Introduction

Define NEC

Necrotising Enterocolitis (NEC) is a life-threatening disease with a high mortality rate (around 50%) that affects newborn babies, predominantly those who are preterm. It has been observed to affect 5-10% of babies born weighing under 1500 grams.⁴ It’s categorised by inflammation of the intestines (gut), which can cause cellular damage and ultimately necrosis (death of body tissue). This can lead to holes within the intestine, otherwise known as intestinal perforations which cause the contents within it to leak into other parts of the body, and cause sepsis (this is when the immune system has an extreme and destructive response to an infection, causing organ failure and ultimately death if not treated promptly).^1,2 The main cause for the inflammation is thought to be a bacterial infection within the premature intestinal wall, though the cause for this infection has not fully been determined.³ 

Although the exact cause of NEC and the molecular disease pathway is not clear, there are three risk factors that have been suggested, these include: formula feeding, prematurity and bacterial colonisation. It has also been suggested that there may be a genetic predisposition that increases risk for the disease as well.³

Why genetics is important

If the main cause of the disease is thought to be a bacterial infection, it may seem strange to involve genetics, especially since the three risk factors can explain how the infection may have started. However, these clinical factors do not explain the variation in susceptibility or the severity of the disease seen between different infants. This indicates that there may be some inherited risk factors present in some infants that make them predisposed or more susceptible to contract NEC.⁴ 

Despite advances in neonatal medicine, the mortality rate and clinical outcomes from the disease appear indifferent, so perhaps research into the genetic risk of NEC and comparing that with clinical risk will help determine a better prevention or treatment plan.

Purpose of outline

Early observations and the data collected showed that there was a difference in the prevalence of disease among various ethnic groups, which could be indicative of a genetic risk.⁴ However, studies done on twins also had similar results, so one twin would get NEC whereas the other would not, despite being genetically identical, indicating there may be more external factors to look at.⁵

Therefore, the purpose of this outline is to investigate the potential genetic risk factors involved in NEC, to determine if they exist, and if they do, identify what they are and how to prevent the disease. 

External risk factors

Mechanism of the disease

The current model for how NEC works suggests that the condition arises due to the immaturity of the infant’s intestines. This underdevelopment comprises both the immune system and the integrity of the epithelial barrier, leaving the gut more vulnerable to external stressors and microbial invasions. The combination of these factors, as well as an imbalance in the naturally occurring gastrointestinal bacteria, is what is thought to cause the inflammation, which can ultimately lead to sepsis.⁶

Gastrointestinal microbiota and the immune system 

The gastrointestinal microbiota refers to the essential microorganisms that live within the gastrointestinal tract. They are usually highly diverse, with most being different types of bacteria, but can also include fungi, viruses and protozoa. They serve multiple functions, including aiding with digestion as well as behaving as an innate immune system. Essentially, these are the ‘good’ bacteria.⁷ 

In infants, this ecosystem is relatively fragile and still developing specially since this is the stage when the infant is now being exposed to the external environment as well as potentially disease-causing bacteria (pathogens). Therefore, during this age, the body will kick start the immune response, and a key part of this is to be able to recognise which are the ‘good’ self bacteria and which ones are the ‘bad’ pathogenic external bacteria.

However, with premature infants, they usually have an immature epithelial barrier as previously mentioned, and this is more sensitive to detecting bacteria. Due to this sensitivity, there is more movement of bacteria to different locations as well as difficulty in being able to detect and differentiate between the essential ‘good’ bacteria belonging to the natural microbiota, and the pathogens. This can trigger an inflammatory response that is not needed and can thus cause damage.

Furthermore, most premature infants are usually given broad-antibiotics which can disrupt the diversity or the timing of formation of the essential bacterial colonies. This disruption is thought to reduce the body's ability to recognise what is self and what is not.

The microbiota also serves a purpose as an innate immune system, which can prevent colonisation from external pathogens. A reduction in the diversity of these colonies leaves the intestines at a higher risk of infection and weakens this innate immunity.⁶

The effects of formula feeding

Another potential cause for inflammation is formula feeding. During the newborn stage, the immune system is still developing. In order to develop properly, breast milk contains a lot of key components that help establish a fully functioning immune system. This includes an antimicrobial protein known as secretory IgA (SigA). IgA remains at high levels in the breast throughout the breastfeeding phase and is vital for building up and protecting the layer of ‘good’ bacteria. This is especially important as infants do not produce their own IgA until they are 2 weeks old, and even then, they do not produce the amount an adult produces until around ages 2-6 years. So, breast milk is important for establishing this protective layer in the intestines.

However, in preterm babies, they are often formula-fed, meaning a lot of NEC cases are associated with this. Furthermore, studies showed that in NEC cases where they are formula fed, there is a reduced presence of beneficial bacteria such as bifidobacteria, which is important for digestion and immune protection. Conversely, these infants tend to have elevated levels of bacteria such as proteobacteria. This group of bacteria is often associated with dysbiosis and microbial imbalance when present in high amounts.⁶

Overall, this suggests formula feeding may increase the risk of NEC.

Genetic susceptibility

Twin studies

In order to evaluate the risk factors, studies were conducted on monochorionic twins. These twins were genetically identical and thus had the same maternal factors and the same care from obstetrics. Therefore, a variation between the infants may rule out a genetic risk. The finding ultimately showed that in pairs that were affected, the twin with NEC had a lower birth weight or lower gestational age when compared to their unaffected sibling. The median birth weight was also lower in affected pairs compared with the unaffected pairs. This suggested that susceptibility to NEC relied more on birth weight and prematurity rather than genetics.⁵

TLR overexpression

TLR, or Toll-like receptors, are receptors in the body that act as surveillance and detection tools for harmful pathogens. They can identify a pathogen by its unique shape and activate an immune response to get rid of it before it can cause any damage. By doing an analysis on intestinal samples of NEC-affected infants, it was found that TLR was overexpressed. 

Overexpression means that it is triggering an unneeded immune response, which can cause inflammation. Further studies done on rodents also identified the link between this receptor and the disorder, therefore supporting the idea.

Interestingly, genetic studies done on 270 infants actually showed no association, so it may be that further investigation involving a larger sample size may need to be conducted.

The SIGIRR gene

The SIGIRR gene is thought to stop overexpression of TLR, as well as reduce any unnecessary inflammation. In a genome sequencing study done on preterm infants with 17 out of the 37 having NEC, it was found that the infants with NEC had brand new variants of the SIGIRR gene compared to their counterparts. Further studies done on epithelial cells showed that these variants increase inflammation and stop the normal function of the gene, as well as causing dysfunctional activation of TLR. Mouse studies, using mice without the functional SIGIRR gene, also showed increased intestinal inflammation, which overall suggests that the SIGIRR gene is important for proper immune response and intestinal function. Therefore, loss of the functional gene or having a dangerous variant may make some individuals more susceptible to getting NEC.

Ethnic disparities

Epidemiological studies conducted showed that certain countries had higher occurrences of NEC than others, for example, the US, UK and Ireland showed a higher incidence of NEC, with more African-American children being affected than other ethnicities, whereas Japan, Austria and Switzerland showed a lower number of cases. However, not enough research has been conducted to determine whether there is a genetic disparity or if this is the result of differences within the neonatal care strategies in each country.⁴

Summary

Overall, it seems that some candidate genes have been suggested and identified, but not enough research has been done to specifically identify a key gene that causes a predisposition to the disorder. Most current evidence seems to suggest that external risk factors such as birth weight, formula feeding and prematurity are more important and easier to identify and prevent. Thus, further research would need to be done to determine an exact genetic link.