Identification of Frameshift Variants in POLH Gene Causing Xeroderma Pigmentosum in Two Consanguineous Pakistani Families.

Xeroderma pigmentosum (XP) is a rare autosomal recessive genetic disorder characterized by severe sensitivity of skin to sunlight and an increased risk of skin cancer. XP variant (XPV), a milder subtype, is caused by variants in the POLH gene. POLH encodes an error-prone DNA-polymerase eta (pol eta) which performs translesion synthesis past ultraviolet photoproducts. The current study documents the clinical and genetic investigations of two large consanguineous Pakistani families affected with XPV. In family 1, whole exome sequencing (WES) revealed a novel frameshift variant, c.1723dupG (p.(Val575Glyfs*4)), of POLH, which is predicted to cause frameshift and premature truncation of the encoded enzyme. Indeed, our ex vivo studies in HEK293T cells confirmed the truncation of the encoded protein due to the c.1723dupG variant. In family 2, Sanger sequencing of POLH exons, revealed a recurrent nonsense variant, c.437dupA (p.Tyr146*). POLH forms a hetero-tetrameric POLZ complex with REV3L, REV7, POLD2 and POLD3. Next, we performed in silico analysis of POLH and other POLZ complex genes expression in publicly available single cell mRNAseq datasets from adult human healthy and aging skin. We found overlapping expression of POLH, REV3L and POLD2 in multiple cell types including differentiated and undifferentiated keratinocytes, pericytes and melanocytes in healthy skin. However, in aging human skin, POLH expression is reduced in compare to its POLZ complex partners. Insights from our study will facilitate counseling regarding the molecular and phenotypic landscape of POLH-related XPV.

着色性干皮病（XP）是一种罕见的常染色体隐性遗传性疾病，其特征为皮肤对阳光极度敏感，且患皮肤癌的风险增加。XP变异型（XPV）是一种症状较轻的亚型，由POLH基因突变所致。POLH基因编码易错的DNA聚合酶η（pol η），该酶可跨越紫外线光产物进行跨损伤合成。本研究对两个罹患XPV的巴基斯坦近亲大家族进行了临床和遗传学调查。在家族1中，全外显子测序（WES）发现了POLH基因的一种新型移码变异c.1723dupG（p.(Val575Glyfs*4)），预计该变异会导致编码的酶发生移码和提前截断。事实上，我们在HEK293T细胞中的体外研究证实，由于c.1723dupG变异，编码的蛋白质发生了截断。在家族2中，对POLH外显子进行桑格测序，发现了一种反复出现的无义变异c.437dupA（p.Tyr146*）。POLH与REV3L、REV7、POLD2和POLD3形成异源四聚体POLZ复合物。接下来，我们对来自成年人类健康皮肤和衰老皮肤的公开单细胞mRNA测序数据集进行了POLH及其他POLZ复合物基因表达的计算机分析。我们发现，在健康皮肤的多种细胞类型中，包括分化和未分化的角质形成细胞、周细胞和黑素细胞，POLH、REV3L和POLD2存在重叠表达。然而，在人类衰老皮肤中，与POLZ复合物的其他成员相比，POLH的表达有所降低。我们的研究成果将有助于就POLH相关XPV的分子和表型情况提供遗传咨询。