SHH gene is a human homologue of the Drosophila gene encoding inductive signals involved in patterning the early embryo, known to be (functionally) highly conserved in many species. The gene was mapped to chromosome 7 (7q36), previously designated as the locus for the gene involved in holoprosencephaly (HPE3) .
The available data provide the evidence that SHH mutations may cause the failure of cerebral hemispheres to separate into distinct left and right halves, leading to HPE, which is one of the most common developmental anomalies of forebrain and midface . Although the majority of HPE are sporadic, familial cases are not rare, with clear autosomal dominant inheritance.
In our first case, the couple presented for PGD with two children showing the clinical signs of HPE  (Figure 3.32). One of them, a female with severe HPE and cleft lip and palate died shortly after birth. The chromosomal analysis performed using peripheral blood lymphocytes of both this child and the parents was normal, but DNA analysis in the child's autopsy material demonstrated the presence of SHH nonsense mutation due to GAG>TAG sequence change leading to premature termination of the protein at position 256 (Glu256^stop)  (Figure 3.33). SHH protein is an intercellular signalling molecule, which is synthesised as a precursor undergoing autocatalytic internal cleavage into a highly conserved domain (SHH-N) with signalling activity, and a more divergent domain (SHH-C), which, in addition to precursor processing, acts as an intramolecular cholesterol transferase crucial for proper patterning activity in animal development. Although the effect of the above nonsense mutation on SHH function is unknown, the resulting protein may fail to fulfill the expected signalling function in early morphogenesis [12,70].
The same mutation was found in their 5-years-old son, who was born after a full-term, normal pregnancy weighing 6 pounds, with a birth length of 183 inches. This child has less severe facial dysmorphisms, which included microcephaly, Rathke's pouch cyst, single central incisor, and choanal stenosis (the last was treated surgically after birth with dilatation). There was also clinodactily of the fifth fingers and incurved fourth toes bilaterally. The child's growth was slow in the first two years, but since then he has been maintaining a reasonably good growth and presently has normal social and cognitive development.
The couple had another pregnancy, which ended in spontaneous abortion due to Turner syndrome (45, X), showing no inheritance of the SHH mutation. The mutation was not found in either parent's genomic DNA, although the paternity testing showed that the father was in fact a biological father of both affected children. This clearly suggested a de novo gonadal mutation in one of the parents, which has been identified by a single sperm genotyping in the present study (see below).
Two PGD cycles were performed based on single blastomere analysis removed from the eight-cell embryos and tested by multiplex nested PCR analysis, involving specific mutation testing simultaneously with linked marker analysis [47, 57]. Of 15 embryos in the first cycle, 12 were available for blastomere biopsy at the eight-cell stage. Blas-tomeres from four embryos failed to amplify, leaving eight with available data for mutation analysis. Seven of these eight embryos appeared to contain the mutant allele, while only one embryo was mutation-free and transferred, yielding no clinical pregnancy.
The second PGD cycle was performed one year later, in which 19 embryos were available, of which 10 were acceptable for blastomere biopsy and DNA analysis. Of these 10 biopsied single blas-tomeres, only one failed to amplify, leaving nine with available data for SHH gene and the marker, to identify the mutation free-embryos for transfer
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.