1. ¹Neuromuscular Unit, Mossakowski Medical Research Centre, Polish Academy of Sciences, Warsaw, Poland
2. ²Department of Genetics, Institute of Psychiatry and Neurology, Warsaw, Poland
3. ³Department of Neurology, Medical University of Warsaw, Warsaw, Poland
4. ⁴Department of Medical Genetics, National Research Institute of Mother and Child, Warsaw, Poland
5. ⁵Department of Medical Genetics, Polish-American Institute of Pediatrics, Jagiellonian University Medical College, Cracow, Poland
6. ⁶Department of Neurology, Medical University of Gdañsk, Gdañsk, Poland

Correspondence: Dr M Jdrzejowska, Neuromuscular Unit, Mossakowski Medical Research Center, Polish Academy of Sciences, Ul. Pawinskiego 5, 02-106 Warsaw, Poland. Tel/Fax: +48 22 658 45 01; E-mail: mariaj@cmdik.pan.pl

Received 17 July 2007; Revised 22 January 2008; Accepted 7 February 2008; Published online 12 March 2008.

Abstract

In this report, we present three families in which we identified asymptomatic carriers of a homozygous absence of the SMN1 gene. In the first family, the bialleleic deletion was found in three of four siblings: two affected brothers (SMA type 3a and 3b) and a 25-years-old asymptomatic sister. All of them have four SMN2 copies. In the second family, four of six siblings are affected (three suffer from SMA2 and one from SMA3a), each with three SMN2 copies. The clinically asymptomatic 47-year-old father has the biallelic deletion and four SMN2 copies. In the third family, the biallelic SMN1 absence was found in a girl affected with SMA1 and in her healthy 53-years-old father who had five SMN2 copies. Our findings as well as those of other authors show that an increased number of SMN2 copies in healthy carriers of the biallelic SMN1 deletion is an important SMA phenotype modifier, but probably not the only one.