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December 20, 2000

Genetics and Heredity

Question from Gillett, Pennsylvania, USA:

My five year old adopted son was diagnosed with transient neonatal diabetes at one month of age. He was on insulin for a year. At that time, his doctor said his pancreas had matured, and he no longer needed insulin. Two months ago, when he was placed with us for adoption, a routine physical showed high blood sugar levels. His case is currently being researched by an endocrinologist. His fasting blood sugars have been 79-171 mg/dl [4.4-9.5 mmol/L] and his after meal levels have been 275-317 mg/dl [15.3-17.6 mmol/L]. He tested negative for islet cell antibodies and GAD antibodies. The doctor feels that he has uniparental bisomy of chromosome 6. We cannot do genetic testing of the parents because of the adoption situation. Can you explain what uniparental bisomy of chromosome 6 is? Is there any predicatable course for this disease?


Transient Neonatal Diabetes (TND) is an exceedingly rare condition with one recent estimate of only 1:400,000 births. For that reason, not a lot is known about the natural history of the condition. However, there has been a recent flurry of interest in a link to uniparental disomy of chromosome 6. What happens here is that whereas in normal human reproduction one half of each of the 23 chromosomes plus one of the sex chromosomes is passed to each gamete (sperm or ovum), in these cases a double dose of chromosome 6 goes to their sperm. In the offspring, there is no maternal DNA in chromosome 6 and a double dose of paternal DNA. This is something of an oversimplification because the paternal disomy may only affect part of the chromosome, particularly the long arm. These infants are often small for gestational age and may have other (usually minor) abnormalities, particularly a large tongue (macroglossia) at birth. This is not an autoimmune process so that the negative antibody tests could be anticipated. Furthermore, not all cases of TND have disomy. A recent report estimated only 20%, and in not every case is the diabetes transient. In your son’s case some of the fasting blood sugar levels are abnormal, and all of the after meal ones seem to be.

A specific diagnosis cannot be made from simple karyotyping and without the biological father, it would be difficult, but not impossible, to do it with the necessary highly specialised DNA techniques. Whilst this may be intellectually disappointing, it really makes no difference to treatment which will be that of type 2 diabetes in a small child, and he may well never require insulin.

If you are inclined to grapple in more detail with this issue you will find a number of recent accounts in PubMed, if you search under ‘parental disomy and neonatal diabetes’, but at the moment there is no clear understanding as to exactly why the overexpression of paternal chromosome 6 causes the diabetes.