What is Phelan-McDermid syndrome?
Phelan-McDermid syndrome is a rare chromosomal condition where individuals are missing a portion of their 22nd chromosome. People Phean-McDermid may have low muscle tone, normal to fast growth, absent or severely delayed speech, intellectual disability, and some characteristic facial features. Some individuals may fall on the autism spectrum. Not everyone with Phelan-McDermid syndrome is affected in the the same way. Contact the Phelan-McDermid Syndrome Foundation to learn more about this condition.
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Are there other names for Phelan-McDermid syndrome?
Other names for Phelan-McDermid syndrome include:
- Chromosome 22q13.3 Deletion Syndrome
- 22q13.3 Deletion Syndrome
- Telomeric 22q13 Monosomy Syndrome
- Deletion 22q13 Syndrome
If you are speaking with your healthcare provider and they use one of these terms, ask for clarification if you are unsure whether it is the same as Phelan-McDermid syndrome.
What are the usual abbreviations for Phelan-McDermid syndrome?
The usual abbreviations for Phelan-McDermid syndrome are PMS or PHMDS. If you are reading an article and you see these abbreviations, be sure to check earlier in the document to confirm that the author is describing Phelan-McDermid syndrome.
What do "q" and "13.3" stand for in chromosome 22q13.3 deletion syndrome?
Our genetic information or DNA is found in every cell of our body. The DNA is organized onto tiny structures called chromosomes. Every chromosome has two parts called arms. The short arm of a chromosome is referred to as the "p" arm. The long arm on each chromosome is called the "q" arm. The "p" and "q" arms are separated by the centromere. "13.3" is the specific location on the long arm ("q" arm) of chromosome 22 where the missing piece (deletion) occurred. If you have a family member with Phelan-McDermid syndrome and you want to learn more about this condition, contact a genetic counselor in your area for an appointment to discuss your family member's test results.
What is the difference between a terminal chromosome 22q13.3 deletion and an interstitial chromosome 22q13.3 deletion?
Our genetic information or DNA is found in every cell of our body. The DNA is organized onto tiny structures called chromosomes. Every chromosome has two parts called arms. The short arm is referred to as the "p" arm and the long arm is referred to as the "q" arm. If part of a chromosome is missing it is called a deletion. There is a difference between a terminal and interstitial chromosome 22q13.3 deletion. A terminal deletion is a deletion that goes all the way to one end of the long arm ("q" arm) of chromosome 22. An interstitial deletion is a deletion within the chromosome 22, which does not involve the end of that chromosome. If you have a family member with Phelan-McDermid syndrome and you want to learn more about this condition, contact a genetic counselor in your area for an appointment to discuss your family member's test results.
Is Phelan-McDermid syndrome more common in males or females?
Phelan-McDermid syndrome is seen equally in males and females. If you are interested in meeting someone who has Phelan-McDermid syndrome contact the Phelan-McDermid Syndrome Foundation to find out how you can attend their family conference.
Can people with Phelan-McDermid syndrome have children?
No individuals with Phelan-McDermid syndrome have been reported to have children. However, it is not thought that people with Phelan-McDermid syndrome are infertile. Speak with your doctor to learn more about how to manage reproductive questions for people with Phelan-McDermid syndrome.
Are any other genetic conditions similar to Phelan-McDermid syndrome?
There are other genetic conditions that are similar to Phelan-McDermid syndrome. Some of these include:
- Ring chromosome 22. In this condition, one copy of chromosome 22 breaks in two places. The ends of the chromosome are lost, and the broken ends fuse together. If the break on the long arm is at chromosome 22q13.3, people with ring chromosome 22 will have similar features to people with Phelan-McDermid syndrome.
- Velocardiofacial syndrome (VCFS/DiGeorge syndrome/22q11.2 deletion syndrome). Overlapping features include low muscle tone, characteristic facial features, speech delay, developmental delay, and kidney problems.
- Williams syndrome. Overlapping features in the newborn period include low muscle tone, global developmental delay, and puffy eyelids.
- "Atypical" Angelman syndrome. Overlapping features include low muscle tone at birth, global developmental delay, absent speech, and an unsteady walk.
- Prader-Willi syndrome. Overlapping features include low muscle tone and feeding issues at birth.
- Smith-Magenis syndrome. Overlapping features include facial features, low muscle tone, developmental delay, intellectual disability, and decreased sensitivity to pain.
- Trichorhinophalangeal syndrome (TRPS). Overlapping features include low muscle tone, intellectual disability, characteristic facial features, and thin toenails.
If you have a family member with features of Phelan-McDermid syndrome, talk to your doctor about the best way to be tested for these conditions.
If a child is found to have Phelan-McDermid syndrome and parents' genetic testing is normal, what is the chance for parents to have another child with Phelan-McDermid syndrome?
If a child is found to have Phelan-McDermid syndrome and the parents' genetic testing results are normal, the chance to have another child with Phelan-McDermid syndrome is likely very low (< 1%). However, there has been a reported case of a mother who did not have Phelen-McDermid syndrome but who had two children with Phelan-McDermid syndrome. It is suspected that some of her egg cells had the chromosome 22 deletion and other egg cells that had normal chromosomes. Having some egg cells (or sperm cells in males) with a genetic change and others that are normal is called germline or gonadal mosaicism. Germline mosaicism can rarely able to be detected through standard genetic blood tests. If a parent has some egg cells or some sperm cells with a chromosome 22q13.3 deletion, their chance to have more children with Phelan-McDermid syndrome is increased, but the exact recurrence risk depends on the number of egg or sperm cells with the deletion. If ALL the egg or sperm cells have the chromosome 22q13.3 deletion, then all biological children would be predicted to have Phelan-McDermid syndrome; however that would very, very rare as it would mean that every precursor cell to the egg or sperm would have had to have both chromosome 22s with 22q13.3 deletions. If half of the egg or sperm cells have the chromosome 22q13.3 deletion, then the risk would be up to 50% chance to have a baby with Phelan-McDermid syndrome. In general, if there is germline mosaicism, the risk for another child with Phelan-McDermid syndrome may be low (around 1%), moderate (as in 6%), or high (up to 50%), depending on both the proportion of egg or sperm cells with the chromosome 22 deletion and the number of embryos with the chromosome 22 deletion that implant, grow, and develop successfully.
If a couple has had one child with Phelan-McDermid syndrome and is concerned about the risk to have another child with this syndrome, it can be very useful to speak with a genetic counselor in their area to learn more about the recurrence risk to have another baby with Phelan-McDermid syndrome, germline mosaicism, and preconceptional/prenatal testing options.
- Tabolacci E, Zollino M, Lecce R, Sangiorgi E, Gurrieri F, Leuzzi V, Opitz JM, Neri G. Two brothers with 22q13 deletion syndrome and features suggestive of the Clark-Baraitser syndrome. Clin Dysmorphol. 2005;14:127-32. PMID:15930901
- University of Miami Gonadal Mosaicism page (http://hihg.med.miami.edu/code/http/modules/education/Design/CoursePageContent.asp?ID=14)
- Baylor College of Medicine Parental Mosaicism Recurrence Risk Calculator http://www.recurrencerisk.org/