Scientists have discovered a new genetic disease, which causes some children’s brains to develop abnormally, resulting in delayed intellectual development and often early onset cataracts. Affected children also developed microcephaly, small head size relative to age-matched peers. Caused by changes in a gene called coat protein complex 1 (COPB1), the disease which has been identified in six individuals is as yet unnamed.
Protein coat complexes include coatomer protein 1(COP1) complex, coatomer protein II (COPII) complex, and a number of adaptor protein (AP) complexes. These protein coats select proteins and lipids for transport and facilitate transport vesicle formation. In recent years, mutations in subunits of these coat proteins complexes, like COP1, have been implicated in various human congenital diseases known collectively as “coatopathies”.
The COPB1 gene encodes the beta-subunit of COP1 (β-COP). It is needed to sort and traffic proteins and lipids from the Golgi apparatus to the endoplasmic reticulum (ER). Searching the genetic cause of the condition in two unrelated families, researchers from the University of Portsmouth and the University of Southampton in England performed whole exome or genome sequencing to investigate potential variants. In their research, biallelic variants in COPB1 were revealed as a potential cause in both families: one a splice variant, the other a missense variant.
Studying the splice COPB1 variant in two affected sisters of Family 1, the team performed CRISPR/Cas9 modeling of this variant in tadpoles. An exon of the tadpole was replaced with an indel of the COPB1 variant injected into tadpole embryos. The tadpoles with the disrupted COPB1 gene variant from Family 1 developed smaller brains than control tadpoles; many had cataracts. These were consistent with the clinical features of the affected family members.
“In our initial experiments to test the link between a genetic variation and a disease we found to our surprise that by altering the DNA of tadpoles, four times out of five we could re-create the disease-related changes seen in human patients,” said study co-author Matt Guille, of the University of Portsmouth. These results support the link between the patient variants and their clinical pathology.
In Family 2, the identified mutation was a COPB1 missense variant. To study it, the researchers cultured human retinal epithelium and embryonic kidney cell lines with a COPB1 expression vector introducing this mutation. The variant in Family 2 was connected to altered trafficking of a mutant β-COP from the Golgi apparatus to the endoplasmic reticulum such that the mutant protein was retained in the Golgi.
“This adds to the growing body of evidence that COP1 subunits are essential in brain development and human health,” the authors write.
The study also provides proof of principle that tadpole CRISPR/Cas modelling can be used as a tool for identification and characterization of novel rare disease genes.
“There is now a large body of evidence showing that making targeted indels in the western clawed frog is so efficient that it is possible to analyze the resulting phenotypes in founder animals,” the authors. “This results in robust but more rapid testing of the causality of a gene variant in disease than waiting for F1 or F2 animals (next generation animals) to be available.”
