USMLE® Step 1 style questions USMLE
An 18-month-old girl is brought to the office by her parent for evaluation of “odd behaviors.” The patient has met all developmental and motor milestones over the past year, including walking and playing with toys. However, over the past four months, she has had difficulty walking and sitting up without support. In addition, while the patient used to babble and say single words, she has not spoken much for the past three months. Weight and height are at the 50th percentile. The patient’s head circumference has decreased from the 40th percentile to the 25th percentile in the past four months. Temperature is 37.0°C (98.6°F), blood pressure is 100/67 mmHg, and pulse is 103/min. On examination, the patient is hypotonic and needs to be held by her parent to sit upright. She wrings her hands together repeatedly. An underlying genetic disorder is suspected. Which of the following best describes the normal function of the protein produced by this gene?
Content Reviewers:Rishi Desai, MD, MPH, Yifan Xiao, MD
Rett syndrome is a rare neurological disorder that mostly in young girls and causes severe impairments in their ability to talk, walk, eat, and even breathe.
A classic feature is that children often make repetitive hand movements - like flapping their hands or clasping their hands together tightly.
The disease was named after Dr. Andreas Rett, a pediatrician who discovered the syndrome in the 1960s.
To begin, the brain is composed of billions of interconnected neurons, each of which is made of up dendrites, that receive signals from other neurons, the soma, or cell body, which has all of the neuron’s main organelles, and the axon which sends signals to other neurons.
In Rett syndrome there’s an X-linked autosomal dominant mutation of the Methyl-CpG-binding protein 2 gene, or MECP2 gene, which codes for MeCP2 protein.
The mutation in the MECP2 gene usually occurs sporadically, meaning that it’s usually not inherited from a parent.
It’s thought that the MeCP2 protein helps to silence or turn off other genes.
Males have only one X chromosome, so if there’s a mutation in the MECP2 gene, then they cannot make functional MeCP2 protein, and that might be why males with the mutation typically die in utero or shortly after birth.
Very rarely a male with Klinefelter syndrome, where there’s an XXY set of chromosomes, might develop Rett syndrome.
Females have 2 X chromosomes, however, so one mutated MECP2 gene can be compensated for by a normal MeCP2 gene on the other X chromosome.
The genes that are regulated by MeCP2 proteins are particularly important for brain development, specifically to help establish neuronal connections.
So in young girls, when they’re first born, the low levels of MeCP2 proteins are sufficient for normal brain development. But as the brain grows more complex, the level of MeCP2 becomes insufficient and the brain fails to develop normally.
In addition to neurologic effects, Rett syndrome is also associated with prolonged QT syndrome, which is when it takes longer than usual for the heart to repolarize.
Rett syndrome can be thought of in four main stages.
The first stage is the early onset stage and occurs between ages 6 to 18 months.
Before 6 months of age, children with Rett syndrome develop normally, but starting around 6 months, children start to lose interest in play and no longer maintain eye contact.
The second stage is the rapid deterioration stage and occurs between ages 1 to 4 years. In this stage there is a dramatic regressions in speech and motor skills.
Rett syndrome, originally termed cerebroatrophic hyperammonemia, is a rare genetic neurological disorder of the gray matter of the brain that almost exclusively affects females, though it has also been found in male patients. It is caused by a mutation of the MECP2 gene and is inherited in an X-linked dominant pattern.
The symptoms of Rett syndrome typically appear between 6 and 18 months of age and include slowed growth, loss of language and social skills, and the development of repetitive hand movements. Other symptoms can include seizures, scoliosis, and breathing abnormalities.
The diagnosis of Rett syndrome is based on a genetic test looking for the MECP2 mutation, and the treatment focuses on managing symptoms with selective serotonin reuptake inhibitors (SSRIs) to help manage behavioral issues. Additionally, a multidisciplinary team that includes occupational, speech, and physical therapists can help optimize a child's life.