PERSPECTIVES FOR TREATMENT OF CHROMOSOMAL DISORDERS
Chromosomal disorders are conditions where, as a result of deletions or
duplications of some chromosomal segments, the organism experiences the excess or absence of a significant number of genes. Manifestations of chromosomal disorders include both morphological defects caused by the abnormal action of genes during embryonal development and functional abnormalities (seizures, hypotonia, aggression, hyperactivity, sleep disturbances, etc.) caused by the constant abnormal action of genes. When we use the term “treatment of chromosomal disorders” we talk about attempts to alleviate functional abnormalities and give patients a better chance to live independently. It is obvious, however, that morphological defects which occur during embryonal development cannot be restored by any drugs.
The existing attempts for treatment of patients with chromosomal
abnormalities may be subdivided into several groups. The potentially most fundamental methods have a goal to restore a normal karyotype or inactivate the action of excessive genes. One strategy involves inactivation of one of the chromosomes 21 in a trisomic patient with Down syndrome. One of the human genes – gene XIST –
inactivates one of X-chromosomes in females. The XIST gene prevents
transcription of the genes on the X-chromosome. If this gene is inserted into one of chromosomes 21 in a patient with Down syndrome, this chromosome will became inactivated, restoring the normal “disomic” condition. Currently the experiments with inactivation of excessive chromosomes are in the “embryonal” stage; if these tests provide good results in laboratory animals, this method will be tested with clinical trials. Although currently XIST-strategy is studied only for
cells with trisomy 21, the same method may be promising for other trisomies as well.
Some types of chromosomal pathology (ring chromosomes, additional
markers, dicentric chromosomes) are relatively unstable. Most patients with these abnormalities have both a clone with an abnormal chromosome and a normal clone. Generation of pluripotent stem cells from the skin cells of a patient with ring chromosome 17 has shown that reprogrammed cells lost the abnormal chromosome 17 and duplicated the wild-cell normal homologue. If this is true for other ring chromosomes and other forms of “unstable” pathology, it will provide an
opportunity to use cellular reprogramming as a way of therapy for certain chromosomal disorders.
A strategy of reprogramming as well as XIST-strategy, however, will (or
may) cause uniparental disomy – a condition when both homologues chromosomes are identical (i.e. two copies of the same maternal or paternal chromosome). If this chromosome carries an autosomal-recessive gene, the patient (or reprogrammed cells of the patient) may become homozygous for this gene.
Other methods for treatment of chromosomal pathology seem to be not so fundamental by nature but more achievable. Down syndrome for instance is a condition where several attempts for treatment of patients with this pathology have been made.
It is well known that the size of the cerebellum in persons with trisomy 21 is significantly less than in persons with a normal karyotype. As a result there is a considerable impairment of cerebellar function in patients with Down syndrome. In the experiments with a specific line of mice Ts65Dn (widely used as an animal model of Down syndrome) the newborn animals who received a stimulation by “Sonic hedgehog” – a growth factor involved in different aspects of development -show both an increase of the size of cerebellum and an improvement of its function. Mice “treated” by “Sonic hedgehog” show an ability to learn their way around a maze – an indicator of function of learning and memory. The idea of early intervention seems very plausible, but there is still a long way to go between experiments with rodents and the clinical usage of “Sonic hedgehog” (or other growth factors) in treatment of Down syndrome.
Experiments with the mice Ts65Dn show that these animals demonstrated excessive inhibitory brain activity. Release of this inhibitory effect may improve cognitive function. One way to reduce this inhibitory effect is to block the activity of GABA – the primary inhibitory neurotransmitter. There are experiments where
Ts65Dn mice were chronically treated with a negative allosteric modulator of GABA RO4938581.
It has been shown that this treatment improves the functional and neuromorphological deficit in a mouse model of Down syndrome. Currently there are some clinical trials ongoing to test this kind of treatment in adult patients with Down syndrome.
An examination of the brains in Down syndrome patients has shown a remarkable similarity between these brains and those in persons with
Alzheimer syndrome. Most Down syndrome patients develop Alzheimer-like plaques and dementia in their 40s. Persons with Down syndrome have high levels of myoinositol – a chemical related to cognitive impairment and involved in formation of plaques in the brain. There is a trial to treat patients with Down syndrome by scyllo-inositol (ELND005) – a drug originally synthesized to break up amyloid plaques in the brains of patients with Alzheimer’s syndrome. The current trial hopes to demonstrate the efficacy of this method for treatment of Down syndrome patients.
There are some publications describing possibility of “pathogenetic”
treatment in other conditions. For example many manifestations of Phelan-McDermid syndrome (deletion of 22q13.3) are caused by the loss of SHANK3 gene, which resides in this area. Neurons of Phelan-McDermid syndrome patients have reduced SHANK3 expression and defects in excitatory, but not inhibitory, synaptic transmission. Treatment of the neurons with insulin-like growth factor I (IGF1) corrects the excitatory function of these cells and shows the pathway to
correct intellectual disability and speech impairment in patients with this disorder.
The more common and more standard ways for treating chromosomal
disorders are related to the replacement of products which are not synthesized in a patient or synthesized at an unusually low level. It is similar to the usage of insulin in patients with diabetes. For example, patients with Klinefelter syndrome have very low levels of testosterone. The administration of testosterone in adolescents with Klinefelter syndrome shows good clinical efficacy in 95% of patients.
Patients with distal deletions of the long arm of chromosome 15 usually reveal significant growth delay, because this deletion involves the gene responsible for synthesis of the growth hormone. The administration of growth hormone significantly improves the growth in these patients. Treatment by growth hormone may be beneficial also for other patients with other chromosomal pathologies experiencing growth delay.
Common drugs for treatment of attention deficit hyperactivity disorder or seizures may be used also for the treatment of these manifestations in patients with chromosomal pathology, although clinical results may vary even in persons with the same chromosomal imbalance.
Dr. Iosif Lurie
CDO Consulting Medical Advisor