Supplementary Material for: Novel variants of HPS6 cause suspected ocular albinism: A report of two cases and the profile of HPS6 variants

Introduction: Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disease characterized by ocular albinism (OA) or oculocutaneous albinism (OCA), platelet dysfunction and other symptoms. This study aimed to analyze the molecular defect in two Chinese families with suspected OA, as well as to investigate the profile of HPS6 variants and their genotype-phenotype correlations. Methods: Seven members from two families were recruited and underwent clinical ophthalmologic examinations. The genomic DNA was extracted from peripheral blood leukocytes. Whole-exome sequencing was performed on the proband of Family JX. The single coding exon of HPS6 was directly Sanger sequenced based on PCR amplification in all available family members. An additional 46 probands from families or sporadic cases with the pathogenic variants of HPS6 reported in the literatures were reviewed. Results: We identified two different compound heterozygous truncating variants of HPS6 in probands with suspected OA from two independent families. The proband of Family JX had c.1674dup and c.503-504del variants, and the other proband from Family CZ had a nonsense variant of c.1114C>T and a frameshift variant of c.1556del. Among them, c.1674dup and c.1556del variants in HPS6 have not been reported previously. Therefore, our patients were diagnosed as HPS6 disease by molecular diagnostics. In the retrospective cohort of HPS6 patients, we delineated the profile of HPS6 variants and revealed a significant overlap between CpG islands and variants of HPS6, suggesting a potential link between DNA methylation and HPS6 variants. We also observed a spatial aggregation of the variants in 3D structure of HPS6 protein, implying the possible functional significance of these structural regions. In addition, we did not find any significant genotype-phenotype correlation of HPS6, and neither did we observe a correlation between the truncation length of the HPS6 protein and the phenotype of HPS6 disease. Conclusion: Our research expands the spectrum of HPS6 variants, providing a comprehensive delineation of their profile and systematically investigating genotype-phenotype correlations in HPS6. These findings could offer potentially valuable clues for investigating the molecular mechanism underlying HPS6 pathogenesis, as well as aiding the clinical diagnosis of HPS6 patients and improving disease prognosis.

Introduction: 赫曼斯基-普德拉克综合征（Hermansky-Pudlak syndrome, HPS）是一种罕见的常染色体隐性遗传病（autosomal recessive disease），以眼白化病（ocular albinism, OA）或眼皮肤白化病（oculocutaneous albinism, OCA）、血小板功能异常（platelet dysfunction）及其他症状为特征。本研究旨在分析两个疑似眼白化病的中国家庭的分子缺陷，并探究HPS6变异的频谱及其基因型-表型关联（genotype-phenotype correlations）。
Methods: 本研究招募了两个家庭共7名成员，均接受了眼科临床检查。从外周血白细胞中提取基因组DNA。对JX家系的先证者（proband）进行全外显子组测序（whole-exome sequencing）。对所有可获取的家系成员，基于聚合酶链式反应（Polymerase Chain Reaction, PCR）扩增，直接对HPS6的单个编码外显子进行桑格测序（Sanger sequencing）。此外，我们对文献中报道的46例携带HPS6致病变异的家系先证者或散发病例进行了回顾分析。
Results: 我们在两个独立家系的疑似眼白化病先证者中，分别鉴定出两种不同的HPS6复合杂合截短变异（compound heterozygous truncating variants）。JX家系的先证者携带c.1674dup和c.503-504del变异，而CZ家系的另一先证者则携带c.1114C>T无义变异（nonsense variant）与c.1556del移码变异（frameshift variant）。其中，HPS6的c.1674dup和c.1556del变异此前未见报道。因此，通过分子诊断，我们将上述患者确诊为HPS6相关疾病。在HPS6患者的回顾性队列（retrospective cohort）中，我们描绘了HPS6变异的频谱，并发现CpG岛（CpG island）与HPS6变异存在显著重叠，提示DNA甲基化与HPS6变异之间可能存在关联。我们还观察到变异在HPS6蛋白三维结构（3D structure）中呈空间聚集现象，暗示这些结构区域可能具有重要功能意义。此外，我们未发现HPS6存在显著的基因型-表型关联，也未观察到HPS6蛋白截短长度与HPS6疾病表型之间存在相关性。
Conclusion: 本研究拓展了HPS6变异的频谱，全面描绘了其变异特征，并系统探究了HPS6的基因型-表型关联。这些发现可为探究HPS6发病的分子机制提供有价值的线索，同时有助于HPS6患者的临床诊断与疾病预后改善。