Congenital Heart Defects (CDHs) are present at birth and affect the structure of a baby's heart as well as its function. One specific CDH, Atrial Septal Defect (ASD), causes a hole to form in the septum that divides the right and left atria. This hole causes irregular blood flow by allowing blood from the left atrium to flow to the right atrium ( Figure 1 & 2 ).
Figure 1. Normal Heart with regular blood flow (Source)
Three months after conception, the baby's heart becomes visible and continues to grow. As the baby's heart grows, several openings in the septum appear. However, these openings close while the fetus is in the mother's uterus or shortly after birth. When at least one of these openings doesn't close, a hole develops, and ASD is diagnosed. According to the Centers of Disease Control and Prevention in Atlanta, Georgia, about 5,240 American babies are born each year with ASD. This is about 1 in every 769 babies in the US.
Figure 2. Atrial septal defect (Source)
Atrial Septal Defect is detected by various tests done by physicians. During pregnancy, an ASD can be seen through screening tests. Screening tests allow doctors to check for any kind of birth defect. During the first trimester screening, ultrasounds can detect the existence of any congenital heart defect by examining the presence of fluid near the baby's neck. During the second trimester screening, the specific heart defect(s) can be found. Fetal echocardiograms ( Figure 3 ) allow the physician to see a detailed image of the fetus' heart, however, sometimes defects are extremely small to be seen. In such cases, the mother may go through another detailed ultrasound.
Figure 3. Atrial Septal Defect shown in a Fetal Echocardiogram (Source)
If ASD isn't treated at a young age, the individual will experience many heart and lung complications. The hole in the septum leads to an increased blood flow through the lungs, damaging the blood vessels. These injured blood vessels can then cause high blood pressure and heart failure.
In order to prevent the symptoms from going to extreme levels, closing the hole in the heart at a young age is the best solution. For large defects, surgery can be done during childhood to close the heart. In addition, patches made of pericardium can be used to close the heart. For smaller defects, suturing the heart is a good solution.
Every individual is unique; they are born with 23 pairs of chromosomes, each chromosome having a distinct DNA sequence. Segments of DNA are called genes. Genes are what give an individual their traits. Traits such as eye color, hair color, and more make an individual special. Certain genes also make proteins that are structural, defensive, and help the body grow. These proteins are made through a process called protein synthesis. Protein synthesis involves two parts: transcription and translation. Transcription occurs when RNA polymerase attaches to DNA, and makes messenger RNA (mRNA). RNA polymerase does this by sequencing RNA bases (adenine, cytosine, uracil, and guanine) that complement with the DNA nucleotides. The mRNA is then processed by editing out certain sections (introns) of mRNA. The strand is then processed by ribosomes, which are organelles responsible for synthesizing peptides. Within the ribosomes, the mRNA is read three nucleotides (called a codon) at a time by transfer RNA (tRNA), which then creates the designated amino acid. ( Figure 4 ). The process continues until the mRNA reaches the "stop" codon. The amin acids are then joined together by peptide bonds, folded by hydrogen bonds, and finally a protein is formed. So, how is this related to ASD?
Figure 4. Protein Synthesis Source: https://biologydictionary.net/protein-synthesis/
The TLL1 gene (Tolloid Like 1) is implicated in Atrial Septal Heart Defects (refer to Figure 6). This protein-coding gene (Figure 5) is located on chromosome 4 at position q32.3 (Figure 7). It is predominantly expressed in mesodermal tissues, particularly fetal heart tissues. TLL1 is crucial for mammalian heart development and the formation of the septum. Research experiments have demonstrated its significance, such as an experiment in which disrupted TLL1 alleles in mice led to cardiac defects and ultimately death due to heart failure.
Figure 5. TLL1 Protein (Source): https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=TLL1
Figure 6. Expressions from the TLL1 gene Source: https://www.gtexportal.org/home/gene/ENSG00000038295
Mutations are alterations in the DNA sequence, including deletions, substitutions, and insertions. A specific mutation associated with ASD is V238A, wherein thymine (T) is substituted with cytosine (C) at position 713 in the TLL1 gene's sequence (refer to Figure 8). This missense mutation changes the amino acid valine (V) to alanine (A) at position 238. Mutations like V238A can disrupt protein formation and function, impacting various biological processes.
