Abstract：
Propionic acidemia (PA) is an autosomal recessive metabolic disorder caused by variants in PCCA or PCCB, both sub-units of the propionyl-CoA carboxylase (PCC) enzyme. PCC is required for the catabolism of certain amino acids and odd-chain fatty acids. In its absence, the accumulated toxic metabolites cause metabolic acidosis, neurologic symptoms, multi-organ dysfunction and possible death. The clinical presentation of PA is highly variable, with typical onset in the neonatal or early infantile period. We encountered two families, whose children were diagnosed with PA. Exome sequencing (ES) failed to identify a pathogenic variant, and we proceeded with genome sequencing (GS), demonstrating homozygosity to a deep intronic PCCB variant. RNA analysis established that this variant creates a pseudoexon with a premature stop codon. The parents are variant carriers, though three of them display pseudo-homozygosity due to a common large benign intronic deletion on the second allele. The parental presumed homozygosity merits special attention, as it masked the causative variant at first, which was resolved only by RNA studies. Arriving at a rapid diagnosis, whether biochemical or genetic, can be crucial in directing lifesaving care, concluding the diagnostic odyssey, and allowing the family prenatal testing in subsequent pregnancies. This study demonstrates the power of integrative genetic studies in reaching a diagnosis, utilizing GS and RNA analysis to overcome ES limitations and define pathogenicity. Importantly, it highlights that intronic deletions should be taken into consideration when analyzing genomic data, so that pseudo-homozygosity would not be misinterpreted as true homozygosity, and pathogenic variants will not be mislabeled as benign.
摘要：
丙酸血症（PA）是由PCCA或PCCB变异引起的常染色体隐性代谢紊乱，丙酰辅酶A羧化酶(PCC)的两个亚基。某些氨基酸和奇数链脂肪酸的分解代谢需要PCC。在缺席的情况下,积累的有毒代谢物引起代谢性酸中毒,神经症状,多器官功能障碍和可能的死亡。PA的临床表现是高度可变的，典型的发病在新生儿或婴儿早期。我们遇到了两个家庭，他们的孩子被诊断患有PA。外显子组测序（ES）未能确定致病变异，我们进行了基因组测序（GS），证明了深内含子PCCB变体的纯合性。RNA分析确定该变体产生具有过早终止密码子的假外显子。父母是变异携带者，尽管其中三个由于第二个等位基因上常见的大良性内含子缺失而显示假纯合性。父母假定的纯合性值得特别注意，因为它一开始掩盖了致病变异，这只能通过RNA研究解决。到达快速诊断，无论是生化还是遗传，在指导救生护理方面至关重要，结束诊断冒险，并允许在随后的怀孕中进行家庭产前检查。这项研究证明了综合遗传研究在达到诊断的能力，利用GS和RNA分析克服ES限制并定义致病性。重要的是,它强调在分析基因组数据时应该考虑内含子缺失，这样假纯合性就不会被误解为真纯合性，和致病变异不会被错误标记为良性。
