Researchers are working to study the new causes of Stargardt's disease

Using a new stem cell-based model of skin cells, the scientists found the first direct evidence that
a mutation in the stargardt-associated ABCA4 gene affects a layer of cells in the eye called the retinal pigment epithelium (RPE).
This finding points to a new understanding of the progression of Stargardt's disease and suggests therapeutic strategies
for diseases that currently lack treatment.
The study was conducted at the National Eye Institute (NEI), which is part of the National Institutes of
Health.
The findings are published today on the website
of Stem Cell Reports.

Dr.
Michael F.
Chiang, director of NEI, said, "This new model will accelerate the development of
treatments for Stargardt's disease.
" We lack a cure for this disease, in part because it's rare
.
Theoretically, this model creates an unlimited supply
of human cells for research.
"In the United States, 1 in 10,000 people have Stargardt

Stargardt disease causes a gradual loss
of central and night vision.
Decreased vision is associated with toxic lipid-rich deposition in RPE, the main role of which is to support and nourish the retina's photoreceptors
.
Under normal circumstances, the ABCA4 gene produces a protein to stop the buildup
of this toxin.
Previous studies have suggested that Stargardt disease is caused
by various mutations in the ABCA4 gene.
More than 800 ABCA4 mutations are known to be associated with
a broad Stargardt disease phenotype.

One way RPE supports photoreceptors is by ingesting their depleted outer segments, which keeps cells trimmed and healthy
.
In Stargardt's disease, many scientists believe that RPE cells die after acquiring toxic byproducts when ingested in the outer segments, which in turn leads to photoreceptor cell death and vision loss
.

Much of what is currently known about Stargardt's disease has been gained by studying mouse models, which are inherently limited
by the wide genetic variability of the disease in humans.
Using a human model of RPE, NEI researchers were able to determine whether ABCA4 gene mutations directly affect RPE
independently of photoreceptors.

To develop the model, the researchers took skin cells from Stargardt patients, converted them into stem cells, and then induced the stem cells to differentiate into RPE cells
.
The researchers examined patient-derived RPE and detected the ABCA4 protein
on the RPE cell membrane.
They used the gene-editing technology CRISPR/Cas9 to generate patient-derived RPEs lacking ABCA4, known as ABCA4 knockouts, to explore the role
of ABCA4 in RPE development.
They found that the absence of ABCA4 did not affect the maturation
of patient-derived RPE.

However, lipid deposits
accumulate within RPE cells when RPEs lacking ABCA4 are exposed to the outer segment of normal (wild-type) photoreceptors.

Further ABCA4 knockout assays showed defective RPE lipid metabolism and impaired ability to digest the outer segments of photoreceptors, resulting in lipid deposition in
RPE cells.

This is the first report that loss of ABCA4 function in human RPE is associated
with intracellular lipid deposition without exposure to the outer segment of the ABCA4 mutant photoreceptor.
Over time, these lipid deposits may cause RPE atrophy, leading to photoreceptor denaturation
.

"Our report provides guidance for gene therapies that target RPE," said the study's lead investigator, Dr.
Kapil Bharti, a senior investigator
in NEI's Division of Ophthalmology and Stem Cell Translational Research.
"Our data suggest that in addition to correcting the loss of ABCA4 function in photoreceptors, gene therapy also needs to target RPE cells
.
"

This study is part of
a larger effort by NEI to address the limited availability of patient-derived stem cell lines for studying Stargardt's disease.
To overcome this hurdle, NEI launched the STGD1-iPSC banking program
from patients with different ABAC4 mutations.
These cells will be made available to society as a whole for mechanistic and genotype-phenotypic studies
.

This work was funded
by the NEI On-Campus Research Program.

Journal Reference:

1. Mitra Farnoodian, Devika Bose, Vladimir Khristov, Praveen Joseph Susaimanickam, Savitri Maddileti, Indumathi Mariappan, Mones Abu-Asab, Maria Campos, Rafael Villasmil, Qin Wan, Arvydas Maminishkis, David McGaughey, Francesca Barone, Rebekah L.
   Gundry, Daniel R.
   Riordon, Kenneth R.
   Boheler, Ruchi Sharma, Kapil Bharti.
   Cell-autonomous lipid-handling defects in Stargardt iPSC-derived retinal pigment epithelium cells.
   Stem Cell Reports, 2022; DOI: 10.
   1016/j.
   stemcr.
   2022.
   10.
   001