Figure 7. Position of TLL1. Source
Figure 8. Nucleotide and Amino Acid Sequence of TLL1. GTT mutates to GCT. Source
TLL1 encodes "astacin-like, zinc-dependent metalloproteases" within the M12A protein family. These proteases generate procollagen C-propeptides, including proteins like chordin, pro-biglycan, and pro-lysyl oxidase. Chordin plays a pivotal role in early vertebrate embryonic tissue development and organogenesis by interacting with BMP proteins. Pro-biglycan stimulates BMP pathways, while pro-lysyl oxidase contributes to connective tissue maturation.
Figure 9. Protein family of position 238. Source
V238A mutation in the astacin family ( Figure 9 )- Astacin is a family of proteins that consists of proteases that cleave peptides. Certain other proteins in this family are also responsible for morphogenesis, a biological process that properly develops an organ or tissue into shape. One such protein is P13497 or bone morphogenetic protein 1 (BMP1). BMP plays a key role in maintaining the extracellular matrix (EMC). Regulating the ECM is important because it helps with cell growth, cell movement and similar functions. BMPs regulate the ECM by processing precursor proteins, which are inactive proteins that can be activated through certain modifications. Such processing involves modifying the proteins into enzymes or structural proteins. One of the main structural proteins is collagen. Collagen is the primary building block of muscles and connective tissues. So, does this relate to the heart? Yes! The heart, a cardiac muscle, is mainly composed of connective tissue. The chart in Figure 10 summarizes this concept. In regards to the flowchart, a mutation in the TLL1 gene can cause a chain effect, altering the function of proteins such as BMP. A malfunction in the BMP protein can prevent the regulation of structural proteins such as collagen and slow the rate of muscle growth. Without sufficient collagen or cardiac muscle, the heart may not be able to form properly.
Figure 10. TLL1 to Heart flowchart
A lot of research regarding atrial septal defect has been done. Such studies help expand physicians' and the audience's knowledge on ASD. These studies also provide an insight into possible solutions to treat the defect.
In a 2022 researchers tested closing the hole through mini thoracotomy and peripheral cannulation. These small incisions are made with the help of median sternotomy, which is a procedure involving the removal of the sternum (breast bone) to access the heart. In this retrospective cross-sectional study, the results of ASD patients who went through mini thoracotomy and peripheral cannulation were recorded. Researchers compared 55 patients who went through peripheral cannulation, and 55 other patients who went through median sternotomy. In the mini thoracotomy group, there were 55 patients, who were compared with 55 other patients who went through median sternotomy. The 23-28 year old patients were then observed for any symptoms; exertional shortness of breath was observed in both groups. The patients were then admitted to the intensive care unit, with the average length of stay being 2-2.5 days. Patients were then shifted to the regular unit for 4.5-5 days. After further observations, the researchers concluded that ASD closure with mini thoracotomy and peripheral cannulation is safe and cost-effective.
In another approach- the outcomes of mini sternotomy and median sternotomy were compared. The one difference between the two sternotomies is mini sternotomy usually done for cosmetic outcomes. Basically, this procedure involves making small incisions on the skin. One positive outcome of this procedure was it resulted in a shorter hospital stay and minimal bleeding. In this research, a highly experienced surgeon performed a mini sternotomy on 55 patients. Researchers then compared those patients with 55 patients who went through median sternotomy. To conclude, the surgeon avoided direct superior vena cava cannulation through mini sternotomy. This allowed a smaller incision, providing adequate exposure. In addition, this mini sternotomy procedure provided a great cosmetic result.
