What is Mosaic Down’s syndrome?

In this article, we look at all aspects of Mosaic Down’s syndrome, including its characteristics, how common it is, what can cause it, and whether or not it’s hereditary.

What is Down’s syndrome?

Down’s syndrome is often characterised as having a full or partial extra copy of chromosome 21 and is sometimes referred to as Trisomy 21. It is one of the most common chromosomal conditions at birth and affects approximately 1 in 1,000 births worldwide. This means that each year in the UK, around 700 babies are born with Down’s syndrome.

(Source:  National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention)

The presentations of Down’s syndrome can vary drastically. Individuals with Down’s syndrome have mild to moderate intellectual disability, characteristic facial features, and may have treatable medical conditions, such as congenital heart defects, low muscle tone, and low thyroid levels.

Nevertheless, many individuals with Down’s syndrome lead healthy and active lives. As our understanding of Down’s syndrome continues to grow, the clinical and social management of the condition improves – allowing many individuals to have jobs, relationships and even live independently.

Different types of Down’s syndrome

There are three different types of Down’s syndrome. People living with all three types of Down’s syndrome show similar presentations in terms of intellectual disability and physical features.

Trisomy 21

Trisomy 21 is the most common form of Down’s syndrome and occurs when a person has a full, separate, extra copy of chromosome 21. It accounts for 95% of people with Down’s syndrome.

Mosaicism

In around 2% of people with Down’s syndrome, individuals will have an extra copy of chromosome 21 in some, but not all of the cells in their body. This is known as mosaic Down’s syndrome.

Translocation

The remaining 3% of Down’s syndrome cases are caused by having an extra critical part of, or an extra whole copy of chromosome 21 that has become attached to another chromosome, rather than existing as its own extra chromosome.

What is mosaic Down’s syndrome?

Mosaic Down’s syndrome is a rare form of Down’s syndrome. The clinical term ‘mosaicism’ is used to describe a person who has two genetically different types of cells. This means that a person with mosaic Down’s syndrome has some cells that contain an extra copy of chromosome 21, and some cells with the normal number of chromosomes.

People with mosaic Down’s syndrome show similar presentations to those with the common form of Down’s syndrome, in terms of intellectual disability and physical features. However, in mosaic Down’s syndrome the percentage of cells containing an extra chromosome may have an effect on the number and severity of Down’s syndrome characteristics present.

How common is mosaic Down’s syndrome?

Mosaic Down’s syndrome accounts for only around 2% of Down’s syndrome cases. It is the rarest form of Down’s syndrome and, in the UK, around 14 babies are born with mosaic Down’s syndrome every year.

What causes mosaic Down’s syndrome?

Mosaic Down’s occurs by a random chance event that occurs shortly after the sperm cell joins with an egg. It is therefore not hereditary, and anyone of any age can have a baby with mosaic Down’s syndrome.

Is mosaic Down’s syndrome hereditary?

Mosaic Down’s syndrome occurs by chance and therefore does not run in families. Mosaic down’s syndrome is a random event that occurs shortly after the sperm cell joins with an egg. It is therefore not hereditary, and anyone of any age can have a baby with mosaic Down’s syndrome.

However, it is important to note that the risk of having a child with Down’s syndrome in general increases with maternal age. This means that older women have an increased chance of having a pregnancy that is affected by Down’s syndrome.

Paternal age is also known to influence the incidence of Down’s syndrome, but only in pregnant women aged 35 and over.

What are the characteristics/features of mosaic Down’s syndrome?

People living with the mosaic form of Down’s syndrome show similar presentations to everybody else with Down’s syndrome in terms of intellectual disability and physical features. However, the severity and number of Down’s syndrome characteristics may be reduced, depending on the percentage of cells that contain an extra chromosome.

On the whole, the severity of Down’s syndrome can vary drastically, but all people with Down’s syndrome have some degree of learning disability. There are several medical conditions that are more common in people with Down’s syndrome, including problems with the heart, hearing, vision, bowel problems and an increased likelihood of infections.

