SMA, thalassemia and hereditary deafness are serious, recessive genetic disorders resulting in numerous health challenges. Carrier screening and prenatal diagnosis are the most effective and direct ways to prevent SMA, severe thalassemia and deafness in children. Therefore, there is an urgent need to understand the prevalence of these three genetic diseases in the population and provide effective screening methods. We tried combined screening of SMA, thalassemia and hereditary deafness, which cost only 50% of the total cost of screening for the three diseases independently. We used oral saliva samples, which are convenient for paternal partners of positive patients to collect and can be sent to the laboratory by mail. We recruited a total of 5310 eligible pregnant women from 7 gestation weeks to 20 gestation weeks. After a detailed consultation, only 2873 pregnant women were willing to undergo combined screening, yielding a screening rate of 54.1%. The main reason for screening refusal was their perception that they were not at risk for the disease since they did not have a family member with any of the three diseases, indicating that education about genetic diseases is very important[10].
This is the first attempt at a combined screening test for SMA, thalassemia, and hereditary deafness in China. Among the 2873 participants screened, 419 were found to carry at least one pathogenic variant associated with the three diseases, with a total positive rate of 14.6%, which is 6.1 times higher than that obtained with SMA screening alone (14.6%/2.4%), 1.7 times higher than thalassemia screening alone (14.6%/8.5%), and 3.3 times higher than hereditary deafness screening alone (14.6%/4.4%), indicating that the positive rate of screening is greatly improved by combined screening. Among the screened pregnant women, 19 carried two pathogenic variants associated with two diseases, and two pregnant women carried all three genetic disease pathogenic alleles. Our experiment shows that the carrier rate of these three hereditary diseases is very high in pregnant women in this region.
There were 70 (70/2873, 2.44%) SMA carriers among the 2873 pregnant women, consistent with the previously reported carrier rate[11,12]. Among them, 56 cases were carriers of exon 7 and exon 8 heterozygous deletions. A total of 67 husbands were recalled for SMA carrier screening, and 3 husbands were found to be carriers. Prenatal diagnosis was conducted in these 3 cases, and 1 fetus had exon 7 and exon 8 homozygous deletions; thus, this fetus was diagnosed with SMA. The pregnancy was terminated after genetic counseling. Prenatal diagnosis and genetic counseling will be available for future pregnancies. We suggest performing prenatal diagnosis by villus puncture in the first trimester.
Thalassemia is a significant public health concern as the carrier rate and disease numbers are increasing worldwide[13]. Presently, carrier screening for thalassemia remains insufficient in most areas of China, except for the Guangdong and Guangxi regions[14]. There is an urgent need to understand the prevalence of thalassemia carriers in the population. In our study, there were 245 thalassemia carriers among the 2873 pregnant women, with a rate of 8.4%. This high rate is partly because some pregnant women with mild anemia are more willing to accept carrier screening. Nevertheless, the carrier rate of thalassemia in pregnant women in the Wenzhou area is high, and carrier screening should be recommended for pregnant women in this area.
In this study, 82 (82/2873, 2.85%) pregnant women and 5 (5/187, 2.7%) parental partner carriers were heterozygous for the Southeast Asian type deletion (-SEA). This finding shows that the -SEA deletion is the most common type of thalassemia in this area. Such couples have a 25% chance of giving birth to fetuses with Hb Bart’s syndrome. Fourteen partners (14/187, 7.4%) were found to be either α- or/and β-thalassemia carriers and had prenatal diagnoses. We detected 1 fetus with -SEA/αQSα and 1 fetus with homozygous -SEA/-SEA, resulting in Hb Bart’s disease. We prevented the two newborn births with α-thalassemia major syndrome by prenatal diagnosis; high healthcare costs would have occurred had these pregnancies continued.
Genetic causes contribute to more than half of congenital hearing loss cases[15]. More than 70 genes have been associated with nonsyndromic hearing loss (NSHL)[16]. We screened for 21 common pathogenic variants in four common genes. In total, 127 (4.4%, 127/2873) participants were found to carry at least one allele of the 21 variants, with a carrier rate of 4.4%, consistent with other studies[17]. It has been reported that approximately 50% of autosomal recessive hearing loss cases harbor mutations in GJB2[18]. In this study, we found 53 carriers of the 235delC variant in the GJB2 gene (41.7%, 53/127), which was the most frequent hearing loss-associated allele detected in this population. Approximately 80% of large vestibular aqueduct syndrome patients have been found to harbor SLC26A4 c.919-2A>G mutations[19]. However, in our study, IVS7-2(A>G) in the SLC26A4 gene was the highest mutation site, with a carrying rate of 0.91 (26/127). m.1555A>G and m.1494C>T are the main pathogenic mutations of the 12S rRNA gene. These rare variants have been shown to have a direct relationship with aminoglycoside-induced hearing loss[20]. One study of the physical screening of newborns found that almost all carriers of 12S rRNA mutations passed the physical test[21]. In our study, 11 pregnant women were m.1555A>G carriers. If such infants are given aminoglycoside antibiotics after birth, irreversible hearing loss will occur[22]. Our study helped these 11 families' children avoid drug-induced deafness from aminoglycoside antibiotics. In our study, m.1494C>T was not detected.
The variants of c.538C > T and c.547G > A sites in GJB3 lead to autosomal dominant nonsyndromic deafness, mainly resulting in high-frequency hearing loss[23], which is believed to cause delayed deafness[24]. In our study, 16 individuals were GJB3 carriers, and spouses did not need to be screened for this mutation.
Finally, 96 hereditary deafness carriers’ paternal partners were recalled, and we used the Sanger sequencing method to screen their corresponding genotypes. Of these, 5 partners (5/96, 5.2%) were hereditary deafness gene carriers. One fetus was confirmed to have homozygous c. 235 del C deletion, and the pregnancy was terminated.
The main benefit of carrier screening is to reduce the burden associated with giving birth to an affected child. Our study has indicated that the carrier rate of these three genetic diseases is high in pregnant women in the Wenzhou area. Our combined carrier screening can reduce screening costs, improve the positive rate of screening, and reduce the burden associated with giving birth to an affected child. Such screening can allow couples to make informed reproductive choices and is a good way to prevent the birth of children with these three genetic diseases, protect patients’ health and improve the population's health. Our workflow and experience of combined carrier screening for three diseases and prenatal diagnosis may facilitate the popularization of combined carrier screening for SMA, thalassemia and deafness in Zhejiang Province and even mainland China.