Molecular prenatal diagnosis of autosomal recessive childhood spinal muscular atrophies (SMAs)

SMA carrier testing using Real-time PCR as a potential preconception screening tool

Background

SMA is a neuromuscular genetic disorder causing irreversible degeneration of the anterior horn cells of lower motor neurons. According to the age of onset and severity of the condition, it is classified into 5 subtypes. SMA carrier’s frequency worldwide is 1:40–80. We used quantitative real-time PCR to determine the copy number of the disease-determining SMN1 gene by rapid and reliable assays. We studied the SMN1 gene copy number in Egyptian sample of 115 individuals, as well as in 10 SMA families.

Results

Our results showed that 57.4% of the couples with the previous history of an affected family members were carriers. On the individual level, carriers of single SMN1 gene copy rate are much higher than the previously reported frequency rates. The effect of consanguineous marriages appears evident in SMA as an autosomal recessive disorder.

Conclusions

In conclusion, the carrier frequency detected in our cohort was high, which possibly corresponds with the worldwide report of SMA as a leading genetic cause of death among infants. Considering the high rate of consanguinity in developing countries confirms the importance of national SMA carrier screening in Egypt. The qPCR carrier screening test is a rapid-cost effective test that can detect approximately 90% of carriers. A population-based preconception prenatal screening for couples will also help reduce the disease burden.

KRT9 gene mutation as a reliable indicator in the prenatal molecular diagnosis of epidermolytic palmoplantar keratoderma

Epidermolytic palmoplantar keratoderma (EPPK) is the most frequent form of such keratodermas. It is inherited in an autosomal dominant pattern and is clinically characterized by diffuse yellowish thickening of the skin on the palms and soles with erythematous borders during the first weeks or months after birth. EPPK is generally caused by mutations of the KRT9 gene. More than 26 KRT9 gene mutations responsible for EPPK have been described (Human Intermediate Filament Database, www.interfil.org), and many of these variants are located within the highly-conserved coil 1A region of the α-helical rod domain of keratin 9. Unfortunately, there is no satisfactory treatment for EPPK. Thus, prenatal molecular diagnosis or pre-pregnancy diagnosis is crucial and benefits those affected who seek healthy descendants. In the present study, we performed amniotic fluid-DNA-based prenatal testing for three at-risk pregnant EPPK women from three unrelated southern Chinese families who carried the KRT9 missense mutations p.Arg163Trp and p.Arg163Gln, and successfully helped two families to bear normal daughters. We suggest that before the successful application of preimplantation genetic diagnosis (PGD), and noninvasive prenatal diagnosis of EPPK that analyzes fetal cells or cell-free DNA in maternal blood, prenatal genetic diagnosis by amniocentesis or chorionic villus sampling (CVS) offers a quite acceptable option for EPPK couples-at-risk to avoid the birth of affected offspring, especially in low- and middle-income countries.

Molecular prenatal diagnosis of autosomal recessive spinal muscular atrophies using quantification polymerase chain reaction

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by degeneration of alpha motor neurons in the spinal cord, resulting in progressive proximal muscle weakness and paralysis. SMA is the second most common neuromuscular disorder and a common cause of infant disability and mortality. About 95% of patients have a homozygous deletion of exon7 in the survival motor neuron 1 gene. About 50 fetuses from 47 Chinese couples at risk of having an affected child were recruited in this study. The homozygous absence of exon7 of the survival motor neuron 1 gene was detected by both polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and the quantitative PCR method. Short tandem repeat microsatellite markers linked to the survival motor neuron 1 gene were used to do linkage analysis. In conclusion, the quantitative PCR method results were as reliable as the results using the PCR-RFLP method in prenatal diagnosis. The quantitative PCR method can give more information on SMA carrier status that coincides with the result of linkage analysis.