The calpains are an evolutionarily ancient family of calcium dependent intracellular proteases that utilize a cysteine residue in the active site to mediate limited proteolysis. The multifunctional calpains require careful regulation, since they target multiple intracellular proteins and pathways 
. Their activity is regulated by intracellular calcium, lipid and protein interactions, subcellular localization, autocatalysis and inhibition by the endogenous peptide calpastatin 
. There is no consensus amino acid sequence or structural motif that is targeted for cleavage by calpains, and as a result it is often difficult to identify the physiologic substrates of these enzymes, including calpain-5 
is expressed during nematode and mouse embryogenesis 
. In adults, CAPN5
is highly expressed in the colon, kidney, liver, trachea, uterus, eye and brain 
. Calpains have been implicated in the pathogenesis of a wide range of human diseases including cancer, multiple sclerosis, Alzheimer's disease, cataract, diabetes and muscular dystrophy 
. Some polymorphisms in CAPN10
have been shown to be risk factors for type II diabetes 
. However, prior to this report, limb girdle muscular dystrophy (LGMD) type 2A was the only disease shown to be caused in a monogenic fashion by variations in a calpain's protein sequence 
The evidence that the two missense mutations we observed in CAPN5 are responsible for ADNIV is compelling. The gene lies within the critical region previously linked to the disease, and all living subjects in the study who are clinically affected were found to harbor a CAPN5 mutation in exon 6. Each of these two mutations alters an amino acid in the catalytic domain that has been highly conserved throughout evolution. Neither of these mutations was found among any of the clinically unaffected members of the three kindreds we studied, or among thousands of normal individuals.
There are interesting similarities and differences between calpain-associated LGMD and ADNIV. In both disorders, the affected cells (skeletal muscle fibers and photoreceptor cells) experience large changes in membrane potential and intracellular calcium concentration as part of their normal behavior. In both disorders, the non-mutant calpain molecules display functionally critical subcellular localization 
. A subset of LGMD is associated with leukocyte infiltration into the tissue 
, and all cases of ADNIV are marked by severe intraocular inflammation. The differences in these diseases are also noteworthy. Calpain-associated LGMD is inherited in a recessive fashion and appears to result from loss of calpain-3 function 
. In contrast, ADNIV is inherited in an autosomal dominant fashion and is caused by missense mutations near the active site. Although these mutations could cause disease through haploinsufficiency, it seems more likely that they result in a gain of function of calpain-5 that causes harm to the photoreceptor cells. Capn5
knockout mice have no observable phenotype 
, and several human neurological disorders have been associated with excess calpain activity 
, including photoreceptor degeneration 
A gain of function mechanism for ADNIV is also supported by the unusual inflammation and neovascularization associated with the disease. There are dozens of monogenic disorders that cause the apoptotic death of photoreceptor cells without causing severe intraocular inflammation or neovascularization of the retina. The latter is much more typical of proliferative diabetic retinopathy than it is heritable photoreceptor disease 
. It is easier to imagine an unregulated or mislocalized calpain promiscuously activating different signaling pathways, or being released into the extracellular space after photoreceptor death and causing inappropriate angiogenesis and leukocyte recruitment, than it is to imagine a 50% reduction of such a protein causing these dramatic complications.
Whether caused by a gain or loss of function of CAPN5
, it is likely that the further elucidation of the pathogenic mechanism of ADNIV will provide important new insight into some of the most important causes of irreversible human blindness: autoimmune uveitis, retinitis pigmentosa, proliferative vitreoretinopathy and diabetic retinopathy. The latter condition alone is responsible for as much as 17% of blindness in some regions of the world 
. The fact that an amino acid change in a single protein can lead to such a phenotype raises the possibility that a common, therapeutically accessible pathway may be shared among these conditions that could be targeted with drugs, antibodies or gene transfer approaches. It is possible that variations in the structure or expression of CAPN5
cause or modify some of these common disorders and this hypothesis will be important to test in future studies. However, given the extreme phenotypic heterogeneity of these disorders, it will be important to study a large number of subjects in such an experiment, to subdivide the patient cohorts into clinically well-characterized groups, and to screen an equal number of ethnically matched controls for each of these groups.