~ What are Related Disorders ~
Nearly 40% of SOFT’s family membership consists of parents whose child does not (or did not) have full trisomy 18 or 13. Mosaicism, partial trisomy, translocations and other related chromosomal disorders are trisomy phenotype variations. Full trisomy refers to three copies of a chromosome being present in all cells. Mosaic trisomy refers to two cell lines in a person; one with trisomy (3 copies of a chromosome), the other with the normal 2 copies. Partial trisomy occurs when only a piece of a chromosome is extra in a person’s cells due to translocation (the attachment of part of one chromosome to another).
In trisomy 13, translocation and mosaicism occur in about 20% of cases (Carey, Trisomy 13 Facts (2015) www.trisomy.org. In trisomy 18, less than 5% have mosaicism and about 2% partial trisomy (Cereda and Carey. 2012, The Trisomy 18 Syndrome). A variable phenotype results in a wider range of outcomes; generally more optimistic, though some might be as affected as a child with a full trisomy. Retesting for mosaicism is sometimes suggested for a child with a full trisomy diagnosis, who is doing better than predicted. Survival statistics for full trisomy are generally not applicable to variable phenotypes.
Studies of groups of individuals with rare disorders are hampered by their small numbers or uniqueness, but there might be case studies for some. The NIH Genetic and Rare Disease Information Center provides information about some disorders at: https://rarediseases.info.nih.gov/. The National Organization for Rare Disorders (NORD) has a search page for rare disorders at https://rarediseases.org/for-patients-and-families/information-resources/rare-disease-information/
Debbie Bruns, PhD, principal investigator for the TRIS project, has worked with SOFT families for a number of years, researching rare chromosomal disorders. See her Tracking Rare Incident Syndromes (TRIS) Project and other information at https://trisomy.wpengine.com/?page_id=10147 “TheTRIS project works to increase the knowledge base on rare incidence trisomy conditions, and to make this information available to families and interested educational, medical and therapeutic professionals.” – Dr Debbie Bruns. You may see your child’s disorder already in her studies. Dr Bruns welcomes participation about any chromosomal disorder from as many parents as possible. Her project includes What is Trisomy 9, and this may be found here. Parents can also find detailed information about chromosome 9 at NORD https://rarediseases.org/rare-diseases/chromosome-9-trisomy-mosaic/.
Drs Anna Cereda and John C. Carey discuss mosaicism and partial trisomy under etiology and pathogenesis in The Trisomy 18 Syndrome (2012). https://www.ojrd.com/content/pdf/1750-1172-7-81.pdf
Dr Stephen Braddock, Geneticist, presented a Related Disorder workshop at the 2009 SOFT conference. https://trisomy.wpengine.com/wp-content/uploads/2015/02/Related-Disorders-Braddock.pdf
SOFT is a member organization of the Global Genes Network which might be a resource of interest to those with a rare disease. There are stories from families on their site.
Related Disorder Support Groups