Homozygosity Mapping and Whole-Exome Sequencing to Detect SLC45A2 and G6PC3 Mutations in a Single Patient with Oculocutaneous Albinism and Neutropenia

Abstract

We evaluated a 32 year-old woman whose oculocutaneous albinism, bleeding diathesis, neutropenia, and history of recurrent infections prompted consideration of the diagnosis of Hermansky-Pudlak syndrome type 2 (HPS-2). This was ruled out due to the presence of platelet delta granules and absence of AP3B1 mutations. Since parental consanguinity suggested an autosomal recessive mode of inheritance, we employed homozygosity mapping, followed by whole exome sequencing, to identify two candidate disease-causing genes, SLC45A2 and G6PC3. Conventional di-deoxy sequencing confirmed pathogenic mutations in SLC45A2, associated with oculocutaneous albinism type 4 (OCA-4), and G6PC3, associated with neutropenia. The substantial reduction of SLC45A2 protein in the patient’s melanocytes caused the mis-localization of tyrosinase from melanosomes to the plasma membrane and also led to the incorporation of tyrosinase into exosomes and secretion into the culture medium, explaining the hypopigmentation in OCA-4. Our patient’s G6PC3 mRNA expression level was also reduced, leading to increased apoptosis of her fibroblasts under ER stress. This report describes the first North American patient with OCA-4, the first culture of human OCA-4 melanocytes, and the use of homozygosity mapping followed by whole exome sequencing to identify disease-causing mutations in multiple genes in a single affected individual.

Recurrent Mutations in the Basic Domain of TWIST2 Cause Ablepharon Macrostomia and Barber-Say Syndromes

“…42 Genomic DNA was extracted from AMS-3.1, -6.1, -7.1 (hyperpigmented, affected), and -7.2 and from BSS-4.2 and -6.1 dermal fibroblasts as well as ATCC adult control, AMS-7.1 (hypopigmented, unaffected), and the mother of AMS-7.2 dermal fibroblasts as unaffected control.…”

Two novel splicing mutations in the SLC45A2 gene cause Oculocutaneous Albinism Type IV by unmasking cryptic splice sites

“…7 MATP is thought to be important for the proper processing and trafficking of melanosomal proteins Tyr, Tyrp1, Dct (Tyrp2), and seems to act as a proton-dependent transporter at the melanosomal membrane, showing a possible relationship with the P protein (OCA2). 21,22 Moreover, recent data based on experiments of heterologous expression of MATP in Saccharomyces cerevisiae suggested that it also acts as a sucrose transporter. 23 OCA4 clinical phenotype is characterized by a high degree of heterogeneity, ranging from profound hypopigmentation (similar to OCA1A) to near normal skin colour.…”

Applications of high-throughput DNA sequencing to benign hematology

“…Cullinane and colleagues studied a woman with oculocutaneous albinism, recurrent infections, bleeding diathesis, and neutropenia with the working clinical diagnosis of HermanskyPudlak syndrome. 23 However, homozygosity mapping and exome sequencing identified mutations in 2 disease loci: the SLC45A2 gene locus associated with oculocutaneous albinism and the G6PC3 gene locus associated with congenital neutropenia. 23 Additional findings of this woman and her sibling were described by Fernandez and coworkers.…”

“…23 However, homozygosity mapping and exome sequencing identified mutations in 2 disease loci: the SLC45A2 gene locus associated with oculocutaneous albinism and the G6PC3 gene locus associated with congenital neutropenia. 23 Additional findings of this woman and her sibling were described by Fernandez and coworkers. 24 Extending disease phenotype-genotype relationships and disease gene discovery…”