Cystinosis Newsletter - Cystinosis Research Network

Page 12 The Cystinosis Research Network Fall/Winter 2006
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Cystinosis Research Update
Taken together, the appearance of cystine crystals is consistent with the data we obtained from the
cystine assay: i) we did not detect cystine crystals in the lens, consistent with the low cystine levels in
this tissue (maximum of 2 nmol 1 /2 cystine/mg protein); ii) we found rare retinal crystals in the oldest
mice when cystine levels reached ~30 nmol 1 /2 cystine/mg protein, and iii) we detected cystine crystals
in the iris and ciliary body prior to 5 months, however, these became abundant from 7 months,
consistent with the sharp 3-fold increase from 30 to 80 nmol 1 /2 cystine/mg protein seen at this age.
Furthermore, the biochemical data provides an insight into the cystine levels of different eye tissues,
which to our knowledge is not known in cystinosis patients, whereas the histological data are consistent
with those reported in patients.
Clinical tests: We used four different tests to determine the biological effect of cystine accumulation.
i) Slit lamp photography: We examined the evolution of cystine crystals in the cornea and iris with
age (1, 3, 5, 7, 9 & 13 months). We first detected rare crystals at 3 months in both tissues and, consistent
with the histological data, crystals were easily detectable and abundant from 7 months. Thus,
although corneal crystals appeared slightly later, their evolution paralleled that of the iris, consistent
with the overlapping profile of cystine levels in these tissues.
ii) Intra-ocular pressure (IOP): Differences in IOP can be due to obstructed flow of aqueous humour
(increased IOP) or defective production by the ciliary body (decreased IOP). Therefore, we indirectly
examined the effect of ciliary body crystals on humour production by measuring IOP. We measured
the IOP of 6 Ctns +/+ and Ctns -/- mice aged 15-19 months using a Tonopen XL and found that the IOP
of the Ctns -/- mice was significantly decreased. These results suggest that the presence of ciliary
body crystals could disrupt aqueous humour production and account for this relative decrease in IOP.
iii) Electroretinogram (ERG) profiles: We studied the retinal function of Ctns +/+ and Ctns -/- mice as
a function of age (3, 5, 7, 9 & 11 months). We did not detect a significant difference in a-wave or bwave
amplitude between the Ctns +/+ and Ctns -/- response at any age. Taken together, these data
suggest that retinal function is preserved in Ctns -/- mice up to 11 months, consistent with the low
cystine levels and the absence of retinal cystine crystals.
iv) Light/dark exploration test: We assayed 17-19 month-old Ctns -/- mice for signs of photophobia.
Both the Ctns +/+ and Ctns -/- mice exhibited a clear preference for the dark compartment, where they
spent almost two thirds of the duration of the session. The Ctns -/- mice, however, showed an increase
in the total time spent in the light compartment, but not in the number of entries into the light section
or the mean duration of each visit. Interestingly, the increased total time the Ctns -/- mice spent in the
light section was due to an increased number of short-duration (inferior to 10 s) visits, which, on average,
lasted 5.1 ± 0.3 s. Thus, although a classic photophobic behaviour was not detected, our results
highlight a need for the Ctns -/- mice to rapidly re-enter the dark compartment significantly more often
than the wild-type mice, which could suggest a mild photophobic state.
Taken together, his temporospatial guide of the ocular anomalies of the Ctns -/- mice, in addition to
validating the mouse model, provides a foundation for the future testing of novel therapeutics to efficiently
restore a normal ocular phenotype. We are now in a position to continue with the next part of
our research proposal, eye targeted in vivo gene transfer studies.
Please note that this work is documented in our manuscript: “A temporospatial guide to the ocular
anomalies in a cystinosis mouse model” by V. Kalatzis, N. Serratrice, C. Hippert, O. Payet, C. Arndt,
C. Cazavieille, T. Maurice, C. Hamel, F. Malecaze, C. Antignac, A. Müller and E.J. Kremer, which is
submitted for publication.
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