Gene Discovery in Mendelian Disorders

We have been involved in numerous gene discoveries for Mendelian disorders.
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This has involved discoveries on a broad list of single-gene disorders including COL11A1 in fibrochondrogenesis a severe neonatal bone phenotype (Tompson et al., 2010), de novo truncating mutations in AHDC1 in intellectual disability and speech delay, the Xia-Gibbs syndrome (Xia et al., 2014) and ACTG2 in visceral myopathy (Wangler and Beaudet, 2015; Wangler et al., 2014).

Model Organisms and the Molecular Pathogenesis of Mendelian Disorders

One of the key problems in clinical genetics and in the use of genomic sequencing is the problem of gene function and difficulty in intrepreting human genomic variation and assigning pathogenicity in the clinic.

We began developing an approach to use Drosophila. Initially our efforts involved
accelerating cross-species studies for novel genes by working simultaneously with a large forward genetic screen in Drosophila and a genomic database from 2,000 individuals ascertained for Mendelian disease. This rich resource was published in Cell (Yamamoto et al., 2014), and this approach was outlined in an article in Genetics which has been selected for Faculty of 1000 (Wangler et al., 2015). We have recently developed an efficient pipeline using Drosophila melanogaster to screen nearly ANY conserved gene and variant of interest from genomic sequencing studies. We have successfully applied these tools to solve cases in the Centers for Mendelian Genomics (, and we are currently applying this pipeline to de novo missense variants from the Simons Simplex Collection as well as unsolved cases from the Undiagnosed Diseases Network (UDN).

The key features of our approach are shown in this figure (from Yamamoto and Bellen, Cell 2015).

  • Variants of interest are screened with bioinformatic prioritization involving prediction of the Drosophila ortholog, and cross-referencing with independent genomic databases.
  • A cassette is introduced into an intron of the gene of interest using CRISPR this cassette relies on latest fly technology to produce both a strong loss-of-function allele, and the expression of a GAL4 from endogenous enhancers. Strong loss of function alleles allowing studies of gene function
  • Human cDNA is introduced into a transgenic strain into a docking site in the fly genome. GAL4 from the endogenous enhancer drives expression of human or variant cDNA from UAS. With phenotypic characterization and then with an integration with the clinical data, to address variant pathogenicity

This process allows for Humanization of the fly for the gene of interest. This pipeline establishes a large screen in human and fly genomes to understand gene function.

Mendelian Disorders of the Peroxisome and Organelle Dynamics

Our lab has developed a Drosophila research program and a clinical research study focused on peroxisomal genes. This work has identified unique phenotypes such as a late-onset ataxia associated with PEX16 (Bacino et al., 2015) and cases of infantile encephalopathy due to DNM1L mutations. The study of these mutations in flies has allowed us to understand loss and gain of function alleles in peroxisomal disease and broadened our understanding of these phenotypes and their biological basis.

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A special area of scientific focus and the mid-term goal of my work is to use these general approaches in understanding the basic function and regulation of peroxisomes in human health and disease. We are conducting a clinical research study at Baylor College of Medicine. The regulation of organelle fission is under intense study as it pertains to mitochondrial fission as this process is altered in an array of neurodegenerative diseases including optic atrophy, neuropathy, Alzheimer’s disease, and Parkinson’s disease. Peroxisomal fission is also critical for neuronal maintenance as peroxisomal dynamics are altered in intellectual disability, hearing loss and neuronal maintenance as demonstrated by our cases of DNM1L variants. However, peroxisomal fission is not well studied. Numerous key questions remain such as how the dynamics and number of peroxisomes are regulated in cells, how altered peroxisomal dynamics perturbs function and what role these processes play in disease.