Down syndrome (DS, Trisomy 21) is the most common chromosomal disorder in humans, affecting 1 in 700 newborns in the United States (Mai et al., 2019). DS is caused by partial or full triplication of human chromosome 21 (HSA21). Individuals with DS typically present with characteristic physical features, reduced immune and endocrine functionality, and delayed cognitive development (Lott and Dierssen, 2010; Roizen and Patterson, 2003). Continued improvements in care, education, and support have led to a progressive increase in life expectancy and life span for people with DS (Bittles et al., 2007).

Over 230 protein-coding genes are located on chromosome 21 (Ensembl GRCh38.p13; Fig. 1A), including the APP gene (21q21.3) that codes for the amyloid precursor protein (APP), the substrate for endoproteolytic cleavages that generate amyloid-β (Aβ) peptides. Abnormal deposition of Aβ peptides underpins Aβ amyloidopathy in the brain and lenses of people with Alzheimer's disease (AD) (Goldstein et al., 2003; Jun et al., 2012; Lott and Head, 2019; Moncaster et al., 2022, 2010; O'Brien and Wong, 2011; Patterson et al., 1988; Tanzi et al., 1988, 1987). In DS, triplication of the APP gene drives early-onset β-amyloid pathology in the brain and lens. Pathological accumulation of Aβ has been reported in brains of children with DS (Leverenz and Raskind, 1998). Young adults with DS exhibit abnormal Aβ levels and AD-related Aβ amyloid pathology in the brain (Mann, 1988). By the fifth decade of life, the majority of people with DS express advanced AD-associated neuropathology, including dense core amyloid plaques and neurofibrillary tangles (Lemere et al., 1996). People with DS in the sixth decade of life and beyond are at high risk of widely distributed AD neuropathology and cognitive decline leading to clinical dementia (McCarron et al., 2014; Wiseman et al., 2015).

Individuals with DS are also at increased risk for ocular complications, including early-onset cataract (Catalano, 1990; Liyanage and Barnes, 2008). The reported prevalence of cataract in DS ranges from 4 to 72 % (Da Cunha and Moreira, 1996; Fong et al., 2013; Jaeger, 1980; Kim et al., 2002; Wong and Ho, 1997), with differences in cohort characteristics, examination procedures, and cataract phenotyping contributing to the wide variation (Little et al., 2020). DS subjects may present with characteristic cerulean blue-dot opacities in the supranuclear (deep cortical) region of the lens (Igersheimer, 1951; Jaeger, 1980; Lowe, 1949; Pearce et al., 1910). The presence of AD-related Aβ amyloidopathy in the brain and lens in patients with AD, and the invariant early-onset and age-dependence of Aβ accumulation and amyloid pathology in the brain in people with DS, led to the hypothesis that the distinctive lens opacities in DS patients were either formed from or related to pathogenic Aβ accumulation in the lens (Goldstein et al., 2003; Moncaster et al., 2010, 2022). Indeed, elevated Aβ tissue levels, AD-linked Aβ deposits and amyloid pathology, and co-localizing supranuclear cataracts have been identified and characterized in postmortem lenses from people with karyotype-confirmed DS, but not in postmortem lenses from non-DS normal controls (Goldstein et al., 2003; Moncaster et al., 2010).

The molecular and ultrastructural pathology, subregional and cellular distribution, clinical phenotypes, and age-related opacification observed in DS lenses recapitulate the Aβ amyloidopathy and corresponding supranuclear cataract phenotype in AD lenses. The underlying molecular pathology in DS and AD lenses shares a common etiology, namely, pathogenic accumulation of highly amyloidogenic Aβ peptides (Moncaster et al., 2022). Progressive, age-dependent Aβ accumulation within the supranucleus (deep cortex) of the lens increases local light scattering that ultimately manifests as the distinctive supranuclear cataract phenotype observed in AD and DS (Moncaster et al., 2010). Accordingly, we hypothesized that measuring light-scattering protein aggregates in the lenses of adolescents with DS would provide insight into lens pathology in this common chromosomal disorder.

Quasi-elastic light scattering (QLS) is a noninvasive technique that can be harnessed to measure changes in the distribution of hydrodynamic particle radii (consistent with molecular size) in the ocular lens in vivo (Fig. 1B) (Benedek et al., 1987; Bursell et al., 1989; Datiles et al., 2008, 2016; Minaeva et al., 2020; Thurston et al., 1997). Cumulative changes in the molecular structure and supramolecular organization of long-lived cytosolic lens proteins lead to changes in both size and aggregation of particles that progressively alter the physical, optical, and functional properties of the lens (Clark, 2004; Costello et al., 2012; Schietroma et al., 2009). We recently deployed a clinically-qualified QLS instrument to detect changes in lens protein molecular size distribution in living humans as a function of age (Minaeva et al., 2020).

The goal of the present study was to investigate molecular changes in the lenses of adolescent DS subjects early in the course of DS-associated lens pathology when the lens is optimally transparent and detection is difficult using conventional ophthalmic techniques (Minaeva et al., 2020). We show that adolescents with DS show QLS signal changes in the lens indicative of abnormal supramolecular size distribution compared to adolescent control subjects without DS.