cone degeneration

Cone-Rod Dystrophy With Decreased Male Fertility

Clinical Characteristics
Ocular Features: 

Features of a cone dystrophy appear first followed by rod damage although the course of degeneration is variable.  Poor vision may be present in some individuals in the first years of life but has a later onset in others.  Evidence for rod dysfunction appears later in most patients.

A hyperfluorescent area centered on the fovea can often be seen although there may be patchy areas elsewhere in the fundus.  Foveal atrophy is present in individuals generally after the 5th decade of life. Full field ERG shows reduced or absent cone responses with variable rod responses with more pronounced changes in older individuals.  Variable pigmentation can be seen in the peripapillary area.

Systemic Features: 

The only systemic abnormality thus far identified is a reduction in sperm count and reduced motility.  The resulting loss of fertility, however, occurs only in male patients with truncating variants in TTLL5 and not in those with missense mutations according to the most recent studies.

Genetics

This autosomal recessive condition results from homozygous or compound heterozygous mutations in the TTLL5 gene (14q24.3).  TTLL5 is localized at the base of the spermatozoal axoneme and at the basal body of the cilia in photoreceptors.  

Interestingly, male mice with mutations in this gene have reductions in sperm motility with evident disruption in the formation of sperm tails.

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

No treatment is known.

References
Article Title: 

Mutations in the polyglutamylase gene TTLL5, expressed in photoreceptor cells and spermatozoa, are associated with cone-rod degeneration and reduced male fertility

Bedoni N, Haer-Wigman L, Vaclavik V, Tran HV, Farinelli P, Balzano S, Royer-Bertrand B, El-Asrag ME, Bonny O, Ikonomidis C, Litzistorf Y, Nikopoulos K, Yioti G, Stefaniotou M, McKibbin M, Ellingford J, Booth AP, Black G, Toomes C, Inglehearn CF, Hoyng CB, Bax N, Klaver CC, Thiadens AA, Murisier F, Schorderet DF, Ali M, Cremers FP, Andreasson S, Munier FL, Rivolta C. Mutations in the polyglutamylase gene TTLL5, expressed in photoreceptor cells and spermatozoa, are associated with cone-rod degeneration and reduced male fertility. Hum Mol Genet. 2016 Aug 22. pii: ddw282. [Epub ahead of print].

PubMed ID: 
27554115

Biallelic variants in TTLL5, encoding a tubulin glutamylase, cause retinal dystrophy

Sergouniotis PI, Chakarova C, Murphy C, Becker M, Lenassi E, Arno G, Lek M, MacArthur DG; UCL-Exomes Consortium, Bhattacharya SS, Moore AT, Holder GE, Robson AG, Wolfrum U, Webster AR, Plagnol V. Biallelic variants in TTLL5, encoding a tubulin glutamylase, cause retinal dystrophy. Am J Hum Genet. 2014 May 1;94(5):760-9.

PubMed ID: 
244791901

Cone-Rod Dystrophies, X-Linked

Clinical Characteristics
Ocular Features: 

Three X-linked forms of progressive cone-rod dystrophies each with mutations in different genes have been identified.  Central vision is often lost in the second or third decades of life but photophobia is usually noted before vision loss.  Cones are primarily involved but rod degeneration occurs over time.  The ERG reveals defective photopic responses early followed by a decrease in rod responses.   All three types are rare disorders affecting primarily males with symptoms of decreased acuity, photophobia, loss of color vision, and myopia.  The color vision defect early is incomplete but progressive cone degeneration eventually leads to achromatopsia.    Peripheral visual fields are usually full until late in the disease when constriction and nightblindness are evident.  The retina may have a tapetal-like sheen.  RPE changes in the macula often give it a granular appearance and there may be a bull's-eye configuration.   Fine nystagmus may be present as well.  The optic nerve often has some pallor beginning temporally.  Carrier females can have some diminished acuity, myopia, RPE changes, and even photophobia but normal color vision and ERG responses at least among younger individuals.

There is considerable variation in the clinical signs and symptoms in the X-linked cone-rod dystrophies among both affected males and heterozygous females.  Visual acuity varies widely and is to some extent age dependent.  Vision can be normal into the fourth and fifth decades but may reach the count fingers level after that. 

Systemic Features: 

None.

Genetics

Mutations in at least 3 genes on the X chromosome cause X-linked cone-rod dystrophy.

CORDX1 (304020) is caused by mutations in an alternative exon 15 (ORG15) of the RPGR gene (Xp11.4) which is also mutant in several forms of X-linked retinitis pigmentosa (300455, 300029).  These disorders are sometimes considered examples of X-linked ocular disease resulting from a primary ciliary dyskinesia (244400).

CORDX2 (300085) is caused by mutations in an unidentified gene at Xq27.  A single family has been reported.

CORDX3 (300476) results from mutations in CACNA1F.  Mutations in the same gene also cause a form of congenital stationary night blindness, CSNB2A (300071).  The latter, however, is a stationary disorder with significant nightblindness and mild dyschromatopsia, often with an adult onset, and is associated with high myopia. Aland Island Eye Disease (300600) is another allelic disorder.   

Pedigree: 
X-linked dominant, father affected
X-linked dominant, mother affected
X-linked recessive, carrier mother
X-linked recessive, father affected
Treatment
Treatment Options: 

There is no treatment for these dystrophies but red-tinted lenses provide comfort and may sometimes improve acuity to some extent.  Low vision aids can be helpful. 

References
Article Title: 
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