Figure 11. ASD Transcatheter Repair Procedure Source
But, is there research about closing the hole? If so, are there side effects? A study that started in 2017 explored tricuspid regurgitation (TR), a type of heart disease where the valves' flaps don't close properly, after ASD transcather repair, a procedure to close the hole in the septum. ( Figure 11 ). Of the patients that went through transcather repair, about 113 patients had severe/moderate TR, and 306 patients had mild TR. The patients' condition with severe/moderate TR decreased in about 30 months because of an improvement in the right ventricle. In about 70% of patients, the severity of TR decreased to mild. However, persistent increase in TR severity was independently associated with atrial fibrillation, or abnormal heart beat. Based on some of the clinical outcomes, 7 patients who continued to have severe/moderate TR and 2 patients with mild TR were hospitalized due to heart failure. On the bright side, 90% of severe/moderate TR patients had no cardiovascular events. The researchers concluded that TR in patients decreased during the long-term follow-up period after this procedure. Heart failure symptoms improved, so transcatheter ASD closure alone can be extremely valuable for patients.
Besides physically closing the hole using devices and surgery, genetics may provide a solution for ASD or any form of defect that begins to develop while the baby is in the womb. One of the possible advancements could be early diagnosis of ASD. Additionally, a fluid called amniotic fluid surrounds the embryo. ( Figure 13 ). Amniotic fluid also contains the embryo's DNA. If the DNA sequences are in the amniotic fluid, is there a way to detect the TLL1 gene?
Figure 12. An ultrasound of a fetus Source:https://www.verywellfamily.com/meaning-of-small-gestational-sac-in-early-pregnancy-2371255
A sample of this fluid can be taken by an intramuscular injection or a similar instrument to obtain the DNA sample. In order to prevent the mother from going through a lot of pain, anesthesia can be used in the form of a pill or ointment. Once a sample of the amniotic fluid is taken, the DNA can be amplified through a process called polymerase chain reaction (PCR). Once the DNA is amplified, it can be processed in a DNA sequencing machine—similar to Illumina's Whole Genome Sequencer. Such machines can provide the nucleotide sequence of the DNA sample. So, can this help detect TLL1? Yes, it can. Based on TLL1's nucleotide sequence ( Figure 5 ) in the DNA, geneticists can observe any mutations (such as V238A) through similar machines/databases that detect genes ( Figure 13 ). With the help of AI and advancements in technology, such creations wouldn't take a miracle. If such an instrument is made, geneticists may be able to diagnose other congenital defects. Now that there is potentially a way to find the presence of a mutated gene; how can the mutated gene be prevented from causing a defect?
Figure 13. A diagram of a potential solution to diagnose ASD.
Can gene therapy be utilized to treat ASD? Similar to the concept of designer babies, the TLL1 gene as well as ones with a similar function can be edited. Once DNA is extracted from the amniotic fluid, it can possibly be edited CRISPR/Cas9 to minimize the effects of ASD on the fetus. This approach could present several challenges- including high cost, non specific side effects, risk to the mother and the fetus etc. While challenges exist, the benefits of such research could revolutionize medical interventions.
Atrial Septal Defect is a congenital heart condition characterized by a hole in the septum between atria. The TLL1 gene plays a crucial role in mammalian heart development, and mutations like V238A can disrupt normal protein function. Research into surgical procedures and genetic interventions provides hope for improving the lives of patients with ASD.
I am Kirthi Gopinath, a senior from Mission San Jose High School (Fremont, CA). Based on the knowledge I learned during the Genetics/Genomics course at Elio Academy of Biomedical Sciences, I wrote a report on Atrial Septal Defect—a congenital heart defect where a prominent hole appears in the septum that divides the right and left atria. The various databases I used were one of the key aspects of this course; I learned how to navigate NCBI, Cosmic, UCSC Genome, and more in order to discover the corresponding gene (TLL1) and the associated mutation (V238A). One of the biggest things I was able to develop was my creative approach for a solution to treat ASD. With the information I learned during the course, I expanded my knowledge on gene therapy, and concluded this method could be a possible treatment for ASD. Thank you so much for all the guidance!
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Project done at Elio Academy of Biomedical Sciences1
The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of Elio Academy.