Test Determines Whether Children Will Respond to Rare Disease Treatment

Researchers created a test to determine which children with Carbamoyl-Phosphate Synthetase 2, Aspartate Transcarbamylase, And Dihydroorotase (CAD) gene deficiency will benefit from receiving a nutritional supplement that historically improved the lives of other children with the condition, according to a study published in Genetics in Medicine.

“CAD encodes a multienzymatic cytoplasmic protein harboring 4 functional domains,” researchers explain. This metabolic pathway is essential for nucleotide homeostasis, cell growth, and proliferation while deficiency occurs when both copies of the CAD gene contain mutations.

Only 17 individuals in the world are known to have CAD deficiency, and 11 of the 17 were included in the current study.

In 2015 researchers discovered uridine supplements corrected a CAD-associated congenital disorder, glycosylation, in an individual with early infantile epileptic encephalopathy. Subsequent reports revealed similar results where uridine treatment resulted in cessation of seizures.

Although there are over 1000 variations in the gene, scientists previously did not know whether a change was just a variant or a mutation which can disrupt uridine production. Now, with the development of a cell-based assay, doctors can determine if CAD variation is pathogenic based on the cell’s ability to grow with or without uridine.

“The effect of uridine for some children with CAD deficiency is nothing short of amazing,” said Hudson Freeze, PhD, a coauthor of the study. “These kids go from bedridden to interacting with people and moving around.”

Investigators set out to create a CAD-knockout (KO) cell line that could be used to assess the pathogenicity of CAD variants. To do so, scientists used CRISPR/Cas9 to generate a human CAD-knockout cell line that requires uridine supplements for survival.

In the cohort, 4 families reported having a family history of affected siblings, while 3 had a sibling with a similar disorder that died. One individual was also noted to have died.

Of the participants:

• All had intellectual and developmental disability
• 9/10 (90%) had seizure activity
• 5/10 exhibited gastrointestinal complications ranging from feeding problems, reflux, and recurrent vomiting
• 5/10 had facial dysmorphism, hypotonia, and ataxia
• 4/10 showed hematological abnormalities

CAD deficiency was determined when each variant was combined based on individual-specific genotyping, marking a stringent prediction based on each single variant. The assay does not test the specific combination of alleles found in each individual, however researchers assumed a combination of 2 variants would not cancel each other to generate a fully capable CAD protein.

“With this test, we can provide hope to some families, while sparing others from unrealistic expectations. That’s incredibly important,” Freeze said.

Reference:

del Caño-Ochoa F, Bobby G, Abedalthagafi M, et al. Cell-based analysis of CAD variants identifies individuals likely to benefit from uridine therapy. Genet Med. Published online May 28, 2020. doi:10.1038/s41436-020-0833-2