In this report, we identify the origin and pathogenic mechanism of cataractogenesis in DS, establish the distinctive lens phenotype in DS as a genetic cataract, and define the molecular correspondence of DS-associated pathology in the lens and brain. Taken together with our previous findings of supranuclear Aβ lens pathology in late-onset sporadic AD 
, these results also establish the pathogenic relationship linking Aβ pathology in the lens and brain in both DS and AD.
AD neuropathology is an invariant feature of DS in subjects over the age of 30 
but may emerge as early as the first decade 
. A critical factor contributing to the association of DS and AD Aβ pathology is the locus of the amyloid precursor protein gene (APP,
21q21) on the long arm of chromosome 21 
. Chromosome 21 triplication leads to increased dosage of the APP
gene, accelerated cerebral Aβ accumulation, progressive AD neuropathology, and age-dependent cognitive sequelae 
. With the exception of a single reported case of an individual with an atypical DS karyotype involving a specific chromosome 21 micro-deletion in the APP
gene locus 
, all subjects with Down syndrome (including the rare cases involving Robertsonian translocation, partial duplications, or trisomic mosaicism) demonstrate triplication of the APP
gene locus (21q21), overexpress amyloid precursor protein (APP) in affected somatic cells, accumulate β-amyloid peptides (Aβ) in the brain, and reveal evidence of early-onset Alzheimer's disease (AD) neuropathology 
In this report, we hypothesize and confirm that this genotype-phenotype concordance extends to Aβ-linked molecular pathology in the supranucleus and deep cortex of the lens. The characteristic cataract phenotype associated with Aβ lens pathology has been identified in only two clinical disorders, Alzheimer's disease 
and as reported here, Down syndrome. This distinctive supranuclear cataract phenotype has not been reported in normal individuals, and is not observed in other non-AD neurodegenerative diseases, nor in normal aged controls 
. The classical cerulean “blue dot” cataracts characteristic of DS initially emerge at the equatorial periphery of the lens, and over time, encompass progressively larger areas of the supranucleus and deep cortex (). While lens pathology observed in both infantile and mature DS lenses reveal opaque regions that are clinically described as supranuclear, the ages of the corresponding fiber cells comprising these regions are different (). In this respect, it is important to note that the lens pathology in DS lenses does not implicate involvement of lens fibers cells in the fetal nucleus.
Age-dependent supranuclear cataractogenesis in Down syndrome.
Distinctive lens pathology is recognized as a characteristic early-onset ocular phenotype in subjects with DS that may be clinically detectable early in life, and in some cases, at birth 
. By contrast, DS neuropathology and cognitive sequelae typically emerge in the third decade of life and age-dependently progress thereafter 
. In the present study, early-onset lens pathology is histochemically documented in illustrative cases of two young adults (21 and 22 years of age) with DS. Lenses obtained from these donors revealed marked Aβ amyloid lens pathology that was evident not only in the supranuclear subregion, but also throughout the anterior and equatorial cortex and epithelium. Aβ biochemical analysis of DS and normal control lenses indicates that increased Aβ expression in the lenses of people with DS may be evident very early in life. In one case included in this study, we detected abnormally high tissue levels of Aβ in the lens of a DS subject at 2 years of age. Although visual impairment may not become clinically apparent due to the peripheral distribution of these lenticular lesions relative to the iris, it is certainly the case that Aβ molecular pathology in the lens may be amongst the earliest expressed age-dependent phenotypes in Down syndrome.
The DS-associated cataract phenotype is not observed in other non-AD neurodegenerative diseases, nor in normal aged controls. In this context, it is important to note that DS-associated cataracts are phenotypically distinct from age-related cataracts (ARC) that commonly emerge starting in the fifth decade of life and typically localize in the central nuclear region. In contrast to the lens pathology in ARC, the DS lens phenotype is often clinically detectable early in life, and in some cases, may be present at birth 
. The distinctive lens phenotype associated with DS is further distinguished from ARC by supranuclear Aβ histopathology, a pathological feature shared with late-onset AD 
The results of the present study support a DS-associated pathogenic pathway linking progressive age-dependent Aβ accumulation in the lens and supranuclear cataractogenesis with corresponding cerebral Aβ accumulation and neuropathology in the brain. We propose the following mechanistic pathway leading to expression of the characteristic lens and brain phenotypes associated with DS (). Increased APP
gene dosage resulting from chromosome 21 triplication (APP,
21q21) leads to increased expression of the amyloid precursor protein (APP) and elevated levels of the pathogenic Aβ cleavage peptides in affected tissues. In the brain, Aβ accumulation leads to early-onset neuropathology and progressive age-dependent neurocognitive sequelae. With respect to the lens, the present investigation supports the hypothesis that increased expression of Aβ in the lens leads to amyloidogenic interactions with other structural proteins (e.g., αB-crystallin) within the cytoplasm of supranuclear lens fiber cells. This pathogenic cascade leads to increased lens protein aggregation, light scattering, opacification, and ultimately, disease-linked cataracts 
. Given the fact that the long-lived lens fiber cells are metabolically sluggish and demonstrate limited capacity to catabolize aggregated protein, we speculate that DS-linked Aβ amyloid pathology in the lens may precede corresponding Aβ pathology in the brain. Regardless, the proposed pathogenic model for Aβ pathology in both tissue compartments (brain and lens) is in general agreement with a broad conception of the amyloid cascade hypothesis 
Model pathogenic pathways in Down syndrome brain and lens.
We hypothesize that the Aβ detected in DS lenses is generated endogenously. The human lens expresses the full complement of enzymes and precursor proteins necessary to generate and process Aβ 
. In the present study, this hypothesis is supported by the finding of intense Aβ immunoreactivity in DS lens epithelial cells. Our finding that lens fiber cell Aβ is exclusively cytoplasmic suggests that this amyloidogenic peptide may be released from intracellular processing compartments during organelle disintegration accompanying epithelial-to-fiber cell terminal differentiation. This model points to the lens as an appropriate target for ascertaining the temporal origin and natural history of AD-linked molecular pathology in vivo.
Our findings suggest an apparent temporal discordance in DS with respect to phenotype expression in the lens and brain, with pathology in the former preceding that in the latter. However, we also observed variability in age-dependent phenotypic expression of Aβ molecular pathology in the lens that comports with analogous variability in the brain of subjects with DS 
. The significance and natural history of variability in Aβ tissue concentration in the lens and its possible relationship to the brain in DS are subjects of ongoing investigation.
In this study, we identify Aβ amyloid pathology as the shared molecular etiology of two defining features of DS, namely, supranuclear cataracts in the lens and AD neuropathology in the brain. The proximal pathogenesis of both pathologies likely derives from the primary chromosomal disorder and associated APP gene-dosage imbalance (). The results reported here support a testable pathogenic mechanism by which the primary chromosomal disorder in DS may be translated into a distinctive phenotype expressed in the lens. In this regard, the phenotypic concordance of Aβ lens pathology in both diseases, DS and AD, is striking. Given that DS-linked pathology may be indicative of aberrant Aβ accumulation in both compartments, the lens may provide an optically accessible target tissue for early detection and longitudinal assessment of early Aβ molecular pathology in DS and AD. Future clinical studies will be required to systematically evaluate the frequency of this supranuclear phenotype in DS patients. Quasi-elastic light scattering instrumentation or other techniques may provide a practical approach for quantitative assessment of specific lens pathology throughout the course of the disease.