Alport syndrome (AS) is the second most common renal genetic disease and is a phenotypic heterogeneous disease of the glomerulus, cochlea, and fundus membrane, which is mainly caused by mutations in the COL4A3, COL4A4, and COL4A5 type IV collagen genes (Miner, 1999; Kashtan, 1999) that encode the α3, α4, and α5 chains of type IV collagen, respectively. Type IV collagen consists of six different α-chains that interact to form three different heterotrimers (α1α1α2, α3α4α5, and α5α5α6) (Warady et al., 2020). Its structure is essential for the structural integrity and function of the glomerular basement membrane of the kidney, cochlea, and eye (Hudson, 2004; Pöschl et al., 2004),where it forms the basement membrane superstructure together with inline/nestin, laminin, and proteoglycan(Miner, 1999; Rheault, 2012). Consequently, patients with AS are at increased risk of renal failure, sensorineural hearing loss, and eye diseases(Watson et al., 2021).
The COL4A5, COL4A3, and COL4A4 genes have 53, 52, and 48 exons, respectively, and it is estimated that only 10% of all possible pathogenic COL4A5 variants have been reported (Gubler et al., 1981). Although more than 400 mutations have been reported in the COL4A5 gene, of which missense mutations account for 30%, there are no hotspot mutations. The α-5 (IV) chain of collagen is 1,685 amino acids in length and is expressed at 225 locations in the human glomerular basement membrane, retinal pigment layer, main visual cortex, and cochlea.
The disease characteristics of AS mainly include progressive glomerulonephritis, renal failure, sensorineural deafness, and specific ocular abnormalities (lenticular nucleus and macular spots). Abnormal α-chain structures destroy the integrity of the basement membrane, which initially leads to hematuria and later develops into moderate-to-severe proteinuria, progressive renal failure, high-pitched sensorineural hearing impairment, and eye disease. The pathology often manifests as thickness of the basement membrane of the kidney, delamination and rupture of the dense layer, damage to the anterior lens of the eye, cataracts, and macular degeneration (Kashtan, 2021; Jia et.al., 2020). Vision is usually unaffected in most patients with AS(Savige et al., 2015). AS remains a genetic disease that is often missed or misdiagnosed and its pathophysiology is still not fully understood, although some studies have reported that it may be related to chronic inflammation and metabolic dysfunction(Warady et al., 2020). There are three general modes of inheritance: X-linked dominant inheritance (X-linked Alport syndrome; XLAS), autosomal recessive inheritance, and autosomal dominant inheritance. XLAS due to a COL4A5 mutation in the Xq22 region accounts for about 80% of AS cases(Barua et al., 2018). In other cases, autosomal recessive inheritance caused by mutations in COL4A3 or COL4A4 accounts for around 15% of cases and autosomal dominant inheritance accounts for around 5% (Jia et al., 2020; Gao et al., 2020; Kruegel et al., 2013).
The clinical manifestations of XLAS in female patients are very different. Some patients have normal renal function and hearing, whereas others rapidly develop end-stage renal disease and deafness (Rheault, 2012). Around 1% of females with XLAS may also have tumors in the bronchus, esophagus, and vagina(Savige et al., 2016). Studies have shown that X-chromosome inactivation is a risk factor for disease progression in women with XLAS(Rheault, 2016).
Diagnosis of AS is mainly based on family history, clinical features, or pathological manifestations, such as renal biopsy, and finally diagnosed by genetic testing(Savige et al., 2016; Savige et al., 2013). Treatment methods include the use of hearing aids, hemodialysis, and peritoneal dialysis to treat end-stage renal failure and the last step is kidney transplantation. There is no specific and effective treatment method for AS(Gao, 2020), although the goal is to delay disease progression as much as possible and avoid renal failure (Kashtan, 2021). Studies have shown that treatment can temporarily delay the development of the disease. For example, antisense oligonucleotides can be used for exon skipping targeted therapy to improve the pathological manifestations and clinical symptoms of males with XLAS(Yamamura et al., 2020) or early treatment with angiotensin converting enzyme inhibitors(Kashtan, 2021). Individualized treatment for AS patients should be performed according to the identified gene mutation type, sex, and clinical stage of the disease (Kruegel et al., 2013). Next-generation high-throughput sequencing technology can identify up to 95% of pathogenic COL4A mutations, although some mutations caused by deep intron splicing or chimerism are difficult to detect(Savige et al., 2019). The development of sequencing technology has led to the discovery of more and more gene mutations.
In the field of assisted reproduction, preimplantation genetic testing (PGT) is defined as a test sample used to analyze a small amount of genetic material from oocytes (polar bodies) or embryos (cleavage or blastocysts) to determine whether there are genetic abnormalities and genetic aberrations, including preimplantation genetic diagnosis/screening before the revision. PGT is the preferred choice for most patients with infertility and genetic diseases.