A new genetic isolate with a unique phenotype of syndromic oculocutaneous albinism: clinical, molecular, and cellular characteristics

Abstract

An extended, highly consanguineous Israeli Bedouin family with at least 20 individuals exhibiting a unique phenotype of oculocutaneous albinism (OCA) was identified. All known OCA genes were excluded in this family. Electron microscopic analysis of platelets revealed absence of dense bodies, suggesting a diagnosis of Hermansky-Pudlak syndrome (HPS). HPS is a rare autosomal recessive disorder of lysosome-related organelle biogenesis, clinically characterized by OCA and platelet dysfunction, sometimes accompanied by other systemic pathologies. All human HPS genes (HPS1-8) and five genes corresponding to murine HPS models were evaluated. Haplotype analysis and homozygosity mapping of the HPS loci revealed linkage to chromosome 10 in the studied family. Subsequently, a novel insertion mutation, c.1066-1067insG was identified in HPS6. Most frameshift mutations generating premature termination codon cause mRNA nonsense mediated decay (NMD), while intronless genes like HPS6 are usually not monitored by NMD. Expression analysis revealed no mRNA decay in patient's fibroblasts, hence truncated protein is most probably produced. Confocal microscopy revealed abnormal distribution of LAMP-3 (lysosomal associated membrane protein-3) in fibroblasts from the patients, indicating abnormal trafficking of lysosomal lineage organelles. So far, a single HPS-6 patient phenotypically similar to HPS-3 and HPS-5 has been identified. The HPS-6 phenotype in the studied family is unique since it resembles OCA and not HPS. Therefore, our finding broadens the phenotypic definition of HPS. Two major genetic isolates of HPS-1 and HPS-3 patients were previously diagnosed in Puerto Rico. The extended Bedouin family is the largest isolate of non-Puerto Rican HPS patients.

An immunoblotting assay to facilitate the molecular diagnosis of Hermansky–Pudlak syndrome

“…Primer pairs were designed for PCR amplification of each exon and their adjacent intron sequences of HPS human candidate genes (Table 1). Primer sequences are available upon request and have been previously described for HPS1 [49], AP3B1 [25], HPS3 [29], HPS4 [20], HPS5 [30] and HPS6 [32]. Standard PCR amplification procedures [48] were employed.…”

“…Therefore, these results suggested that patients 88 and 156 suffer from HPS due to mutations causing BLOC-2 deficiency, and that patients 45, 94 and 128 suffer from HPS due to BLOC-3 deficiency. Based on reported differences in the severity of HPS disease caused by BLOC-2 deficiency (in HPS-3, -5 or -6 [8,11,[29][30][31][32]) or BLOC-3 deficiency (in HPS-1 or -4 [18][19][20][21][22][23]), one would then expect that patients 88 and 156 should suffer from a mild form of HPS and that patients 45, 94 and 128 should be at a higher risk of developing pulmonary fibrosis or GI disease. Unfortunately, only two of these patients (88 and 45) were examined at an age in which the validity of these predictions could begin to be evaluated: at the time of their last evaluation at the NIH Clinical Center, patients 88 (age 40 years), 94 (age 2 years), 128 (age 2 years) and 156 (age 19 years) had no signs of interstitial lung disease of GI disease, while patient 45 (age 35 years) presented with a history of GI disease but no interstitial lung disease.…”

“…HPS-2, and not the other HPS types, is associated with recurrent infections due to chronic neutropenia and other deficiencies in the innate immune system [24][25][26][27][28]. In contrast, HPS-3, -5 and -6 have been clinically characterized as a mild form of HPS [8,11,[29][30][31][32]. Taken together, these observations underscore the potential significance of identifying the disease type -or at least the deficient protein complex -for prognosis and, in the future, for consideration of eventual treatment options.…”

“…By analogy, focused mutation analyses could also be undertaken for HPS patients from other genetic isolates [32]. Nevertheless, the molecular diagnosis (and determination of disease type) of HPS patients of non-Puerto Rican origin is largely based on sequencing of all coding regions and intron-exon boundaries of every single candidate gene.…”

Early Origin of Genes Encoding Subunits of Biogenesis of Lysosome-related Organelles Complex-1, -2 and -3

“…However, a series of recent observations has brought this idea into question. First, deficiencies in each BLOC in human and mouse fibroblasts, which normally express all BLOC subunits but lack LROs, resulted in phenotypes that, albeit mild, revealed that these complexes can play physiological roles in the 'conventional' endosomal-lysosomal system (17)(18)(19)(20)(21)(22). Second, characterization of acidocalcisomes, which are specialized organelles found in a wide variety of non-metazoan eukaryotes, revealed several properties consistent with their classification as LROs (23,24), thus casting doubt on the assumption that LROs would be unique to metazoans.…”