Physical characteristics

The physical features of Down’s syndrome include:

• Shorter than average height
• Shorter than average neck
• Flattened facial features
• Eyes that slant upward
• Small ears, hands and feet
• A tongue that tends to stick out of the mouth
• Palmar Crease – a line across the palm of the hand
• Low muscle tone (hypotonia) or loose joints

Developmental and behavioural characteristics

Developmental and behavioural features of Down’s syndrome include:

• Slower than average to meet developmental milestones such as talking and walking
• Mild to moderate intellectual disability
• Problems with verbal memory
• Expressive communication
• Short attention span
• Children with Down’s syndrome tend to be more impulsive and stubborn
• Children with Down’s syndrome tend to have strong social skills

Medical complications

There are several medical conditions that are more common in people with Down’s syndrome.

These can include:

• Around half of children born with Down’s syndrome are affected by a heart condition
• More than half of children born with Down’s syndrome have vision problems
• Up to three quarters of children with Down’s syndrome have some degree of hearing loss
• Children born with Down’s syndrome are more susceptible to Autism
• Adults with Down’s syndrome may develop dementia at a younger age

However, a huge amount of support is available to help people with Down’s syndrome live fulfilling and healthy lives.

What is the life expectancy for people with Down’s syndrome?

The life expectancy for people with Down’s syndrome has increased dramatically as our knowledge and understanding of the condition has grown.

In 1960 the life expectancy of a person with Down’s syndrome was only 10 years.

Today, the life average expectancy of an individual with Down’s syndrome is almost 60 years, and this figure is continuing to climb.

Source: Down Syndrome Life Expectancy Is Higher, But Not For Everyone (nationwidechildrens.org)

What is the treatment for Down’s syndrome?

The severity and presentation of Down’s syndrome varies drastically between individuals with Down’s syndrome. Therefore, there is no single standard treatment for Down’s syndrome. Treatments are based on each individual’s medical, developmental and intellectual needs.

What can impact the probability of having a child with Down’s syndrome?

The risk of having a child with Down’s syndrome increases with maternal age. This means that older women have an increased chance of having a pregnancy that is affected by Down’s syndrome.

For example, a 35-year-old woman has a 1 in 350 risk of having a pregnancy affected by Down’s syndrome, and this risk increases year-on-year, whereby the risk at the age of 45 increases to around 1 in 30:

Other risk factors include being a carrier of a genetic translocation for Down’s syndrome. Men and women can carry this genetic predisposition asymptomatically.

Paternal age is also known to influence the incidence of Down’s syndrome, but only in pregnant women aged 35 and over.

Parents who have has one child with Down’s syndrome are slightly more likely to have  another child with Down’s syndrome.

How can I get Down’s syndrome screening during pregnancy?

A traditional screening test for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome is available at 11-13 weeks of pregnancy. Known as a ‘combined’ test, it comprises a blood test and an ultrasound scan.

The blood test looks at markers found in the blood that can indicate an increased chance of carrying a baby with a chromosomal disorder, while the ultrasound scan is used to determine something known as the ‘nuchal translucency’.

The results of both these tests, along with the age of the pregnant woman, determine the risk of carrying a foetus with Down’s, Edwards’ or Patau’s syndrome.

In the case of a high-risk result, the pregnant woman will be offered the chance to receive non-invasive prenatal genetic testing (NIPT), another screening test, or diagnostic testing.

Diagnostic testing typically involves amniocentesis or chorionic villus sampling (CVS). For amniocentesis, a long, thin needle is inserted into the abdomen to obtain a sample of amniotic fluid for testing. For CVS, a needle is inserted into the womb to obtain a sample of cells from the placenta.

These tests will provide a definite answer as to whether or not the baby has Down’s, Edwards’ or Patau’s syndrome. However, both tests are invasive and carry a small risk of miscarriage.

For this reason, many pregnant women are choosing private NIPT screening, as it is non-invasive, and available to all expectant mothers irrespective of whether they were classified as high risk by the combined test.

For an AlphaBiolabs NIPT, all that’s required is a blood sample, which is collected from the mother’s arm. This makes the test 100% safe for mum and baby, with no risk of miscarriage.

The test uses next-generation sequencing to analyse cell-free foetal DNA (cffDNA) – fragments of the baby’s DNA that naturally circulate in mum’s bloodstream during pregnancy – and detect chromosomal abnormalities, including Down’s syndrome.