Recessive mutations in the polyglutamylase TTLL5, present in photoreceptor cells and spermatozoa, cause cone-rod degeneration and incompletely penetrant male infertility.

Abstract

Purpose: To identify the genetic causes of an atypical form of cone-rod dystrophy, sometimes associated with male infertility.

Methods: We performed whole-genome-sequencing (WGS) in an affected male individual from a consanguineous family who was negative for mutations in genes known to be associated with retinal diseases. Further screening of TTLL5 exons and flanking intronic sequences was performed by direct Sanger sequencing. Immunofluorescence was used to detect TTLL5 within human control spermatozoa and murine retinal sections.

Results: WGS of the index patient identified approximately 4 million DNA changes, which were reduced to 19 putative disease causing variants following the use of an internal in silico pipeline. Among these, we identified a homozygous frameshift mutation (c.1782del; p.Asp594Glufs*29) in the polyglutamylase gene TTLL5, confirming the recent published data that associate this gene to retinal dystrophy. Screening of TTLL5 exons and flanking intronic sequences in 49 patients with a diagnosis of cone or cone-rod dystrophy (33 from Sweden, 12 from Greece, as well as one English and three Dutch patients who displayed homozygosity for the chromosomal region containing TTLL5) identified four additional biallelic mutations in five individuals including another frameshift (c.2132_2133insGATA; p.Met712Ilefs*15), a nonsense (c.349C>T; p.Gln117*) and two missense (c.1627G>A; p.Glu543Lys and c.2266A>T; p.Ile756Phe) changes. Interestingly, two of the six patients with mutations in TTLL5 were infertile due to reduced sperm motility, a phenotype that is also displayed by Ttll5^-/- male mice. Immunofluorescence with anti-TTLL5 antibody in healthy human spermatozoa revealed that TTLL5 is localized at the base of the flagellum, near the centrioles, while in murine photoreceptors it is present at the basal body of the sensory cilium.

Conclusions: Our observations suggest that TTLL5 is a component of both cilia and flagella and that its deficiency causes cone/cone-rod degeneration and incompletely penetrant male infertility.