Homozygous mice have a phenotype similar to human Chediak-Higashi syndrome patients, exhibiting lysosomal dysfunction with resultant protein storage; diluted coat color; abnormal melanogenesis; immune cell dysfunction resulting in increased susceptibility to bacterial, viral, and parasitic infections and decreased cytotoxic activity against tumor cells.
Figure 1. Charzard mice (right) exhibit hypopigmentation of the fur. A wild-type littermate (left) is shown for reference.
The charzard phenotype was identified among G3 mice of the pedigree R1793, some of which exhibited hypopigmentation of the fur (Figure 1).
Nature of Mutation
Whole exome HiSeq sequencing of the G1 grandsire identified 123 mutations. Among these, only one affected a gene with known effects on fur pigmentation: Lyst. The mutation in Lyst was presumed to be causative because the charzard hypopigmentation phenotype mimics other known alleles of Lyst (see MGI for a list of Lyst alleles as well as the entry for souris). The Lyst mutation is a T to C transition at base pair 13,647,083 (v38) on chromosome 13, or base pair 56,745 in the GenBank genomic region NC_000079 for the Lyst gene. The mutation corresponds to residue 4,222 in the mRNA sequence NM_010748 within exon 11 of 53 total exons.
The mutated nucleotide is indicated in red. The mutation results in substitution of cysteine (C) 1,347 to a premature stop codon (C1347*) in the Lyst protein.
Figure 1. Domain structure of the Lyst protein. The Lyst protein is a 3788-amino acid protein whose biochemical functions remain unknown. The N-terminal portion of the protein contains approximately twenty repeats with homology to ARM and HEAT repeat motifs and a perilipin domain (PD). The C-terminal portion of Lyst contains a BEACH domain and seven WD40 motifs. The charzard mutation results in substitution of cysteine (C) 1,347 to a premature stop codon (C1347*). This image is interactive. Click on the image to view other mutations found in Lyst (red). Click on the mutations for more specific information. There are two additional alleles in the Lyst gene: sooty and grey wolf. The locations of their mutations is unknown.
The Lyst gene encodes the protein Lyst (also CHS/Beige), a 3788-amino acid protein whose biochemical functions remain unknown (Figure 2). A large N-terminal portion of the protein (amino acids 1-3,132) contains approximately twenty repeats with homology to ARM (Armadillo) and HEAT (huntingtin, elongation factor 3, A subunit of protein phosphatase A, target of rapamycin) repeat motifs (1;2). ARM and HEAT motifs are α-helical domains of about 50 amino acids that pack together to form elongated “solenoids” (3); evidence suggests they mediate protein associations at the membrane (4), and vesicle transport (5), respectively. A perilipin domain (PD; amino acids 1079-1313) may interact with lipids. The C-terminus of Lyst contains two distinct domains, a BEACH (beige and chediak) domain (amino acids 3132-3472) and seven WD40 motifs (1). The BEACH domain is a 345-amino acid region of unknown function (1), and WD40 motifs are protein interaction motifs that typically form β sheets arranged in a 7-bladed β propeller fold (6). The charzard mutation results in substitution of cysteine (C) 1,347 to a premature stop codon (C1347*). Amino acid 1,347 is within the ARM/HEAT domain, C-terminal to the perilipin domain.
Please see the record for souris for information about Lyst.
In humans, mutations in the LYST gene cause Chediak-Higashi Syndrome (CHS, OMIM #214500), a rare autosomal recessive disorder characterized by oculocutaneous albinism, severe immune deficiency, bleeding tendency, recurrent pyogenic infection, progressive neurologic defects and a lymphoproliferative syndrome [(7;8); reviewed in (2)]. These defects are caused by the aberrant formation of giant granules within a variety of cell types, and disrupted intracellular protein trafficking (2;9;10). The enlarged granules consist of organelles such as lysosomes, melanosomes, cytolytic granules and platelet dense bodies, and it is thought that the increased size of these organelles inhibits their migration and fusion at the cell surface and/or organelle-organelle fusion. There is no clear understanding of the molecular mechanisms of Lyst protein function, or how its loss leads to the formation of enlarged lysosomes and lysosome-related organelles.
In mice, mutations in Lyst cause the beige phenotype (7;8). As in humans, beige mice exhibit hypopigmentation, bleeding tendency, and defective immune cell function resulting from the formation of giant granules in melanosomes, lymphocytes, neutrophils, and other cell types (10-12). Beige mice have defective NK cell (13) and cytotoxic T lymphocyte function (14), and increased susceptibility to infections including MCMV (15;16). However, beige mice do not develop lymphoproliferative disorder, even after challenge with infection (15). The charzard hypopigmentation phenotype mimics other known alleles of Lyst indicating that there is loss of function in the Lystcharzard protein. Immune cell phenotypes were not observed in charazard.
charzard(F):5'- CGTCTGTCACTTAAGTTCCATGTAAG -3'
charzard(R):5'- CAAACATAAAACATGGTACTGGGC -3'
charzard_seq(F):5'- GTAGTTATAACCTGAACAATTGCCG -3'
charzard_seq(R):5'- ATAAAACATGGTACTGGGCTAAATAG -3'