Subtelomere deletions and translocations are frequently familial.

In recent years, strategies have been developed to investigate the possible role of chromosomal subtelomere regions in genetic disorders. The present study was to determine the incidence of familial subtelomeric abnormalities among individuals with developmental delay, idiopathic mental retardation, or non-specific congenital abnormalities. A review was conducted for patients and their relatives on whom subtelomeric DNA fluorescence in situ hybridization (telo-FISH) studies were performed. Patients were identified through a search of the Mayo Genetics System (MGS) database. Of 2,170 consecutive telo-FISH index case studies completed in our laboratory between January 2002 and December 2003, 121 or 5.6% had abnormalities of the subtelomere region. The present report includes 18 other abnormal index cases seen prior to 2002 to yield a total of 139 abnormal index cases. This represents 71 index patients with deletions, 53 index patients with derivative chromosomes, and 15 index patients with balanced rearrangements. A familial abnormality was identified in 29 (51.8%) of 56 families in whom parents and/or sibs were available for testing. Among 28 patients with deletions, 9 (32%) had an inherited deletion, whereas 19 (68%) were de novo. Family members of 20 index patients with derivative chromosomes were tested. Of these, 13 (65%) patients inherited the abnormality from a parent (12 from a parent who had a balanced translocation and 1 from a parent with the same abnormality), while 7 (35%) apparently arose de novo. Seven (88%) of 8 with balanced translocations inherited the translocation from one parent. The most common familial abnormalities involved 8pter deletion or rearrangement. The incidence of familial subtelomeric abnormalities is significantly high making parental telo-FISH studies an essential part of the investigation of patients with subtelomeric chromosome abnormalities. (c) 2005 Wiley-Liss, Inc.