy confirmed the structural defects observed by light microscopy, showing tail coiling, dislocation of the head-tail connecting piece from the basal plate, excess cytoplasm, absent posterior portion of the perinuclear theca calyx, and loosening of the periacrosomal region. In addition, motility of Cylc2 -/- sperm was markedly reduced (7% motile sperm), and that of Cylc1 -/7;Cylc2 -/- sperm was even lower (2%); the little motility observed was not progressive, but circular. Analysis of spermiogenesis in cylicin-deficient males showed that Cylc1- and Cylc2-null mice had gaps in the forming acrosome, as well as an irregular shape of the cap. In Cylc1 -/y;Cylc2 +/- and Cylc1 -/y;Cylc2 -/- mice, most round spermatids were deformed or displayed irregularly localized caps, and Cylc2 -/- and Cylc1 -/y;Cylc2 -/- mice showed detachment of the acrosome from the nuclear envelope. Immunofluorescence staining to analyze formation and development of the manchette revealed abnormal elongation and disassembly in cylicin-deficient spermatids, associated with loosening of the acrosome.