FISH

SMALL SUPERNUMERARY MARKER CHROMOSOMES

-  sSMC -

BASICS - EFFECTS

 

What do sSMC?

 

- In 74% of the cases a de novo sSMC has no phenotypic effects! {156}

- In 1974 it was postulated "that a bisatellited chromosome by association with acrocentric chromosomes may interfere with mitosis at a critical stage of fetal development". {22}

- Of prenatally ascertained cases with sSMC the following percentage shows abnormal phenotypes:
18% of 33 cases {13} 30% of 27 cases {42}; 13% of 123 cases {43}.

- The overall risk for abnormal phenotype is 10.9% for cases with satellited sSMC and 14.7% for cases with non-satellited sSMC {43}.

- The risk of an abnormal phenotype associated with a de novo sSMC (excluding those derived from chromosome 15) is 7% (if sSMC is from 13, 14, 21 or 22) and 28% (for non-acrocentric chromosomes) {44}.

-  44% of prenatally ascertained cases with sSMC are familial cases {42} - first report on a 3 generation sSMC without clinical effect described in {71}.

-  "The phenotypes associated with the presence of a marker vary from normal to severely abnormal." {66}

-  It is discussed that sSMC may lead to reduced fertility in males without additional clinical symptoms in connection with the sSMC {57; 65; 69; 90}

- Of 123 cases with sSMC detected prenatally between 1970 and 1989 in USA 37 were electively terminated, while only 4 of the remaining 86 pregnancies ended with a still birth or spontaneous abortion. 9 additional cases of the 86 cases were born with abnormalities (= 10.5%) {43}.

- In {160} in 1/2 prenatally detected sSMC the MOM of ßC-hCG:Cr was reduced to 0.31 and in both cases the MOM of free ß-hCG:Cr was enhanced to ~1.5.

- In one male with normal sperm (normozoospermia) sSMC present in 26% of sperm and 42% of fertilized embryos {168}

 

References